How Is Ideal Body Weight Calculated? (Devine, Robinson, Hamwi Formulas)
"Ideal body weight" sounds like it should be a single, precise number - but ask three different calculators and you will often get three slightly different answers. That is because there is no single universally agreed formula. Instead, several formulas were developed decades apart, mostly for medical dosing purposes, and each uses a slightly different approach.
Why Ideal Weight Formulas Exist
Most ideal body weight formulas were originally created to help estimate medication dosages, not as a fitness or aesthetic target. Because of this clinical origin, they tend to be conservative and height-based, rather than accounting for muscle mass, frame size, or individual body composition.
The Devine Formula
One of the most widely used formulas in medical settings:
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
Worked Example (Devine)
For a man who is 5'11" (11 inches over 5 feet):
50 + (2.3 x 11) = 50 + 25.3 = 75.3 kg
The Robinson Formula
A later refinement of Devine, using slightly different base weights:
Men: 52 kg + 1.9 kg for each inch over 5 feet
Women: 49 kg + 1.7 kg for each inch over 5 feet
Worked Example (Robinson)
Same man, 5'11" tall:
52 + (1.9 x 11) = 52 + 20.9 = 72.9 kg
The Hamwi Formula
An older, simpler formula still commonly referenced:
Men: 48 kg + 2.7 kg for each inch over 5 feet
Women: 45.5 kg + 2.2 kg for each inch over 5 feet
Worked Example (Hamwi)
Same man again:
48 + (2.7 x 11) = 48 + 29.7 = 77.7 kg
Why the Three Formulas Disagree
Notice that for the exact same height, the three formulas produced results ranging from about 73 kg to 78 kg - a real difference, not rounding error. This is exactly why it makes more sense to think of "ideal weight" as a range spanning multiple formulas rather than chasing one precise number, since no single formula is definitively "correct."
What These Formulas Do Not Account For
- Muscle mass: A muscular, athletic person will naturally weigh more than these formulas suggest, without carrying excess fat.
- Frame size: Someone with a naturally larger or smaller bone structure will reasonably fall outside the calculated range.
- Age-related changes: None of these formulas adjust for age, even though healthy body composition shifts somewhat over a lifetime.
How to Use an Ideal Weight Range in Practice
Rather than treating any single formula's output as a target to hit exactly, use the spread across all three as a general reference range - and weigh it alongside other indicators like BMI, waist circumference, and how you actually feel and perform physically, rather than as a strict number to chase.
Step-by-Step: Calculating Your Own Ideal Weight Range
- Measure your height in feet and inches.
- Calculate the number of inches over 5 feet (if under 5 feet, formulas typically subtract instead).
- Apply the Devine, Robinson, and Hamwi formulas using your gender-specific base and multiplier.
- Compare the three results to see your reasonable ideal weight range, rather than a single number.
Skip the Manual Math - Use Our Free Ideal Weight Calculator
Our free Ideal Weight Calculator runs all three formulas at once and shows you the full range instantly, based on your height, age, and gender.
Pair it with our BMI Calculator for a second reference point on the same question.
Final Thoughts
There is no single correct "ideal weight" formula - Devine, Robinson, and Hamwi were all developed for slightly different clinical purposes and will each give you a somewhat different number for the same height. Treat the spread between them as a reasonable range, not a precise target, and remember that none of them account for muscle mass or frame size.