What the BMI actually is
Body mass index is a single number derived from height and weight — in adults, weight in kilograms divided by height in metres squared. It gives a general indication of whether weight falls in a healthy range. A BMI between 18.5 and 25 is conventionally called normal, 25 to 30 overweight, and above 30 obesity.
Its virtue is simplicity: it takes two measurements anyone can obtain and produces a number that correlates, across large populations, with the risk of diabetes, cardiovascular disease, and several cancers. That is genuinely useful. But it was designed to describe populations, not to judge individuals, and that distinction gets lost constantly.
The exact equations this calculator uses
Two separate calculations, both named, so you can check the arithmetic yourself.
1. Body mass index
BMI = weight (kg) ÷ height (m)²
The 18.5 / 25 / 30 boundaries are the World Health Organization's international classification, adopted for US clinical practice by the NHLBI.12
2. Ideal body weight — the Devine formula
Men: 50 kg + 2.3 kg × (inches over 5 ft)
Women: 45.5 kg + 2.3 kg × (inches over 5 ft)
Published by B.J. Devine in 1974.3 It lands near a BMI of 22 for most heights.
A worked example
A woman who is 5 ft 6 in (1.676 m) and weighs 165 lb (74.8 kg):
- BMI = 74.8 ÷ (1.676 × 1.676) = 74.8 ÷ 2.809 = 26.6 — just into the overweight band.
- Healthy range = BMI 18.5 to 25 at that height = (18.5 × 2.809) to (25 × 2.809) = 52.0 to 70.2 kg = 115 to 155 lb.
- Devine ideal weight = 45.5 + (2.3 × 6 inches over 5 ft) = 45.5 + 13.8 = 59.3 kg = 131 lb.
Notice how far apart those last two answers are. The Devine figure of 131 lb sits near the bottom third of a healthy range that runs to 155 lb. Both are "correct"; they are answering different questions. This is exactly why the calculator shows you a range and not just the single number.
Who these numbers are wrong for
I would rather you know this before you act on the result than after.
- Muscular people. Muscle is denser than fat, so BMI reads high in anyone who trains seriously. A lean rugby player and a sedentary person of the same height and weight get an identical BMI and have nothing else in common.
- Older adults. Muscle is lost with age and fat is gained at the same weight, so an unchanged BMI at 70 does not describe the same body it did at 40. In older people a BMI slightly above 25 is associated with better, not worse, outcomes.
- People of South and East Asian descent. The risk of type 2 diabetes and cardiovascular disease rises at lower BMIs. A 2004 WHO expert consultation identified additional public-health action points at 23 and 27.5 rather than 25 and 30, while keeping the international classification unchanged.4 If this is you, read the 25 on this page as closer to a 27.5.
- Pregnancy, and children or teenagers. Neither calculation applies. Children are assessed on age-and-sex percentile growth charts instead; in pregnancy, weight gain is judged against pre-pregnancy BMI by a separate standard.
- Anyone under about 5 ft. The Devine formula is only defined for height above five feet and becomes nonsense below it — it was never built to extrapolate downward.
- Amputation or limb difference. Both equations assume a whole body of average proportions, and neither adjusts for missing mass.
Where the "ideal weight" number actually came from
The Devine formula was not derived from any study of health, longevity, or body composition. It was published in 1974 as a way to estimate creatinine clearance so that gentamicin — an antibiotic — could be dosed safely in obese patients.3 It has since been cited in hundreds of papers as an ideal-weight equation, a use its author never claimed for it.5
It persists because it is convenient and roughly sensible, not because anyone demonstrated that people at their Devine weight are healthier. Treat it as a reference point, never a target.
That is why the calculator also gives you a healthy range — the span of weights corresponding to a BMI of 18.5 to 25 at your height. A range is a far more honest answer than a single number, and for most people any weight inside it is fine.
What if my BMI is over 25?
For most people, a BMI over 25 does indicate excess fat. There are real exceptions. A muscular athlete may have a BMI over 25 with a low body fat percentage, because muscle is denser than fat. Older adults lose muscle and may carry more fat at the same BMI than they did at forty.
BMI does not measure body fat, and it says nothing about where fat sits — which matters. Fat around the abdomen carries more metabolic risk than fat on the hips and thighs. A waist measurement adds useful information that BMI alone misses. As a rough guide, a waist over 40 inches in men or 35 inches in women signals increased risk regardless of what BMI says.
If you want to change the number
Weight is more responsive to consistent, unglamorous habits than to any particular diet. If you want to work on it, two things on this site are more useful than another calculator: understanding how many calories your body actually burns, and reading through the weight loss series, which covers why exercise alone so often disappoints and why plateaus are normal rather than a sign of failure.
If your weight is bound up with blood sugar — prediabetes or type 2 diabetes — the remission guide I am writing addresses that specifically, in plain language.
A healthy weight is built from what you eat, not what you weigh
BMI tells you where you are. It says nothing about whether your diet is actually feeding you. VitalFill tracks 40+ nutrients against targets set for your age and sex — free on the App Store.
Questions people actually ask about this
What is my ideal body weight?
There is no single ideal weight, only a range. For your height, the healthy range is the span of weights giving a BMI between 18.5 and 25 — for someone 5 ft 6 in, that is roughly 115 to 155 lb. The single "ideal body weight" number this and every other calculator shows comes from the Devine formula, which sits near a BMI of 22 and was originally an antibiotic-dosing tool. Anywhere in the range is fine for most people.
How do I calculate BMI by hand?
Divide your weight in kilograms by your height in metres, then divide by your height in metres again. In pounds and inches: multiply weight in pounds by 703, then divide twice by height in inches. A 165 lb person who is 66 in tall: 165 × 703 = 115,995; ÷ 66 = 1,757; ÷ 66 = 26.6.
Is a BMI of 26 bad?
On its own, barely. A BMI of 26 sits just inside the overweight band, where the increase in risk over a BMI of 24 is small for most people and easily outweighed by fitness, waist size, blood pressure and blood sugar. It matters more if your waist is over 40 in (men) or 35 in (women), or if it is climbing year on year. It matters less if you are muscular, over 65, or it has been stable your whole adult life.
Why does BMI say I am overweight when I am not fat?
Because BMI cannot tell muscle from fat — it only knows your weight and your height. If you lift, sprint, or carry a naturally heavy frame, BMI will overstate your fat. The quickest cross-check is a tape measure at the navel: a normal waist alongside a "high" BMI usually means the BMI is the thing that is wrong.
Is BMI different for men and women?
The BMI calculation and its 18.5/25/30 cutoffs are identical for both. Only the Devine ideal-weight figure differs by sex, by a fixed 4.5 kg. Women do carry more body fat than men at the same BMI, which is normal physiology and not something the number adjusts for.
What is a healthy BMI for someone over 65?
Slightly higher than the standard band. In older adults, a BMI in the mid-20s is consistently associated with better survival than a BMI at the low end of "normal", partly because low weight in later life often reflects lost muscle or undiagnosed illness. Unintentional weight loss after 65 is a reason to see a doctor, not a success.
Is BMI accurate for Asian people?
It is calculated the same way, but the risk thresholds sit lower. A 2004 WHO expert consultation found that diabetes and cardiovascular risk in Asian populations rises substantially below the standard 25 cutoff and identified 23 and 27.5 as additional action points.4 Practically: if you are of South or East Asian descent, treat a BMI of 23 the way this page treats 25.
Should I aim for my ideal body weight?
Not as a target in itself. For nearly everyone with a BMI above 30, losing 5–10% of current weight delivers most of the measurable health benefit — blood pressure, blood sugar, triglycerides, sleep apnoea — and that is a far more achievable goal than a formula's number. Chasing the Devine figure is how people end up on their fourth failed diet.
References
The two equations on this page and every threshold quoted with them.
- World Health Organization. Obesity: Preventing and Managing the Global Epidemic. Report of a WHO Consultation. WHO Technical Report Series 894. Geneva: WHO; 2000. iris.who.int The source of the 18.5 / 25 / 30 classification used throughout this page.
- National Heart, Lung, and Blood Institute. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. NIH Publication 98-4083; 1998. nhlbi.nih.gov Where the same cutoffs, and the 40 in / 35 in waist thresholds, entered US clinical practice.
- Devine BJ. Gentamicin therapy. Drug Intelligence & Clinical Pharmacy. 1974;8:650–655. The original publication of the ideal-body-weight equation — as an antibiotic dosing aid.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet. 2004;363(9403):157–163. https://doi.org/10.1016/S0140-6736(03)15268-3 The 23 and 27.5 action points, and the decision to retain the international classification.
- Pai MP, Paloucek FP. The origin of the "ideal" body weight equations. Annals of Pharmacotherapy. 2000;34(9):1066–1069. https://doi.org/10.1345/aph.19381 Traces how a gentamicin dosing formula became the standard "ideal weight" number.