About the Ideal Weight Calculator
Four formulas are in common use for "ideal body weight". This ideal weight calculator runs all four, and the single most useful thing it can tell you is what each one was actually built for.
Hamwi (1964) — diabetic diet planning
Devine (1974) — calculating drug dosages
Robinson (1983) — a correction to Devine against actuarial data
Miller (1983) — another correction to DevineNot one of them was designed to tell an individual what to weigh.
Devine in particular was a dosing convenience — a way to scale drugs like gentamicin to something more stable than total body weight. It was never validated as a health target and its author never claimed it was. The other three are adjustments to a formula that was already not doing the job people now use it for.
How to Use the Ideal Weight Calculator
Enter your height and sex. That is all any of these formulas take, which is itself worth noticing.
Step-by-Step Example
Male, 183cm (6ft).
Hamwi: 48 + 2.70 × 12 = 80.5 kg
Devine: 50 + 2.30 × 12 = 77.7 kg
Robinson: 52 + 1.90 × 12 = 74.9 kg
Miller: 56.2 + 1.41 × 12 = 73.2 kgA spread of 7.3 kg — about 10%. That is larger than most people's entire target, from four formulas answering the same question.
The healthy BMI range for the same height is 62kg to 83.7kg.
The Four Lines Cross
All four share the same shape: a base weight at five feet, plus an increment for each inch above it. But they differ in both terms, and in opposite directions.
Miller has the highest base and the smallest increment. Hamwi has the lowest base and the largest.
So the lines cross, and the disagreement is not a simple widening:
| Height | Spread between the four |
|---|---|
| 5ft 0in | 8.2 kg |
| 5ft 6in | 1.1 kg |
| 6ft 0in | 7.3 kg |
| 6ft 6in | 15.0 kg |
They converge to within about 1.1 kg at 168.6cm for men — almost exactly the historical average height each was calibrated around — and fan out in both directions from there.
Which is the usual behaviour of a regression pressed into service as a rule: it agrees most about the people it was fitted to, and least about everybody else. The calculator finds that convergence point by scanning rather than quoting it.
Why It Refuses Below Five Feet
Every one of these formulas measures height above five feet. Below that, the increment term goes negative and the formula starts subtracting.
The answers fall, then keep falling, for a group the formulas were never fitted to in the first place. A naive implementation will hand back a confident number that is worse than no number.
So this calculator refuses, and points to the healthy BMI range instead — which works at any height because it scales with height rather than counting inches from an arbitrary floor.
They Cannot See Your Build
Look at the inputs again: height and sex. Nothing else.
A heavily muscled person and a sedentary person of the same height get identical answers. So do a wide-framed person and a narrow-framed one. The formulas have no way to distinguish them, because no information about build ever enters.
This is not a flaw that careful use can work around. It is what happens when a number that depends on body composition is estimated from a quantity that contains none.
If build matters to you — and it should — body fat percentage and lean body mass are the measures that see it.
Use the Range Instead
A BMI of 18.5 to 25 gives a weight range for any height. For 183cm that is 62kg to 83.7kg.
Three things to notice about it.
It is wide. Nearly 22kg at this height — wider than the disagreement between all four formulas put together. That width is honest: there genuinely is no single correct weight for a height.
It scales properly. No arbitrary floor at five feet, no increments to extrapolate past.
It was actually built for this. BMI has real limitations — it cannot tell muscle from fat and it is biased by height — but describing population health was at least what it was designed to do.
Understanding Your Result
Ideal weight gives the span across all four formulas, because a single figure would be a false precision.
All four formulas lists them individually.
How far apart they are gives the spread and where the formulas converge.
Healthy BMI range is the wider, more honest band.
Worth knowing says whether the formulas fall inside that band.
When Should You Use This Calculator?
To see how much the formulas disagree. That is the genuinely useful output.
When a doctor or a form references one. Now you know which, and why.
Not as a target. Use the BMI range, and pick a point in it you can sustain.
Not if you are muscular. None of these formulas can see it.
Common Mistakes
Treating one formula as authoritative. Four serious attempts disagree by 10%.
Assuming they were designed for this. Devine was for drug dosing, Hamwi for diet sheets.
Reading a single number as a target. The honest answer is a range.
Using them below five feet. They subtract there, and the calculator refuses for good reason.
Expecting them to account for muscle. They take height and sex, and nothing else.
Chasing the lowest of the four. Miller is not a stricter standard; it is a different regression.
These formulas are historical, were built for other purposes, and take no account of build, body composition, age or health. Nothing here is medical advice — discuss a target weight with a doctor, who can consider the rest of your health alongside it.