# BMI calculator, with what it does and does not tell you

Body mass index (BMI) is weight in kilograms divided by height in metres squared. The WHO adult categories are under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight and 30 or more obesity. BMI does not separate muscle from fat, so for active people waist and the trend over time say more.

## How it works

### The formula

BMI = kg ÷ m². In pounds and inches: 703 × lb ÷ in².

### Limits

It does not account for muscle, age, sex or ethnicity. Some health bodies use lower cut-offs for people of South Asian, Chinese and other Asian backgrounds.

## Questions

### Is BMI accurate for athletes?

Often not. Muscular people can have a high BMI with low body fat. Waist circumference and trends are better day-to-day signals.

### Does BMI apply to children?

No. Children use age- and sex-specific percentiles.

### What should I track instead?

A 7-day average weight, waist in cm, and how training is going. All three are easy to log daily.

## Sources

- [WHO. Obesity and overweight fact sheet](https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight)

Accurate as of November 12, 2026.

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