Why Ideal Weight Formulas Disagree With Each Other
Put a 1.80 m man into the ideal weight calculator and three answers come back. Devine returns 75.0 kg. Robinson returns 72.6 kg. The healthy BMI band stretches from 59.9 kg to 80.7 kg. Nothing is broken. The formulas were built for different purposes by different people, and only one of them was ever about health.
Devine was designed for drug dosing
B. J. Devine published his formula in 1974:
- Men: 50 kg + 2.3 kg for each inch over 5 feet
- Women: 45.5 kg + 2.3 kg for each inch over 5 feet
He was not describing a healthy physique. He needed a standardised lean body weight so that gentamicin, an antibiotic with a narrow safety margin, could be dosed without poisoning heavier patients. Fat tissue takes up much less of the drug than lean tissue, so dosing on actual body weight overshoots. Devine’s equation gave clinicians a defensible estimate of how much of a patient was metabolically active.
Hospitals still use it that way today. It was never validated as a body composition target, and the round numbers give away how it was constructed: Devine himself described the figures as a convenient approximation rather than a regression result.
Robinson tried to fix the fit
Robinson and colleagues revisited the numbers in 1983 against actuarial height and weight tables:
- Men: 52 kg + 1.9 kg per inch over 5 feet
- Women: 49 kg + 1.7 kg per inch over 5 feet
The base is higher and the per-inch increment is lower, so Robinson comes in above Devine for short people and below it for tall ones. The crossover for men sits near 5 feet 8. Miller and Hamwi published their own variants in the same era, all fitted to the same insurance data, all disagreeing by a few kilograms.
None of them account for age, frame size, or muscle. Every one is a straight line through height, which is why a 1.90 m former rower and a 1.90 m sedentary accountant get identical targets.
The BMI range is the honest answer
The 59.9 to 80.7 kg span looks uselessly wide next to a precise-sounding 75.0 kg, and that width is the point. It reflects genuine uncertainty about frame size and body composition that the single-value formulas paper over with false precision.
Where you should sit inside that range depends on how much muscle you carry, which height cannot tell you and a tape measure can. If the Navy tape method puts you at 14% body fat, the top of the range is fine. At 28%, aim lower in it.
Treat the number as a direction
The practical use of these formulas is spotting when you are a long way from any of them. If every formula says 73 to 75 kg and you weigh 74, stop optimising and go train. If they say 75 and you weigh 103, the direction is clear and the exact target does not matter yet; you will reassess long before you get there.
What the formulas cannot do is tell you what you will look like at the target. Two people at 75 kg and the same height can differ by 10 percentage points of body fat, which is a completely different body. This is the same limitation that makes BMI a weak signal for individuals, and it applies with equal force here.
A better goal than a goal weight
Pick a body fat percentage and a set of performance markers instead. "Get to 15% and hold my current squat" leads somewhere useful. "Get to 75 kg" can be achieved by losing muscle, which is technically success and practically a worse outcome.
If you do want a calorie target to move toward any of this, work from your maintenance rather than your goal weight, using the difference between BMR and TDEE as the starting point.