Why Ideal Weight Formulas Disagree With Each Other
At 180 cm, the four ideal weight formulas hand a man 75.0 kg, 72.6 kg, 71.5 kg and 77.7 kg. They disagree by 6.2 kg, and every one of them was written before 1984. “Ideal weight” is the name of the equations and nothing more. What you should weigh depends on your training history, your health and your own measurements, and four pre-1984 lines on a chart have no access to any of it. The ideal-weight calculator prints all four, so you see the disagreement instead of one tidy number.
Four formulas, different assumptions
Let x be your height in inches above five feet. The tool uses the commonly reproduced versions:
- Devine: male 50 + 2.3x kg; female 45.5 + 2.3x kg.
- Robinson, rounded coefficients: male 52 + 1.9x kg; female 49 + 1.7x kg.
- Miller: male 56.2 + 1.41x kg; female 53.1 + 1.36x kg.
- Hamwi: male 106 + 6x lb; female 100 + 5x lb, converted to kilograms for comparison.
All four trace back to insurance weight tables and clinical dosing conventions, so what your body is currently made of sits entirely outside them. Robinson’s original paper carries more decimal places than the rounded version everyone quotes, which is why two calculators can land a few hundred grams apart and both be working correctly. The equations and the comparison come from Robinson and colleagues, 1983 and a review comparing ideal-weight equations.
A 180 cm example
At 180 cm, x works out to roughly 10.866 inches. Run it through the male equations and Devine returns about 75.0 kg, rounded Robinson 72.6 kg, Miller 71.5 kg and Hamwi 77.7 kg. Quote any one of those to a decimal place and it looks far more precise than a 6.2 kg spread deserves. A chart of all four figures by height shows how wide the spread runs at every height. A chart of all four figures by height shows how wide the spread runs at every height. A chart of all four figures by height shows how wide the spread runs at every height. A chart of all four figures by height shows how wide the spread runs at every height.
The main comparison keeps the midpoint of Devine and Robinson and lists all four underneath. That midpoint averages two guesses, and nobody has validated it as a fifth formula.
How the BMI interval differs
At 1.80 m, a BMI of 18.5 to just under 25 covers 59.94 kg to just under 81.00 kg, and the BMI calculator will work that out for your height. Read it as a screening interval for adults from age 20, and not as a promise that every weight inside it suits you. The BMI guide covers what your muscle, your age and your fat distribution do to that number.
Where these equations run out
Your age, your training history, your frame and your symptoms are invisible to all four. Below five feet the tool withholds their results instead of extending the lines into a height range the original papers never covered. These numbers must never be used to choose a medication dose.
Pick a target with a clinician who can weigh your health history, your strength and your function against the output of a 40-year-old equation. Pregnancy, childhood and medical nutrition needs all require a different assessment.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.