Ideal Weight: Why Every Formula Disagrees

Ask four "ideal weight" formulas about the same person and you'll get four different answers, sometimes six kilograms apart. That's not a bug — it's the record of four clinicians each inventing a quick estimate for a different purpose. Understanding why they disagree is more useful than picking a favorite.

Four formulas, one skeleton

The classic formulas share a structure: a base weight at 5 feet (60 inches), plus a fixed amount per inch above it, with different constants for men and women. All report kilograms:

Devine (1974):   male 50 + 2.3 × over    female 45.5 + 2.3 × over
Robinson (1983): male 52 + 1.9 × over    female 49   + 1.7 × over
Hamwi (1964):    male 48 + 2.7 × over    female 45.5 + 2.2 × over
Miller (1983):   male 56.2 + 1.41 × over female 53.1 + 1.36 × over
(where "over" = height in inches above 60)

Devine's version was built for calculating drug dosages — it estimates lean body mass so pharmacists can scale medication, not to hand patients a target. Hamwi's came from dietetic practice and is the punchiest per inch. Robinson and Miller were later revisions that deliberately flattened the slope, because their authors thought Devine and Hamwi were too aggressive for taller frames.

The same man, four answers

Take a man who is 180 cm tall — 70.9 inches, so 10.87 inches over the 60-inch base. Running him through each formula (the calculator does all four at once):

Devine:   50 + 2.3 × 10.87 = 75.0 kg
Robinson: 72.6 kg
Hamwi:    77.3 kg
Miller:   71.5 kg

That's a spread of nearly 6 kg — almost 13 pounds — between the lowest and highest "ideal." A woman at 165 cm (65 inches, 5 over) gets 56.9, 57.4, 56.4, and 59.8 kg respectively. Notice the pattern: the formulas agree more near 5 feet (where they're all just their base constant) and disagree more as height climbs, because their per-inch slopes differ. The taller the person, the wider the spread — and the less any single number deserves your trust.

Why they all disagree — and what none of them knows

All four formulas are linear in height and nothing else. They can't see muscle versus fat, frame size, age, or body composition — a 190 cm rugby forward and a 190 cm sedentary man get identical numbers. They were each derived from the populations and purposes at hand: drug dosing, dietetic counseling, insurance tables. So "ideal weight" formulas are best read as rough historical consensus rather than prescription. The honest answer they converge on is a range — for the 180 cm man, roughly 71–77 kg by these estimates — and ranges are what tools like the Ideal Weight Calculator show you, all four side by side.

Better questions than "what's my ideal weight?"

Two screening numbers answer more practical questions with less history attached. BMI relates weight to total height and flags statistical risk bands: a 70 kg person at 175 cm has a BMI of 22.9 — comfortably in the normal band of 18.5–24.9. The BMI formula guide walks through that calculation and its limits. Waist-to-height ratio asks where you carry the weight, which matters more than the scale does: keep your waist under half your height. A person 170 cm tall with an 80 cm waist is at 0.47 — healthy territory, and their ceiling is 85 cm. That measure is explained in the waist-to-height guide, and both numbers together tell you more than any single "ideal" figure.

Use the range, not the number

A practical way to hold all this: compute the four formula answers, note the band they span, and treat the whole band as context — not a target with decimals. Bodies are managed by habits, not by hitting a formula's output; if BMI and waist-to-height sit in healthy ranges and you feel strong, the "ideal weight" debate is academic. If any number concerns you, the conversation to have is with a clinician, not a calculator — these tools screen, they don't diagnose.

Try it yourself

The Ideal Weight Calculator shows all four classic formulas at once for your height and sex, so you can see the honest spread instead of one arbitrary number. The BMI Calculator screens weight against height in metric or imperial units. The Waist-to-Height Ratio Calculator turns one tape-measure number into a risk category and your personal waist ceiling.