How this instrument works
Ideal body weight is not a wellness target — it began life as a dosing scalar. In 1974 B. J. Devine needed a fast way to size gentamicin doses, so he pinned a base figure at five feet — 50 kg for men, 45.5 kg for women — and added 2.3 kg for each further inch of stature. That is the entire equation: a straight line through height, with sex shifting the intercept by 4.5 kg and nothing else entering.
Because the line is straight, the instrument behaves predictably. Every extra inch adds exactly 2.3 kg; every extra centimetre adds about 0.9 kg. A man of 180 cm reads 75.0 kg, and at 190 cm he reads 84.0 kg. There is no curvature, no plateau, and no allowance for frame or muscle — which is exactly why the figure works as an arithmetic input to a dose and fails as a personal goal.
The same two lines survive well outside pharmacy. Critical-care ventilation protocols size tidal volume from predicted body mass using these identical coefficients, because lungs scale with stature rather than with what a patient happens to weigh today. Sizing a breath from actual mass would over-inflate a heavier patient's lungs; sizing it from height does not.
- Set the Sex switch — it selects the base figure, 50 kg for male or 45.5 kg for female.
- Enter your Height. The field accepts centimetres, metres, inches or feet, and converts before it computes.
- Read the Ideal body weight output, switching its unit to pounds or stone if that is how you think.
- Below five feet the sheet declines to answer: the Devine line is defined only from 60 inches upward.
Worked example — a man of 172 cm
Take a man measuring 172 cm. Convert first: 172 ⁄ 2.54 = 67.716535 inches, which is 7.716535 inches past the five-foot anchor. Multiply by the slope — 7.716535 × 2.3 = 17.748031 kg — then add the male base of 50 kg. The sheet reads 67.748031 kg, shown as 67.7 kg at the displayed precision.
Switch the output unit and the same figure becomes 149.4 lb, or 10 st 9 lb. Note what never entered the arithmetic: build, age, and current mass. Two men of 172 cm — one a rower, one entirely sedentary — receive the identical 67.7 kg, because the equation only ever knew how tall they are.
Questions
Is ideal body weight the same as a healthy weight?
No. The Devine equation returns a single point derived from stature alone, while a healthy range spans many kilograms and depends on composition, age and history. Clinicians use this figure to size drug doses and ventilator settings, not to set patient goals. If you want a range rather than a point, the BMI sheet in this series draws one for your height.
Why does the formula stop below five feet?
Because 60 inches is where Devine pinned the base, and the line was fitted for adults above it. Extrapolate downward and the arithmetic still runs — a woman of 140 cm would read 34.3 kg — but the answer drifts below anything clinically sensible. The instrument therefore declines under 152.4 cm rather than hand you a confident wrong number.
Which formula should I use — Devine, Robinson, Miller or Hamwi?
Devine, if you want the one clinical practice actually runs on. Hamwi came first in 1964; Robinson and Miller both published revisions in 1983 with gentler slopes. All four agree within a few kilograms across ordinary adult heights, and all four are straight lines — they differ in base and slope, not in shape. Devine persists because dosing references, ventilation protocols and hospital software were built around it.
Does this account for muscle, frame size or age?
None of the three. Stature and sex are the only inputs, so a powerlifter and a marathon runner of the same height receive the same figure. Older tables sometimes added a frame-size adjustment of roughly ten percent either way, but that correction was eyeballed rather than measured and modern practice drops it. Read the output as a reference scale, not a description of your body.
Why do clinicians dose drugs on ideal rather than actual body weight?
Many drugs distribute through lean tissue and body water rather than fat, so dosing a heavier patient on total mass can deliver far more drug than the tissue clearing it can handle. Aminoglycosides — the class Devine was working on in 1974 — are the classic case. Some regimens use an adjusted figure that adds a fraction of the excess mass back; the number computed here is the starting point those adjustments build on.
References
- NIH NHLBI — Aim for a healthy weight
- MedlinePlus (NIH National Library of Medicine) — Weight control
- NIH NIDDK — Weight management
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.