How BMI is calculated
BMI divides your weight by the square of your height. In metric units: weight (kg) ÷ height (m)². In imperial units: 703 × weight (lb) ÷ height (in)². A person who is 170 cm and 70 kg has a BMI of 70 ÷ 1.7² = 24.2. The same formula applies to everyone; there is no term for sex, age, or frame size.
The measure is old. The Belgian statistician Adolphe Quetelet worked it out in the 1830s to describe the average build of a population. The physiologist Ancel Keys gave it its current name in 1972 and recommended it for population studies, not for judging individuals. That original purpose still defines what the number can and cannot do.
BMI categories for adults
| Category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | 40.0 and above |
These cutoffs come from the World Health Organization and apply to adults aged 20 and over. Across large populations, the 18.5–24.9 band is where rates of type 2 diabetes, heart disease, and several cancers are lowest. That is a statement about groups of millions of people. Your own risk depends on far more than one ratio.
The limits of BMI
BMI is a screening tool. It flags when weight might deserve a closer look; it does not diagnose anything. Four blind spots are worth knowing about:
- It cannot tell fat from muscle. A 90 kg rugby player and a 90 kg office worker of the same height both score 27.8. Athletes and regular lifters routinely land in the overweight band while carrying less fat than average. A body fat estimate separates the two.
- It ignores where fat sits. Fat stored around the organs carries more metabolic risk than fat under the skin, and BMI cannot see the difference. Waist circumference is the usual companion measure for exactly this reason.
- The cutoffs do not fit every ethnicity equally. A 2004 WHO expert consultation found that adults of South and East Asian descent face raised metabolic risk at lower BMIs, and proposed action points of 23 and 27.5 for those populations. Several countries use adjusted thresholds in clinical practice.
- It says nothing about fitness, diet, or bloodwork. Two people with an identical BMI can have completely different blood pressure, cholesterol, and cardiorespiratory fitness.
This is why clinicians read BMI alongside waist size, blood tests, and history rather than on its own. Use it the same way: one data point among several.
BMI for women and by age
BMI for women
The calculation and the categories are identical for women and men. Body composition is not: at the same BMI, women typically carry 8–10 percentage points more body fat, which is healthy and expected. So a woman and a man who both score 23 do not have the same amount of fat, and neither needs to. During pregnancy the number stops being meaningful; antenatal guidance works from pre-pregnancy BMI instead.
BMI by age
For adults 20 and over, one chart covers all ages, but the evidence gets more nuanced at the upper end. Several large cohort studies of adults over 65 found the lowest mortality around a BMI of 23–27, slightly above the standard band, partly because a little reserve is protective during illness. Muscle loss with age also lets an older adult hold a normal BMI while carrying more fat than the number suggests. Under 20, BMI is read against age-and-sex percentile charts, not adult cutoffs.
What to do with your result
If your BMI sits outside 18.5–24.9, the useful response is curiosity rather than alarm. Cross-check with a second measure first: waist circumference, or the body fat calculator if you want to know how much of your weight is lean mass. The ideal weight calculator shows how wide the reasonable range for your height actually is.
If you then decide to change your weight, small and steady wins. Find your maintenance calories with the TDEE calculator and adjust by 250–500 calories a day, which moves the scale by roughly 0.25–0.5 kg per week. A BMI below 18.5 is worth raising with a doctor before anything else, since unintended weight loss has medical causes that food alone will not fix.
