How to use it
- Pick metric or imperial units and your sex – each formula has a male and a female version.
- Enter your height. The average of the four formulas appears as the headline, with each formula and the BMI range listed below it.
- Compare the spread: if the formulas disagree by several kilograms, the BMI range is the more useful guide.
- Copy the summary or the link to keep or share the result.
Example
A man who is 180 cm tall is 70.87 inches, or 10.87 inches above 5 ft. Devine: 50 + 2.3 × 10.87 = 75.0 kg. Robinson: 52 + 1.9 × 10.87 = 72.6 kg. Miller: 56.2 + 1.41 × 10.87 = 71.5 kg. Hamwi: 106 lb + 6 × 10.87 lb = 171.2 lb = 77.7 kg. The average is 74.2 kg and the BMI 18.5–24.9 band is 59.9–80.7 kg, so every formula sits inside the upper half of the healthy band.
Formulas and their origin
Devine (1974): men 50.0 kg + 2.3 kg × (inches over 5 ft) women 45.5 kg + 2.3 kg × (inches over 5 ft)
Robinson (1983): men 52.0 kg + 1.9 kg × … women 49.0 kg + 1.7 kg × …
Miller (1983): men 56.2 kg + 1.41 kg × … women 53.1 kg + 1.36 kg × …
Hamwi (1964): men 106 lb + 6 lb × … women 100 lb + 5 lb × … (converted to kg exactly)
BMI range: 18.5 × height(m)² to 24.9 × height(m)²Hamwi published his rule in a 1964 diabetes handbook; Devine (1974) proposed his version for gentamicin dosing; Robinson (1983) and Miller (1983) re-fitted the slope to Metropolitan Life insurance tables. None was derived from mortality data in a modern population, and all were fitted for adults of about 5 ft and taller – the calculator shows a warning below 152.4 cm, where the lines extrapolate towards unrealistically low weights (the legacy version refused to calculate at all below 153 cm). Age, frame size, muscle mass and ethnicity are not part of any formula. The review by Peterson et al. (2016) concluded that a BMI-based range describes “ideal” weight better than any single-number formula.
Sources: Pai & Paloucek 2000, Ann Pharmacother – The origin of the "ideal" body weight equations, Peterson et al. 2016, Am J Clin Nutr – Universal equation for estimating ideal body weight, WHO – BMI classification.
The results are statistical estimates for healthy adults, not medical advice or a weight target. Discuss weight goals with a doctor or dietitian, especially if you are pregnant, under 18, an athlete or have a medical condition.
Why the formulas disagree
Each author fitted a straight line to a different reference table, so the base weight at 5 ft ranges from 45.5 to 56.2 kg and the slope from 1.36 to 2.7 kg per inch. For short people the lines are far apart at the bottom; for tall people the steeper Hamwi slope pulls its result several kilograms above Miller. Treat the spread as the uncertainty of the concept itself, and use the BMI range when you need a defensible target.
Questions
- Which ideal weight formula is the most accurate?
- None is "accurate" in a medical sense: they are conventions. Devine is the most widely used in clinical dosing, Robinson and Miller were fitted to later insurance tables. For a health target the BMI range of 18.5–24.9 is better supported by evidence.
- Does the calculator take age or frame size into account?
- No, because the published formulas do not. Older adults and people with a large frame or high muscle mass often sit above these values without any excess fat.
- Why is there a warning for heights under 5 ft?
- The formulas subtract from the base weight for every inch below 5 ft and were never validated there; at 140 cm Devine gives only 34 kg for a woman. The BMI range still works at any adult height.