Calculators guide
How to Calculate BMI — and What the Number Does and Doesn't Tell You
Updated 28 September 2026 · 3 min read
Body mass index (BMI) is a quick way to relate weight to height. Doctors and public-health agencies use it to screen adults for weight-related health risks — but it's a starting point, not a verdict on anyone's health. The BMI Calculator works it out in metric or imperial units and shows the healthy weight range for your height. This guide is general information, not medical advice.
Open the free BMI CalculatorThe BMI formula
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Example: 70 kg and 1.75 m tall. 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.9.
Getting an accurate reading
Weigh yourself at the same time of day — first thing in the morning is most consistent — in light clothing, and measure your height without shoes, standing straight against a wall. Weight naturally moves up and down by a kilogram or more from day to day, so look at the trend rather than a single reading.
What the numbers mean for adults
| BMI | WHO category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 and over | Obese |
For a height of 1.75 m, the healthy range of 18.5–24.9 works out to about 57–76 kg. The calculator shows this range for your own height.
Healthy weight range by height
| Height | Healthy weight (BMI 18.5–24.9) |
|---|---|
| 1.55 m (5 ft 1 in) | 44–60 kg (98–132 lb) |
| 1.60 m (5 ft 3 in) | 47–64 kg (104–141 lb) |
| 1.65 m (5 ft 5 in) | 50–68 kg (111–149 lb) |
| 1.70 m (5 ft 7 in) | 53–72 kg (118–159 lb) |
| 1.75 m (5 ft 9 in) | 57–76 kg (125–168 lb) |
| 1.80 m (5 ft 11 in) | 60–81 kg (132–178 lb) |
| 1.85 m (6 ft 1 in) | 63–85 kg (140–188 lb) |
| 1.90 m (6 ft 3 in) | 67–90 kg (147–198 lb) |
Where BMI falls short
- Muscle weighs more than fat. Athletes and very muscular people can have a high BMI without excess fat.
- It doesn't show where fat is. Fat around the waist carries more health risk; waist measurement adds useful information.
- Ethnicity matters. The WHO has noted that some Asian populations have higher health risks at lower BMIs, and some countries use lower cut-offs.
- Age changes the picture. Older adults naturally lose muscle, so BMI can understate body fat.
- It isn't for children or pregnancy. Children are assessed with age- and sex-specific percentiles, and pregnancy changes weight for other reasons.
A useful second check: waist-to-height
Because BMI can't see where weight is carried, some health services add a simple waist check. The UK's NICE guidance, for example, suggests keeping your waist measurement to less than half your height. Measure around your middle, halfway between the bottom of your ribs and the top of your hips, after breathing out normally.