BMI Calculator
Body mass index from your height and weight, in metric or US units, with the weight range for your height and an optional waist-to-height ratio.
Optional: age and waist
Measure the waist at the level of the navel (belly button), after breathing out.
- BMI "normal range" for your height
- The weights that give a BMI of 18.5 to 24.9 at this height.
- Waist-to-height ratio
Metric or US units
Switch between kilograms and centimetres, or pounds, feet and inches. Your numbers convert with you.
Plain about the limits
BMI is a rough indicator. We show what it can and cannot tell you, and add waist-to-height ratio for a second view.
Private
Your measurements stay in your browser. Nothing is uploaded, stored or tracked.
BMI categories for adults
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Normal range | 18.5 to 24.9 |
| Overweight | 25 to 29.9 |
| Obesity class I | 30 to 34.9 |
| Obesity class II | 35 to 39.9 |
| Obesity class III | 40 and above |
Source of the categories: World Health Organization, adult BMI classification.
Related: estimate daily energy use with the calorie calculator, or work out proportions with the percentage calculator.
Frequently Asked Questions
How is BMI calculated?▼
Body mass index is your weight in kilograms divided by your height in metres, squared. For 70 kg and 1.75 m that is 70 ÷ (1.75 × 1.75) = 22.9. If you use pounds and inches, the calculator converts them first (1 lb = 0.45359237 kg and 1 inch = 2.54 cm exactly), so you get the same answer in either unit system.
What do the BMI categories mean?▼
For adults, the World Health Organization groups BMI into underweight (below 18.5), normal range (18.5 to 24.9), overweight (25 to 29.9) and three classes of obesity (30 and above). These are population-level groupings used for screening. They describe a number, and they are not a diagnosis of any individual.
Is BMI accurate for everyone?▼
No. BMI uses only height and weight, so it cannot tell muscle from fat or show where weight is carried. It can read high for muscular people and low-risk for people who have lost muscle, which becomes more common with age. The thresholds also differ for some ethnic groups. Treat it as one rough indicator among many.
What is waist-to-height ratio and why is it shown?▼
It is your waist measurement divided by your height. Because it reflects where weight is carried, many health organisations discuss it alongside BMI, and a ratio of 0.5 is commonly used as a reference point (waist about half of height). Enter your waist to see your ratio. It is information only, not a verdict.
Does this calculator work for children and teenagers?▼
No. The categories here are for adults aged 18 and over. In children and teenagers BMI is interpreted against growth charts for their age and sex, which a doctor or nurse can do.
Should I use this to decide what to do about my weight?▼
Please don't use any online calculator to make health decisions. This tool gives general information only. If you are worried about your weight or health, or before you change your diet or exercise, speak to a doctor or another qualified health professional who can consider your whole situation.
Is my information stored or sent anywhere?▼
No. The calculation happens in your browser, and your height, weight and other entries are never uploaded or saved.