How To Calculate Ideal Body Weight

An ideal body weight formula takes a height and returns a single number of kilograms. Devine's version, the one most often quoted, adds 2.3 kg for every inch a person stands above five feet, starting from a base of 50 kg for men and 45.5 kg for women. That is the whole calculation. It contains no measurement of the person beyond height and sex, and no reference at all to muscle, fat, frame size or age, which is the first clue that the word ideal deserves suspicion.

The formula was published by B. J. Devine in 1974, in a paper about drug dosing rather than nutrition. Clinicians needed a reproducible weight to plug into calculations for aminoglycoside antibiotics such as gentamicin, tobramycin and amikacin, which are dosed per kilogram of body weight. A standard reference weight made those doses consistent between patients and between hospitals. Nothing in the original purpose concerned how much a person should weigh for their own health or appearance, and the paper made no claim about wellbeing at all; it presented a way to estimate a number for a formula sheet.

That origin is why the label misleads. Ideal suggests an optimum to be reached, a target a doctor would endorse and a scale would confirm. The formulas offer nothing of the kind. They are arithmetic averages drawn from small populations decades ago, smoothed into a straight line so that a dose could be estimated quickly and identically by anyone. Calling the output ideal was a convenience of language that hardened into an assumption, and the assumption is the part worth discarding. The arithmetic is fine; it is the noun attached to it that has caused the trouble.

The clearest evidence that no single ideal weight exists is that the formulas disagree with one another. At 175 cm, the Devine formula gives a man 70.4645669 kg while the Robinson formula gives 68.9055118 kg. Same height, same sex, a difference of 1.5590551 kg. Robinson uses a 52 kg base and a 1.9 kg increment per inch instead of Devine's 50 kg and 2.3 kg. Both are in clinical use; neither is more correct, because the quantity they describe is not a physical constant that could be measured and settled.

Every one of these formulas is written in inches, not centimetres, and anchored at five feet. The structure is base plus increment times the number of inches above 60. That threshold is a historical convenience, roughly the average adult height of the reference group when the equations were fitted. Feed a metric height straight into the equation and the result is meaningless, because 175 is then being treated as a count of inches rather than centimetres. Conversion has to come first, every time. Anyone whose height is already quoted in feet and inches has the unit the formula wants, and only has to count how many inches sit above the five-foot mark.

The sex adjustment is a fixed constant rather than a measurement. Devine adds 50 kg for men and 45.5 kg for women, a flat 4.5 kg gap applied to everyone regardless of their actual body. A tall, lightly built woman and a short, heavily built man are each pushed in the same direction by a rule that knows only their height and a binary label. The difference is real in a statistical sense, reflecting average body composition differences in the original samples, but it is crude at the level of an individual.

What the formulas ignore is everything that actually varies between people. Muscle weighs more than fat for the same volume, so a trained athlete and a sedentary person of identical height can differ by many kilograms while both are healthy. Frame size, age, ethnicity, pregnancy and limb loss all shift what a reasonable weight is, and none of them appears in the equation. Two people who receive the same ideal weight can have entirely different bodies and entirely different health. A weight that sits comfortably on one body can be far from comfortable on another of the same height, because a scale cannot tell muscle from fat.

A range is a more honest answer than a point. Body mass index does a similar job but returns a band rather than a single figure. At 175 cm, a BMI between 18.5 and 25 corresponds to roughly 56.65625 to 76.5625 kg. That interval spans twenty kilograms, and every weight inside it is, by that measure, unremarkable. Presenting one number as the ideal collapses that honest width into a precision the underlying data never supported.

The formulas have not disappeared, because they remain genuinely useful for the purpose they were built for. Drug dosing, some mechanical ventilation calculations and a handful of clinical scores still use Devine's result as a reference weight, precisely because a standardised figure is what those calculations need. What they should not do is migrate from the dosing sheet to the bathroom scale. For assessing an individual's health, a BMI range, waist circumference, body composition and clinical context all carry more information than any ideal weight formula can. The formulas are worth knowing, and worth keeping in their proper place.