Ideal Weight Calculator

An ideal weight calculator is a mathematical tool that uses established scientific formulas to estimate a person's optimal body mass based primarily on their height and biological sex. While popular culture often associates the concept of an "ideal weight" with aesthetic goals, the medical and scientific communities utilize these calculations to determine safe medication dosages, assess nutritional needs, and evaluate baseline metabolic functions. By understanding the origins, mechanics, and clinical applications of these formulas, individuals and healthcare professionals can make precise, data-driven decisions regarding physiological health and pharmacological treatments.

What It Is and Why It Matters

The concept of an "ideal body weight" (IBW) refers to a mathematically derived target weight that represents a person's optimal lean body mass, independent of excess adipose (fat) tissue. In clinical and physiological contexts, a person's total body weight is divided into two primary compartments: lean body mass (muscle, bone, organs, water) and fat mass. Because fat tissue is far less metabolically active than lean tissue and receives significantly less blood flow, using a person's total body weight to make medical decisions can lead to catastrophic errors. The ideal weight calculation solves this problem by providing a standardized estimate of a patient's functional, metabolically active tissue mass.

This metric matters profoundly in the fields of pharmacology, anesthesiology, and clinical nutrition. When a physician prescribes a hydrophilic (water-soluble) medication, the drug distributes primarily into lean tissue and bodily fluids, not into fat. If a physician doses a highly toxic antibiotic like gentamicin based on the total body weight of a 300-pound patient, the patient will receive a massive overdose, potentially resulting in permanent kidney failure or hearing loss. The ideal weight calculation provides the safe baseline for that dosage. Furthermore, in clinical nutrition, dietitians use ideal body weight to calculate basal energy expenditure for patients on ventilators or feeding tubes, ensuring they receive the exact caloric intake required to maintain organ function without overfeeding, which can stress the respiratory system. Beyond the hospital setting, ideal weight serves as a vital epidemiological benchmark, allowing researchers to study population health, track obesity trends, and establish baseline risk factors for cardiovascular disease, type 2 diabetes, and metabolic syndrome.

History and Origin

The quest to define an optimal human body weight began in the late 19th century, driven not by medicine, but by the burgeoning field of physical anthropology. In 1871, the French surgeon and anthropologist Paul Broca created "Broca's Index," the first widely recognized formula for estimating ideal weight. Broca's rule was famously simple: a person's ideal weight in kilograms was equal to their height in centimeters minus 100. While revolutionary for its time, Broca's Index failed to account for the physical differences between biological sexes and produced highly inaccurate targets for exceptionally tall or short individuals.

The modern understanding of ideal weight shifted dramatically in 1943 when the Metropolitan Life Insurance Company published its first "Desirable Weight" tables. Actuaries analyzed millions of life insurance policies to determine which height-to-weight ratios correlated with the longest lifespans and the lowest mortality rates. These tables categorized individuals by frame size (small, medium, large) and became the global standard for decades. However, the medical community found tables cumbersome for rapid clinical calculations. In 1974, Dr. Ben J. Devine published a landmark paper in the Drug Intelligence and Clinical Pharmacy journal. Devine needed a quick, reliable mathematical formula to calculate the clearance of the drug gentamicin. He essentially reverse-engineered the Metropolitan Life Insurance tables to create a simple algebraic equation that could be memorized by pharmacists and physicians.

Following Devine's breakthrough, other researchers identified slight mathematical biases in his formula and attempted to perfect it. In 1983, Dr. J.D. Robinson published a modified formula to better account for the physiological proportions of women, while Dr. D.R. Miller published another variation in the same year aimed at correcting the weight curve for taller individuals. Decades earlier, in 1964, Dr. G.J. Hamwi had introduced a simple "rule of thumb" for endocrinologists and dietitians to quickly estimate ideal weight for diabetic patients. Today, these four formulas—Devine, Robinson, Miller, and Hamwi—form the indisputable foundation of all modern ideal weight calculations, representing a fascinating evolution from 19th-century anthropology to actuarial statistics, and finally to precise pharmacological mathematics.

Key Concepts and Terminology

To deeply understand ideal weight calculations, one must master the specific terminology utilized by physiologists and clinicians. The most fundamental concept is Total Body Weight (TBW), which is simply the absolute measurement of a person's mass as determined by a standard scale, encompassing bone, muscle, fat, water, and undigested food. In contrast, Ideal Body Weight (IBW) is the theoretical target weight derived from height and sex formulas, representing an optimal physiological state.

Lean Body Mass (LBM) refers to the total weight of your body minus all fat mass. While IBW is an estimation of a healthy weight, LBM is an actual physiological measurement of everything in the body that is not adipose tissue. Adipose Tissue is the scientific term for body fat, which serves as energy storage, thermal insulation, and an endocrine organ. In pharmacological contexts, understanding the difference between lean and adipose tissue is critical due to Pharmacokinetics, the study of how the body absorbs, distributes, metabolizes, and excretes drugs. A key pharmacokinetic concept tied to ideal weight is the Volume of Distribution (Vd), which describes how extensively a drug disperses throughout the body's tissues versus remaining in the blood plasma.

Another vital concept is Body Frame Size, which refers to the structural robustness of an individual's skeleton. Because bone density and skeletal width vary, two people of the exact same height can have different optimal weights. Frame size is typically categorized as small, medium, or large, and is accurately measured via wrist circumference or elbow breadth. Finally, Basal Metabolic Rate (BMR) represents the minimum number of calories a body requires to perform basic life-sustaining functions at rest. Because muscle burns significantly more calories than fat, clinical calculations of BMR often rely on Ideal Body Weight rather than Total Body Weight to prevent overestimating the caloric needs of obese individuals.

How It Works — Step by Step

The mathematics of ideal weight calculations rely on linear equations that establish a baseline weight for a specific height (universally set at 5 feet, or 60 inches) and add a specific weight increment for every inch above that baseline. To demonstrate exactly how these mechanics operate, we will break down the four primary formulas and apply them to a single, concrete example: a biological male who is 5 feet 10 inches tall (70 inches total).

The Devine Formula

The Devine formula is the most widely used equation in clinical medicine.

Male Formula: 50.0 kg + 2.3 kg for every inch over 5 feet.
Female Formula: 45.5 kg + 2.3 kg for every inch over 5 feet.
Worked Example (5'10" Male): The subject is 10 inches over 5 feet. Multiply 10 inches by 2.3 kg to get 23.0 kg. Add this to the male baseline of 50.0 kg. The result is exactly 73.0 kg (160.9 lbs).

The Robinson Formula

The Robinson formula was designed to slightly adjust the Devine formula, producing slightly higher weights for men and slightly lower weights for women.

Male Formula: 52.0 kg + 1.9 kg for every inch over 5 feet.
Female Formula: 49.0 kg + 1.7 kg for every inch over 5 feet.
Worked Example (5'10" Male): The subject is 10 inches over 5 feet. Multiply 10 inches by 1.9 kg to get 19.0 kg. Add this to the male baseline of 52.0 kg. The result is exactly 71.0 kg (156.5 lbs).

The Miller Formula

The Miller formula utilizes a higher baseline weight but a lower incremental addition per inch, flattening the curve for taller individuals.

Male Formula: 56.2 kg + 1.41 kg for every inch over 5 feet.
Female Formula: 53.1 kg + 1.36 kg for every inch over 5 feet.
Worked Example (5'10" Male): The subject is 10 inches over 5 feet. Multiply 10 inches by 1.41 kg to get 14.1 kg. Add this to the male baseline of 56.2 kg. The result is exactly 70.3 kg (155.0 lbs).

The Hamwi Formula

The Hamwi formula is unique because it was originally written using imperial units (pounds) rather than metric units (kilograms), making it a favorite among American dietitians.

Male Formula: 106 lbs + 6 lbs for every inch over 5 feet.
Female Formula: 100 lbs + 5 lbs for every inch over 5 feet.
Worked Example (5'10" Male): The subject is 10 inches over 5 feet. Multiply 10 inches by 6 lbs to get 60 lbs. Add this to the male baseline of 106 lbs. The result is exactly 166.0 lbs (75.3 kg).

As demonstrated, the same 5-foot 10-inch male has an ideal weight ranging from 155.0 lbs (Miller) to 166.0 lbs (Hamwi), highlighting that "ideal weight" is a calculated estimate rather than an absolute biological truth.

Types, Variations, and Methods

Because no single mathematical equation can perfectly capture the vast diversity of human physiology, professionals utilize different types and variations of ideal weight calculations depending on the specific problem they are trying to solve. The four primary formulas (Devine, Robinson, Miller, and Hamwi) represent the "Linear Incremental Method." These are best suited for rapid clinical decision-making where precise, reproducible numbers are required across different medical staff. Among these, the Devine formula is universally recognized as the gold standard for pharmacological dosing, while the Hamwi formula remains the dominant method in nutritional science and dietetics for establishing baseline caloric targets.

Another entirely different method for determining ideal weight relies on the Body Mass Index (BMI) Range Method. BMI is calculated by dividing a person's weight in kilograms by the square of their height in meters (kg/m²). The World Health Organization defines a "normal" or "healthy" BMI as falling between 18.5 and 24.9. To find an ideal weight using this method, a practitioner selects a target BMI—often the exact midpoint of 21.7—and reverse-engineers the formula. For example, for a person who is 1.75 meters tall (approximately 5'9"), the calculation is 21.7 multiplied by (1.75 squared). 1.75 squared is 3.0625. Multiplying 3.0625 by 21.7 yields a target ideal weight of 66.45 kilograms (146.5 lbs). This method is highly favored by epidemiologists because it provides a healthy range rather than a single rigid number.

A third variation involves adjusting the linear formulas based on Frame Size. Because the standard Devine or Hamwi formulas assume a "medium" skeletal frame, practitioners will apply a percentage modifier for individuals with visibly different bone structures. The standard protocol dictates subtracting 10 percent from the calculated ideal weight for individuals with a small frame, and adding 10 percent to the calculated ideal weight for individuals with a large frame. A large-framed 5'10" male who calculates a Hamwi ideal weight of 166 pounds would add 16.6 pounds, resulting in a frame-adjusted ideal weight of 182.6 pounds. This variation provides a much-needed layer of personalization to otherwise rigid algebraic equations.

Real-World Examples and Applications

To grasp the true utility of ideal weight calculations, one must observe how they are applied in high-stakes, real-world scenarios. Consider the field of critical care pharmacology. A 55-year-old male patient is admitted to the Intensive Care Unit with a severe, life-threatening bacterial blood infection. The patient is 5 feet 8 inches tall and weighs 320 pounds (145.1 kg). The critical care physician needs to prescribe an intravenous dose of Vancomycin, a powerful antibiotic. If the physician uses the patient's Total Body Weight of 145.1 kg, the resulting massive dose of medication will overwhelm the patient's kidneys, causing acute renal failure. Instead, the physician calculates the patient's Ideal Body Weight using the Devine formula: 50 kg + (8 inches × 2.3 kg) = 68.4 kg. The physician doses the medication based on a physiological mass of 68.4 kg, entirely ignoring the excess 76.7 kg of adipose tissue. The medication clears the infection, and the patient's kidneys remain perfectly healthy.

In the realm of clinical nutrition and dietetics, ideal weight is equally vital. Imagine a 40-year-old female who is 5 feet 4 inches tall and currently weighs 210 pounds. She consults a registered dietitian for a medically supervised weight loss plan. The dietitian needs to calculate her Basal Metabolic Rate (BMR) to establish a safe daily caloric deficit. If the dietitian calculates BMR using her current weight of 210 pounds, the formula will assume that all 210 pounds are metabolically active muscle and organ tissue, resulting in a massive overestimation of her daily caloric burn. The patient would be instructed to eat too many calories and would fail to lose weight. Instead, the dietitian calculates her Hamwi Ideal Body Weight: 100 lbs + (4 inches × 5 lbs) = 120 lbs. The dietitian calculates her baseline caloric needs based on 120 pounds of lean mass, ensuring the prescribed diet creates a genuine, effective energy deficit.

Another critical application occurs in anesthesiology. When an anesthesiologist prepares a patient for surgery, they must administer a paralytic agent (neuromuscular blocker) such as Rocuronium. Paralytic agents are highly hydrophilic; they do not enter fat cells. If a 5-foot 2-inch, 250-pound female patient is given a dose of Rocuronium based on her 250-pound total weight, her muscles will remain paralyzed long after the surgery is completed, requiring prolonged mechanical ventilation and occupying a valuable ICU bed. The anesthesiologist calculates her Devine IBW at 50.1 kg (110 lbs) and doses the paralytic strictly to that number, ensuring the patient wakes up and begins breathing independently exactly as the surgeon finishes suturing.

Common Mistakes and Misconceptions

The most pervasive and psychologically damaging misconception regarding ideal weight calculations is the belief that the resulting number represents an aesthetic ideal or a universal standard of physical attractiveness. The term "ideal" is a medical artifact from the 1970s; it means "pharmacologically optimal for calculating drug clearance," not "how you should look in a bathing suit." When healthy, muscular individuals input their metrics into an ideal weight formula and see a number that is 20 or 30 pounds lighter than their current weight, they often experience unnecessary distress, mistakenly believing they are severely overweight.

A closely related mistake is the failure to account for muscle mass. The Devine, Robinson, Miller, and Hamwi formulas are entirely blind to body composition. They assume a completely average ratio of muscle to fat. Therefore, a professional bodybuilder, a collegiate athlete, or an avid weightlifter will almost always weigh significantly more than their calculated ideal weight. If a 6-foot-tall male athlete has 15 percent body fat and weighs 200 pounds, he is in peak physical condition. However, the Devine formula will state his "ideal" weight is 171 pounds. Attempting to force an athlete to drop to an ideal weight target inevitably requires the destruction of healthy, metabolically vital muscle tissue.

Beginners also frequently misunderstand how to apply the formulas to women, assuming that the biological sex differences in the formulas are arbitrary. In reality, the formulas account for the biological fact that women naturally carry a higher essential body fat percentage than men for reproductive health and hormonal balance. Men generally require a minimum of 2 to 5 percent body fat for basic physiological function, while women require 10 to 13 percent. The formulas reflect this difference in lean mass distribution. Attempting to apply a male formula to a female patient, or assuming a woman should strive for the same weight-to-height ratio as a man, represents a fundamental misunderstanding of human endocrinology and physiology.

Best Practices and Expert Strategies

Expert practitioners do not rely on ideal body weight as a standalone, absolute metric; instead, they use it as a foundational component within a broader clinical framework. The most critical expert strategy is the utilization of the Adjusted Body Weight (AjBW) formula. When a patient is significantly obese (typically defined as weighing more than 120 percent of their calculated Ideal Body Weight), using pure IBW for drug dosing can actually lead to under-dosing. While fat tissue is less metabolically active than muscle, it is not completely inert; it still requires some blood flow and contributes to the volume of distribution for certain medications.

To solve this, professionals calculate Adjusted Body Weight. The standard medical formula for AjBW is: Ideal Body Weight + 0.4(Total Body Weight - Ideal Body Weight). The "0.4" represents a 40 percent correction factor, acknowledging that excess adipose tissue contributes roughly 40 percent as much to drug clearance as lean tissue. For example, consider a 5'10" male who weighs 260 pounds, with an IBW of 160 pounds. His excess weight is 100 pounds. The practitioner multiplies the 100 pounds of excess weight by 0.4 to get 40 pounds. They then add that 40 pounds back to the 160-pound ideal weight, resulting in an Adjusted Body Weight of 200 pounds. The physician will then dose medications based on 200 pounds, striking the perfect balance between avoiding toxicity and ensuring therapeutic efficacy.

Another best practice among top dietitians and sports scientists is to cross-reference ideal weight calculations with actual body composition data. Rather than blindly prescribing a weight loss target based on the Hamwi formula, an expert will use a tool like a DEXA scan or bioelectrical impedance scale to determine the patient's actual lean body mass. If a patient's measured lean body mass is already higher than their mathematically calculated Ideal Body Weight, the practitioner will immediately discard the formula. The rule of thumb among experts is that physiological reality always supersedes algebraic estimation. The formulas are meant to be a proxy for lean mass when actual lean mass cannot be directly measured.

Edge Cases, Limitations, and Pitfalls

While ideal weight formulas are highly functional for the majority of the adult population, they break down dramatically when applied to specific edge cases. The most glaring mathematical limitation occurs with extremely short individuals. The Devine, Robinson, Miller, and Hamwi formulas all use a baseline height of 5 feet (60 inches). They were designed to add weight for every inch over 5 feet. When a patient is under 5 feet tall, practitioners attempt to subtract weight for every inch under 60 inches. However, because the formulas are linear, subtracting the standard increments rapidly produces absurdly low, medically dangerous target weights. For a woman who is 4 feet 8 inches tall, strict adherence to the Devine subtraction would yield an ideal weight of 36.3 kg (80 lbs), which borders on severe clinical malnutrition.

Another significant pitfall involves amputees. Ideal weight formulas assume a complete, anatomically standard human skeleton. If a patient has undergone a major amputation, calculating their ideal weight based on their height will yield a dangerously inaccurate number. Medical professionals must use specific percentage deductions to adjust the target. For example, an entire leg accounts for approximately 16 percent of total body weight, an entire arm is 5 percent, a lower leg below the knee is 5.9 percent, and a foot is 1.5 percent. If a 5'10" male with an ideal weight of 160 pounds has had a full leg amputation, the practitioner must reduce the 160-pound target by 16 percent (25.6 pounds), resulting in a true anatomical ideal weight of 134.4 pounds.

Elderly populations present another critical limitation. As human beings age, they naturally lose bone density (osteopenia or osteoporosis) and muscle mass (sarcopenia), while often experiencing a decrease in height due to spinal compression. An 80-year-old man who has shrunk from 5'10" to 5'8" will have vastly different physiological ratios than a 30-year-old man who is naturally 5'8". Applying standard ideal weight formulas to geriatric patients often overestimates their lean body mass, potentially leading to medication overdoses. In these edge cases, geriatricians rely heavily on direct clinical observation, renal function tests, and frailty indexes rather than relying solely on the rigid mathematics of Devine or Hamwi.

Industry Standards and Benchmarks

In the medical and health sciences industries, specific organizations dictate how and when ideal weight calculations should be utilized. The American Society of Health-System Pharmacists (ASHP) sets the standard for clinical dosing. According to ASHP guidelines, Ideal Body Weight is the mandatory benchmark for dosing narrow-therapeutic-index drugs such as aminoglycosides (gentamicin, tobramycin), theophylline, and acyclovir. However, the ASHP standard explicitly dictates that if a patient's Total Body Weight is less than their calculated Ideal Body Weight, the pharmacist must always use the Total Body Weight for dosing to prevent accidental toxicity.

In the realm of clinical nutrition, the American Society for Parenteral and Enteral Nutrition (ASPEN) provides the definitive benchmarks. ASPEN guidelines state that for critically ill obese patients (BMI > 30), feeding formulas should not be based on actual total body weight. Instead, the industry standard is to provide 60 to 70 percent of the caloric needs calculated using the patient's Ideal Body Weight. This specific benchmark ensures "permissive underfeeding," a strategy that forces the obese patient's body to mobilize stored fat for energy while providing enough protein to prevent the breakdown of vital muscle tissue during hospitalization.

Globally, the World Health Organization (WHO) sets the benchmarks for population health using the BMI-derived ideal weight method. The WHO rigidly categorizes a BMI of 18.5 to 24.9 as normal, 25.0 to 29.9 as overweight, and 30.0 or higher as obese. When public health agencies, such as the Centers for Disease Control and Prevention (CDC) in the United States, report on the percentage of a population that is "at their ideal weight," they are universally referencing the percentage of individuals falling within this specific WHO BMI benchmark, rather than analyzing individual Devine or Hamwi calculations.

Comparisons with Alternatives

Ideal weight calculations are just one of several tools used to assess physical health and body composition. The most common alternative is Body Mass Index (BMI). While ideal weight formulas provide a specific target weight in pounds or kilograms, BMI provides a dimensionless ratio. The advantage of BMI is its universal standardization and ease of calculation; it requires no memorization of complex linear increments. However, the disadvantage is that BMI is purely a measure of mass relative to height. It cannot distinguish between a 250-pound sedentary individual with high body fat and a 250-pound professional linebacker with immense muscle mass; both are classified as "obese" under BMI. Ideal weight formulas, while also blind to muscle mass, are generally preferred in clinical settings because they yield an actionable metric (a specific weight) that can be directly plugged into drug dosing software.

A far superior, though less accessible, alternative is Direct Body Composition Analysis, such as Dual-Energy X-ray Absorptiometry (DEXA) scans, hydrostatic weighing, or Bioelectrical Impedance Analysis (BIA). These methods physically measure the exact percentage of fat, muscle, bone, and water in a specific human body. The advantage is absolute precision; there is no guessing or algebraic estimation involved. A DEXA scan will tell a practitioner exactly how many pounds of metabolically active lean mass a patient possesses. The profound disadvantage of these alternatives is cost, time, and availability. A physician in a busy emergency room cannot order a $150, 20-minute DEXA scan for every patient who needs an antibiotic. Ideal weight formulas remain ubiquitous precisely because they are free, instantaneous, and mathematically robust enough for triage and immediate clinical action.

Another functional alternative used primarily in cardiovascular health assessment is the Waist-to-Hip Ratio (WHR) or Waist Circumference. Rather than trying to calculate an optimal total weight, these metrics focus entirely on fat distribution. Visceral fat (fat stored around the abdominal organs) is vastly more dangerous to cardiovascular health than subcutaneous fat (fat stored under the skin, such as on the thighs). A waist circumference over 40 inches for men or 35 inches for women is a massive red flag for metabolic syndrome. The advantage of WHR over ideal weight is that it directly measures the specific type of fat that causes disease. However, WHR is useless for pharmacological dosing or calculating basal metabolic rates. Ultimately, ideal weight formulas excel at estimating lean mass for metabolic and medical purposes, while alternatives like WHR excel at predicting chronic disease risk.

Frequently Asked Questions

Why do men and women have different ideal weight formulas? Biological males and females possess fundamentally different physiological compositions due to endocrinological (hormonal) differences. Men naturally produce high levels of testosterone, which promotes greater bone density, broader skeletal frames, and increased skeletal muscle mass. Women produce higher levels of estrogen, which promotes the storage of essential body fat necessary for reproductive health and menstruation. The formulas account for this biological reality by setting higher baseline weights and higher incremental additions for men, ensuring that the estimated lean mass accurately reflects standard human dimorphism.

Can I use these formulas if I am under 18 years old? No, the Devine, Robinson, Miller, and Hamwi formulas are strictly designed for fully grown adults. Children and adolescents are in active phases of skeletal growth, hormonal development, and rapid body composition changes. Applying adult linear formulas to a 12-year-old will yield wildly inaccurate and potentially dangerous weight targets. Pediatricians use specialized growth charts developed by the CDC or WHO, which plot a child's height, weight, and head circumference against age-matched percentiles to ensure they are developing along a healthy, natural trajectory.

Why does my calculated ideal weight seem incredibly low? Most people are shocked by their calculated ideal weight because modern societal norms have significantly skewed our perception of what a healthy weight looks like. In many Western countries, over 70 percent of the adult population is overweight or obese. Because we are surrounded by heavier bodies, a truly lean, medically optimal body often appears "too skinny" to the modern eye. Furthermore, the formulas represent your lean body mass plus a very minimal amount of healthy fat; they do not account for the extra 10 to 15 pounds of cosmetic muscle or recreational weight many healthy people carry.

Which of the four formulas is the most accurate? "Accuracy" depends entirely on the context of the application. If you are a pharmacist calculating the clearance rate of a potentially toxic medication, the Devine formula is the universally accepted standard and is therefore the "most accurate" for that purpose. If you are a dietitian trying to establish a baseline caloric need for a diabetic patient, the Hamwi formula is the industry standard. For general personal use, all four formulas will produce a cluster of numbers within a 5 to 10-pound range. Taking the average of all four formulas often provides the most reasonable, balanced target for a layperson.

What should I do if my current weight is much higher than my ideal weight? If your actual weight significantly exceeds your calculated ideal weight, you should not immediately panic or adopt an extreme starvation diet. First, assess your body composition; if you lift weights heavily, the excess mass may be healthy muscle. If the excess mass is adipose tissue, use the ideal weight as a long-term, multi-year goal rather than a short-term requirement. Consult a healthcare provider or registered dietitian to calculate a safe, sustainable caloric deficit that aims for a weight loss of 1 to 2 pounds per week, prioritizing the preservation of muscle mass through adequate protein intake.

Does my ideal weight change as I get older? Mathematically, the standard formulas do not change with age; they are based solely on height and sex. However, physiologically, your optimal weight absolutely changes. As humans age past 60, carrying a slight amount of extra weight (a BMI between 25 and 27) is actually associated with lower mortality rates and better outcomes during hospitalizations or falls—a phenomenon known as the "obesity paradox" in geriatrics. Therefore, while the calculator will give an 80-year-old the same number as a 30-year-old, a geriatric physician will generally prefer the 80-year-old to weigh 10 to 15 pounds more than the formula suggests.