Body Mass Index (BMI) is the world's most widely used quick screening tool for weight categories. One number — your weight relative to your height — summarises whether you fall in a range associated with the lowest health risk.
The BMI formula
BMI = weight (kg) ÷ height (m)²
Imperial version: BMI = 703 × weight (lb) ÷ height (in)²
Both give the same number.
How to calculate BMI step by step
- Square your height in metres: 1.75 m → 1.75 × 1.75 = 3.0625
- Divide your weight by that value: 70 ÷ 3.0625
- Round to one decimal place: 22.9
In imperial: 703 × 160 ÷ (70 × 70) = 703 × 160 ÷ 4,900 = 22.9 — the same BMI.
What the ranges mean
- Below 18.5 — underweight
- 18.5 to 24.9 — normal weight
- 25 to 29.9 — overweight
- 30 and above — obesity (often split into classes I, II and III)
What BMI does not tell you
BMI cannot distinguish muscle from fat. A lean, muscular athlete and a person with low muscle mass can share the same BMI with very different body compositions. It also ignores where fat is stored — visceral fat around the abdomen carries higher risk than fat elsewhere. For older adults, children and athletes, read BMI alongside body fat percentage or waist circumference.
How to use BMI sensibly
- Treat it as a screening signal, not a verdict.
- Track the trend over time rather than one reading.
- Pair it with related measures like body fat percentage.
Put it into practice
Run your numbers on the free BMI Calculator, then go deeper with the Body Fat Calculator.
