What Is My Ideal Weight?

They were built to dose drugs

This is the thing almost nobody is told, and it reframes everything else.

Devine (1974) and Hamwi (1964) were developed so clinicians and pharmacists could estimate lean body mass quickly at the bedside — because some medications are dosed on lean mass rather than actual weight.

They were never designed as a personal target.

That explains why they're so crude, why height is the only input, and why nobody involved cared whether the answer would feel motivating.

The disagreement grows with height

Every one of these formulas has the same shape: a base at 5 feet, plus a fixed amount per inch above it.

• Devine: 50 kg (men) / 45.5 kg (women), + 2.3 kg per inch — that's 5.07 lb/inch
• Hamwi: 106 lb (men) / 100 lb (women), + 6 lb/inch for men, 5 for women

That 0.93 lb per inch difference accumulates:

Height (men)	Devine	Hamwi	Gap
5'6″	140.7	142	1.3
5'10″	160.9	166	5.1
6'2″	181.2	190	8.8
6'6″	201.5	214	12.5

They nearly agree around average height and diverge steadily the further you get from it — which is precisely where you'd most want a formula to be reliable.

The women's formulas barely disagree at all

Devine adds 5.07 lb/inch; Hamwi's women's constant is 5. Almost identical — so the two stay under a pound apart across most of the range.

The men's formulas disagree substantially and the women's hardly do. That's an artefact of how each was fitted, not anything about bodies — which is itself a reason to hold any of these numbers loosely.

What they cannot see

Height is the only input. So invisible to all of them:

Body composition · muscle mass · frame size · age · ethnicity · pregnancy · your actual health

A muscular athlete and a sedentary person of identical height get an identical answer. That isn't a flaw in the arithmetic — it's a direct consequence of the design.

They're also least reliable at the extremes: very tall, very short and very lean people are where the linear per-inch structure breaks down.

Why "base plus per inch" is the wrong shape anyway

All four formulas assume weight should rise linearly with height — a fixed number of pounds per inch, forever.

Bodies don't work that way. A person who is 10% taller is roughly 10% larger in every dimension, so their mass scales closer to the cube of height than to a straight line. That's the same reason BMI divides by height squared rather than by height.

A linear formula can only be a decent approximation over a narrow band of heights — which is exactly why all of them behave worst at the extremes, and why the two here drift apart as you get taller. The divergence isn't two formulas disagreeing about a fact; it's two straight lines approximating a curve, and separating as they leave the region they were fitted to.

Which one is right?

None of them, in the sense you mean.

They're different reasonable fits to the same crude relationship, built for a clinical estimating task rather than to be correct about individuals.

The calculator shows a midpoint for convenience, and it's no more authoritative than either — it's just the middle of two approximations.

What to use instead

For a personal target, none of these alone.

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 isn't a number. It's that the number you came looking for is less solid than it appeared — and any calculator handing you one confident figure is hiding that from you.

Print the reference sheet — the second section is a list of everything these formulas can't see, which is the part worth reading first.

Sources: CalcVita — ideal weight formulas compared · FatCalc — IBW calculator

Arithmetic on published formulas, not medical advice, and not a target.