Ideal Weight Calculator

Every formula this calculator compares was originally published to solve a clinical dosing problem, not to define a personal health goal. Hamwi’s formula, presented at a 1964 diabetes conference, was meant to give clinicians a fast bedside estimate for diet planning. Devine’s, published in 1974, existed to help calculate gentamicin and other drug doses that needed to be based on lean body size rather than actual weight, since dosing an overweight patient by total weight risks overdose for drugs that do not distribute into fat tissue. Robinson’s and Miller’s, both from 1983, refined the same underlying idea with adjusted constants.

None of them were derived from studies of what weight is associated with the best health outcomes. Their later adoption as personal “goal weights” is a repurposing their authors never intended, which is the main reason they disagree with each other and with modern weight guidance as much as they do.

All four share the same basic shape — a base weight defined at five feet of height, plus a fixed increment for every inch above it — and differ only in the constants plugged into that shape. Because the increments were fitted to different clinical samples decades apart, the formulas track closely for people near average height and spread further apart the taller or shorter someone is.

None of these equations accounts for muscle mass, frame size, sex-specific fat distribution or age — a bodybuilder and a sedentary person of the same height receive the identical “ideal weight” from every one of them. That is a structural limitation, not a rounding error: weight alone, without a measure of composition, cannot distinguish a healthy body from an unhealthy one at the same number on a scale. The healthy BMI range, spanning roughly 18.5 to 24.9, is deliberately wider and is generally treated as the more defensible reference point precisely because it does not imply a false precision these older formulas carry.

Despite their limitations as personal targets, ideal-body-weight formulas remain in routine clinical use for exactly the purpose Devine designed his for: calculating doses of drugs — certain antibiotics, chemotherapy agents and sedatives among them — where dosing by total body weight would be inaccurate for an overweight or obese patient. That clinical role is also why the constants have barely changed in decades: the formulas do not need to be “right” about health, only consistent and well understood by the people using them.