How "ideal weight" is estimated
Two views, deliberately shown together because they answer different questions.
The BMI healthy range is every weight producing a BMI between 18.5 and 24.9 at your height. At 1.70 m that spans 53 to 72 kg. This is the more honest answer, because it reflects the genuine breadth of healthy human variation.
The Devine formula returns one classic reference figure:
- Men: 50 kg + 2.3 kg for each inch over 5 ft
- Women: 45.5 kg + 2.3 kg for each inch over 5 ft
Worked example: a woman of 5'6″ is 6 inches over five feet, so 45.5 + (6 × 2.3) ≈ 59.3 kg. Her BMI-based healthy range at that height (about 1.68 m) runs from roughly 52 to 70 kg — so the Devine figure lands in the lower middle of the span, which is typical.
Where the Devine formula actually came from
This is worth knowing before you treat its output as a goal. The formula was published by Dr B.J. Devine in 1974, and it was not designed to tell anyone what they should weigh. It was created to standardise drug dosing — some medications are dosed on lean body mass, and clinicians needed one reproducible number per height rather than an honest range.
Its journey from pharmacy calculation to consumer fitness target is an accident of convenience. Robinson and Miller were later fitted to the same kind of height-weight data for the same dosing purpose, and Hamwi's 1964 rule of thumb has no published derivation at all. If a website hands you one precise "ideal weight," this is almost certainly where it comes from, and the range is the better answer to the question you were actually asking.
Why a range, not a number
Look at the bars in the figure. Every one of them is wide, and every point along them is a healthy weight for that height. Where you sit within your own span depends on things no height-based formula can see: your muscle mass, frame size, bone density, sex, age, and where your body stores fat.
A muscular person belongs near the top of their range. A small-framed person may feel and function best near the bottom. Note too that the BMI ranges are identical for men and women, even though men typically carry more muscle at the same height — which in practice places them higher in the span without anything being wrong.
Where this does not apply
- Athletes and muscular people — muscle mass pushes weight above the range without excess fat.
- Older adults — some evidence suggests slightly higher body weight is protective in later life, partly because reserves matter during illness.
- Children and adolescents — use age- and sex-specific growth charts.
- Pregnancy — weight gain is expected and guided by starting BMI.
- People of South Asian, Chinese and some other Asian ancestry — lower BMI thresholds are often applied, shifting the healthy span downward.
- Anyone with a history of disordered eating — target weights can be harmful; please see our medical disclaimer.
Better questions than "what should I weigh?"
Weight is among the least informative health numbers available and the one people fixate on hardest. Your waist measurement, blood pressure, blood glucose and lipids, how easily you climb two flights of stairs, how well you sleep, and whether your strength is trending up or down over months all tell you considerably more.
Someone a few kilograms above their range who sleeps well, trains regularly, and has clean bloodwork is in better shape than someone inside the range who does none of those things. A scale cannot distinguish them. Pair this estimate with a body-fat estimate and the healthy weight chart for a fuller picture.
Four formulas, and their coefficients do not agree either
The result shows Devine next to the three formulas usually quoted with it. Robinson and Miller were fitted to life-insurance height-weight tables, and Devine matches an older rule of thumb drawn from the same tables. A 2000 review in the Annals of Pharmacotherapy found them so similar that it concluded any one of them may be used.
Checking them for this page turned up something more telling. Robinson's 1983 paper gives its equations as 51.65 kg + 1.85 kg per inch over five feet for men and 48.67 kg + 1.65 kg for women. The version repeated in most calculators, and in a 2021 clinical-nutrition review, is a rounded 52 + 1.9 and 49 + 1.7. At six feet that rounding alone moves a man's answer by about 1 kg. We use the published figures. Miller and Hamwi are shown as tabulated in that 2021 review, because the original papers are not openly available to check.
When a figure's last digit depends on which secondary source someone copied, it is not a sound basis for a personal goal. The range is the answer, and these formulas are reference points inside it.