How to use
Choose imperial (feet, inches, stones and pounds) or metric (centimetres and kilograms), then enter your height and weight. You get:
- Your BMI and adult weight category.
- Healthy weight for your height: the weights that give a BMI of 18.5 to 24.9, shown in stones and pounds or in kilograms.
- To reach a healthy weight: how much to lose or gain to get into that range.
Waist size is optional. Pick your sex and enter your waist to check for extra fat around the middle. Measure halfway between the bottom of your ribs and the top of your hips, after breathing out normally.
How BMI is calculated
BMI = weight (kg) ÷ height (m)²
In imperial, your weight is first turned into pounds (stones × 14 + pounds) and then:
BMI = 703 × weight (lb) ÷ height (in)²
Adult BMI categories used by the NHS and WHO:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obese (class 1) |
| 35 to 39.9 | Obese (class 2) |
| 40 or more | Severely obese (class 3) |
Waist: the NHS advises keeping your waist to less than half your height, so someone 5 ft 9 in (175 cm) should aim for under 34.5 in (87.5 cm). This calculator flags a waist over 102 cm (40 in) for men or 88 cm (35 in) for women, the level linked with a substantially higher risk of type 2 diabetes and heart disease.
Examples
| Height | Weight | BMI | Category | Healthy weight range |
|---|---|---|---|---|
| 5 ft 9 in | 11 st 6 lb | 23.6 | Healthy weight | 9 st – 12 st 1 lb |
| 5 ft 4 in | 13 st 0 lb | 31.2 | Obese (class 1) | 7 st 10 lb – 10 st 5 lb (lose 2 st 9 lb) |
| 5 ft 11 in | 15 st 10 lb | 30.6 | Obese (class 1) | 9 st 7 lb – 12 st 11 lb (lose 2 st 13 lb) |
| 165 cm | 70 kg | 25.7 | Overweight | 50.4 – 68.0 kg |
For the first row: 11 st 6 lb is 160 lb and 5 ft 9 in is 69 inches, so 703 × 160 ÷ 69² = 23.6.
Keep in mind
BMI is a quick screening tool, not a diagnosis. It doesn’t measure body fat directly, and the same BMI can mean different health risks depending on muscle mass, age, sex and ethnicity. It isn’t suitable during pregnancy. If your BMI is outside the healthy range, your GP can check your weight and health properly and tell you about local NHS weight management support.