What BMI measures — and what it does not
Body Mass Index is your weight divided by the square of your height. That is the entire calculation. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, not as a measure of individual health but as a way to describe the distribution of weight across a population.
That origin explains both its usefulness and its limits. Across large groups, BMI correlates well with body fat and with the risk of conditions like type 2 diabetes, cardiovascular disease and some cancers. For any one person, it can be badly wrong.
BMI cannot distinguish muscle from fat. A rugby forward and a sedentary person of the same height and weight receive the same score, though their health risks are entirely different. Nor does it know where fat is stored, which matters — abdominal fat carries substantially more metabolic risk than fat on the hips and thighs.
The BMI formula
In metric units the formula is direct:
- kg
- Body weight in kilograms
- m
- Height in metres — divide centimetres by 100
In imperial units a conversion factor of 703 is applied:
The 703 converts pounds-per-square-inch into kilograms-per-square-metre. Both formulas give the same answer for the same person.
BMI categories
The World Health Organization defines these bands for adults aged 20 and over:
| BMI | Category | Associated risk |
|---|---|---|
| Below 16.0 | Severe thinness | High |
| 16.0 – 16.9 | Moderate thinness | Moderate |
| 17.0 – 18.4 | Mild thinness | Mild |
| 18.5 – 24.9 | Healthy weight | Lowest |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese class I | Moderate |
| 35.0 – 39.9 | Obese class II | Severe |
| 40.0 and above | Obese class III | Very severe |
Different thresholds for South and East Asian populations
Research consistently finds that people of South Asian, Chinese, Japanese and Korean descent develop metabolic disease at lower BMIs than European populations, because they tend to carry more visceral fat at the same BMI.
The WHO therefore suggests lower action points for Asian populations, and India's national guidelines adopt them formally:
| Category | International | Asian / Indian |
|---|---|---|
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25.0 – 29.9 | 23.0 – 24.9 |
| Obese | 30.0 and above | 25.0 and above |
A worked example
The healthy weight range for 1.70 m is 53.5 kg to 71.9 kg.
Measures that tell you more than BMI
If your BMI is near a boundary, or you are muscular, these give a clearer picture.
Waist circumference
Measured at the navel, this captures abdominal fat directly — the fat that matters most for metabolic risk. Risk rises above 94 cm for men and 80 cm for women in European populations, and above 90 cm for men and 80 cm for women in South Asian populations.
Waist-to-height ratio
Divide your waist by your height, in the same units. The rule of thumb is simple: keep your waist under half your height. Several large studies have found this predicts cardiovascular risk better than BMI, and it needs no reference table.
Body fat percentage
The most direct measure of the thing BMI is trying to approximate. Our body fat calculator uses the US Navy circumference method, which needs only a tape measure.
For a fuller picture, pair this with the ideal weight calculator and the TDEE calculator, which tells you the calorie level that maintains your current weight.
Frequently asked questions
What is a healthy BMI?
For adults, the WHO defines 18.5 to 24.9 as the healthy range. For people of South Asian, Chinese, Japanese or Korean descent, 18.5 to 22.9 is the equivalent range, because metabolic risk begins to rise at a lower BMI in these populations.
"Healthy" here means the range associated with the lowest population-level risk. An individual at 26 who is active and metabolically healthy may well be at lower risk than someone at 23 who is not.
Is BMI accurate for athletes?
No. Muscle is roughly 18% denser than fat, so a muscular person weighs more for the same volume. Many professional athletes have BMIs above 25 or even 30 while carrying under 10% body fat.
If you train seriously with weights, use body fat percentage or waist circumference instead. BMI will systematically overstate your risk.
Does BMI work for children?
Not in the same way. Children's body composition changes constantly with growth, so a raw BMI number is meaningless on its own. Paediatric BMI is plotted on age- and sex-specific percentile charts instead.
The CDC classifies children aged 2 to 19 as underweight below the 5th percentile, healthy weight from the 5th to 85th, overweight from the 85th to 95th, and obese at or above the 95th. This calculator is for adults aged 20 and over.
Is BMI accurate for older adults?
It becomes less reliable after about 65. People lose muscle and bone density with age while gaining fat, so BMI can stay flat while body composition worsens considerably. Height loss from spinal compression also inflates BMI.
Some research suggests a slightly higher BMI — around 23 to 27 — is associated with better outcomes in older adults, as some reserve helps with recovery from illness.
How much weight do I need to lose to reach a healthy BMI?
The calculator shows this directly once you enter your measurements. Broadly, each BMI point is about 3 kg for someone 1.70 m tall, or roughly 3.5 kg at 1.80 m.
A safe rate of loss is 0.5 to 1 kg a week, which requires a deficit of roughly 500 to 1,000 calories a day. Our calorie calculator works out the intake for a target rate.
Can BMI be too low?
Yes. A BMI under 18.5 is classified as underweight and carries real risks: weakened immune function, reduced bone density, anaemia, fertility problems and slower recovery from illness. Below 16 the risks are serious.
Unintentional weight loss leading to a low BMI warrants medical investigation. If you are deliberately restricting food and find it hard to stop, that is worth raising with a doctor.