🎯 Ideal Body Weight Calculator

Three Formulas, One Question: What Should I Weigh?

Walk into any hospital and ask a pharmacist, nurse, or dietitian to calculate a patient's ideal body weight. Chances are they'll reach for the Devine formula — a tidy equation that's been scribbled on clinical notepads since 1974. But ask a different clinician and they might prefer Robinson's refinement, or the older Hamwi scale. Each of these gives a slightly different number, and none of them are the same as what the BMI chart suggests. So which one is right?

The honest answer is that none of them are "right" in any universal sense. They're tools — each designed for a specific clinical context — and the meaningful question isn't which formula wins, but what these numbers collectively tell you about a healthy weight range for your height.

Where These Formulas Came From

The Hamwi formula is the oldest of the three, published in 1964 primarily to help endocrinologists calculate caloric needs for diabetic patients. Its logic is straightforward: for men, start at 106 pounds for the first five feet of height, then add six pounds per inch beyond that. For women, start at 100 pounds and add five pounds per inch. In metric terms, that translates to a base of roughly 48 kg for men and 45.5 kg for women, with increments per inch above five feet.

Dr. B.J. Devine published his formula a decade later, in 1974. It was developed primarily to help clinicians estimate drug dosing — many medications are calibrated to lean body mass rather than total weight, and a simple height-based estimate was more practical than a body composition scan. Devine's formula has a slightly gentler slope than Hamwi's for men (2.3 kg per inch versus 2.7), which tends to produce more conservative estimates for taller individuals.

The Robinson formula came in 1983, built from an analysis of Metropolitan Life Insurance tables that had themselves been a reference standard for decades. Robinson's version is the most conservative of the three for men — using 1.9 kg per inch above five feet — but slightly more generous for shorter women. The practical differences between Robinson and Devine are rarely more than a few pounds, but they reflect genuinely different underlying assumptions about body composition across heights.

How the Numbers Actually Compare

Consider a 5'10" (178 cm) man. The Devine formula gives 66.4 kg (146 lbs). Robinson gives 65.3 kg (144 lbs). Hamwi gives 72.9 kg (161 lbs). The healthy BMI range for that height spans from about 57.9 kg to 77.9 kg (128–172 lbs). That's a 17-pound spread just between the three formulas, and the BMI range is wider still.

For a 5'5" (165 cm) woman, the comparison looks like this: Devine gives 59.1 kg (130 lbs), Robinson gives 57.5 kg (127 lbs), and Hamwi gives 56.6 kg (125 lbs). The healthy BMI range for that height runs from 50.3 to 67.6 kg (111–149 lbs).

What you notice immediately is that the clinical formulas often converge near the lower end of the BMI-healthy range. This isn't a flaw — it reflects their original purpose. These formulas were created to estimate lean body mass, not to define an optimal weight for health. A person who weighs more than their Devine formula suggests isn't necessarily unhealthy; they might simply have more muscle mass, denser bones, or a larger body frame than the formula assumes.

The BMI Range: A Different Kind of Estimate

The healthy BMI range (18.5–24.9) approaches the question from the opposite direction. Rather than starting from height and estimating a target weight, it defines a weight band within which population-level health outcomes have generally been favorable. It's epidemiologically grounded rather than clinically derived.

The lower bound of the healthy BMI range (18.5) marks the threshold below which the risks of undernutrition and associated conditions begin to rise meaningfully. The upper bound (24.9) sits at the conventional cut-off for "overweight," though it's worth noting that this threshold has been criticized for underestimating risk in some populations and overestimating it in others.

For most people of average height, the BMI-based weight range is substantially wider than any single clinical formula. A 5'9" man has a healthy BMI weight range spanning from about 125 to 168 pounds — a 43-pound window. The Devine formula gives him a single number: 143 pounds. These are answering different questions, and understanding that difference helps you use both more intelligently.

Height Matters More Than You'd Expect

One interesting property of these formulas is how strongly height drives the outcome. The equations are strictly linear in height — every inch above five feet adds a fixed increment regardless of whether you're 5'1" or 6'5". In reality, body weight doesn't scale linearly with height; it scales more like height to the power of two or three (since body volume grows in three dimensions while height grows in one). This is actually one of the known limitations of these formulas for people who are very short or very tall.

At the extremes of height, the formulas can give results that seem clinically unreasonable. A man who is 6'6" would get a Devine estimate of about 96 kg (211 lbs), which is plausible. But a woman who is 4'10" would get a Devine estimate of only about 39 kg (86 lbs) — below the healthy BMI threshold for that height. This is why medical professionals use these formulas as starting points within a clinical context, not as standalone prescriptions.

Body Frame Size and Why It's Rarely Discussed

All three clinical formulas were originally intended to be adjusted for body frame size — small, medium, or large — using measurements like wrist circumference or elbow breadth. The Hamwi formula, for example, suggests adjusting the base estimate by 10% in either direction for small or large frames. In practice, these adjustments are almost never made, partly because frame measurements require a separate assessment and partly because the adjustments themselves weren't rigorously validated.

This matters because two people of the same height and sex can have meaningfully different bone structures. A person with a large frame carries more bone mass and will naturally weigh more at a given level of leanness. The formulas, as typically applied without frame adjustment, assume a medium frame and can underestimate a healthy weight for people with naturally larger builds.

Interpreting Your Result

The most useful way to read the output of these calculators is as a convergence range rather than a single target. When the Devine, Robinson, and Hamwi formulas cluster within five to ten pounds of each other — which they usually do for people of average height — that cluster represents a plausible lean body weight estimate. Sitting slightly above that range while still within the healthy BMI window is entirely consistent with good health, particularly if the extra weight reflects muscle mass.

Where these estimates become more practically useful is in clinical settings where you're tracking medication dosing, calculating nutritional needs, or monitoring weight changes over time. For those purposes, the consistency of the formula matters more than its absolute accuracy — you want a stable reference point, not a philosophically perfect number.

For personal health goals, the most important thing these calculators can do is reframe the question. Rather than asking "what should I weigh?" — a question with no universal answer — they invite a more productive question: "what weight range is associated with good health at my height?" That range is usually broader than most people assume, and landing anywhere within it, while maintaining adequate muscle mass and physical fitness, is a reasonable goal that serves health better than chasing any single number.