Ideal Weight Calculator (Devine & Hamwi)

How this is calculated

⚠️ THESE FORMULAS WERE NOT BUILT TO TELL YOU WHAT TO WEIGH. They were developed decades ago to help clinicians and pharmacists DOSE MEDICATION - some drugs are dosed on lean body mass rather than actual weight, and a quick height-based estimate was needed at the bedside. They were never intended as a personal target, and reading them as one is using a tool for a purpose its authors did not have. That is the single most useful thing to know about them, and almost nobody is told it. HOW THEY WORK: every one of these formulas has the same shape - a base weight at 5 feet, plus a fixed amount for each inch above that. DEVINE (1974): men 50 kg, women 45.5 kg, plus 2.3 kg per inch over 5 feet. HAMWI (1964): men 106 lb, women 100 lb, plus 6 lb per inch for men and 5 lb for women. ⚠️ THEY DISAGREE, AND THE DISAGREEMENT GROWS WITH HEIGHT. This is worth stating precisely rather than as a vague range, because the shape of it is the interesting part. Devine adds 5.07 pounds per inch over five feet; Hamwi adds 6 for men. That 0.93 per inch accumulates, so the two formulas nearly agree at 5 foot 6 - a gap of about 1.3 pounds - and are 5.1 apart at 5 foot 10, 8.8 apart at 6 foot 2, and 12.5 apart at 6 foot 6. For women the per-inch constants are almost identical (5 against 5.07), so the two barely diverge at all - under a pound across most of the range. The men's formulas disagree substantially and the women's hardly do, which is an artefact of how each was fitted rather than anything about bodies. Robinson and Miller, two other well-known formulas, sit in and around the same territory. If several formulas built for the same job by serious people cannot agree - and disagree MORE the further you get from average height - no single one of them is telling you a precise fact about your body. Showing two side by side is deliberate. A calculator that presented one number would be implying a confidence that does not exist. ⚠️ WHAT THEY CANNOT SEE, which is nearly everything that matters: body composition, muscle mass, frame size, age, sex beyond a binary switch, ethnicity, pregnancy, and your actual health. A muscular athlete and a sedentary person of identical height get identical answers. That is not a flaw in the arithmetic - it is a consequence of using height as the only input. THEY ARE LEAST RELIABLE AT THE EXTREMES, for very tall, very short, and very lean people - which is a known limitation of the linear per-inch structure rather than a rounding issue. WHAT TO USE INSTEAD, if you want a number: none of these on its own. Body composition, how you feel, what your clothes do, blood markers and what a doctor says are all more informative than a height-based estimate from the 1960s. If you want a single screening figure, BMI is at least designed for populations - though it has its own well-known problems with muscular builds. This is arithmetic on published formulas, presented with their disagreement visible. It is not medical advice and it is not a target.

Common questions

What were these formulas actually for?

Dosing medication. They were developed decades ago so clinicians and pharmacists could estimate lean body mass quickly at the bedside, because some drugs are dosed on lean mass rather than actual weight. They were never designed as a personal weight target, and that is the single most useful thing to know about them - it explains why they are so crude, why they use only height, and why nobody involved was worried about whether the answer would feel motivating.

Why show two numbers?

Because showing one would imply a confidence that does not exist. The two here nearly agree at 5 foot 6 - about 1.3 pounds apart - and diverge steadily with height, reaching 5.1 pounds at 5 foot 10 and 12.5 at 6 foot 6. Robinson and Miller sit in and around the same territory. If several formulas built for the same job by serious people disagree, and disagree MORE the further you get from average height, no single one is telling you a precise fact about your body. The gap between them is the honest content of this page.

Which one is right?

None of them, in the sense you mean. They are different reasonable fits to the same crude relationship, and they were built for a clinical estimating task rather than to be correct about individuals. Devine is the most widely used, Hamwi is older and runs higher for taller people. The midpoint is shown for convenience and is no more authoritative than either - it is just the middle of two approximations.

What can't they see?

Nearly everything that matters. Body composition, muscle mass, frame size, age, ethnicity, pregnancy, and your actual health. A muscular athlete and a sedentary person of the same height get an identical answer, because height is the only input. That is not a flaw in the arithmetic - it is a direct consequence of the design. They are also least reliable at the extremes: very tall, very short and very lean people are where the linear per-inch structure breaks down.

Why does the gap grow with height?

Because the formulas disagree on the per-inch increment, not just the base. Devine adds 2.3 kg - about 5.07 pounds - for each inch over five feet, while Hamwi adds 6 pounds for men and 5 for women. Those differences accumulate, so two people of different heights get answers that are not just different but differently different. The calculator shows the per-inch divergence for exactly that reason.

So what should I use instead?

For a personal target, none of these on their own. Body composition, how you feel, what your clothes are doing, blood markers and what a doctor actually says are all more informative than a height-based estimate from the 1960s. If you want one screening figure, BMI is at least designed for populations - though it has its own well-known problems with muscular builds. The genuinely useful takeaway here is not a number; it is that the number you were looking for is less solid than it appeared.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Ideal Weight Calculator (Devine & Hamwi) at https://www.bfcbrilliance.com/tools/ideal-weight-calculator.

What I entered:
- Height (in): ___
- Formula variant: ___

What it calculated:
- Devine (1974): ___
- Hamwi (1964): ___
- The two disagree by: ___
- Midpoint of the two: ___
- Devine in kilograms: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Keep this general and do not give medical advice — where it matters, tell me what to raise with a clinician.

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