BMI Calculator

What this BMI calculator does

Body Mass Index (BMI) is a quick screening number that relates your weight to your height. This calculator computes it instantly and tells you which standard category you fall into — underweight, normal weight, overweight or obese. It is the same measure used by doctors, public-health bodies and the World Health Organization as a first, low-cost indicator of whether your weight may pose a health risk.

BMI is a screening tool, not a diagnosis. It doesn't measure body fat directly and can't tell muscle from fat, so a very muscular athlete may register as "overweight" despite being lean. Use it as a starting point and a way to track changes over time, not as the final word on your health.

How to use it

1. Enter your weight in kilograms. If you weigh in pounds, divide by 2.205 (e.g. 154 lb ÷ 2.205 ≈ 70 kg).
2. Enter your height in centimetres. If you measure in feet and inches, convert first (e.g. 5 ft 9 in ≈ 175 cm).
3. Calculate. Your BMI and category appear instantly. Re-enter different values to compare.

The formula explained

Dividing weight by height squared adjusts for the fact that taller people naturally weigh more. The result is a single number that can be compared against population-wide categories. The squaring is what makes BMI roughly independent of height for people of average build.

BMI categories chart

Worked examples

When BMI is useful and when it isn't

• Useful for screening large groups, tracking your own trend over months, and as a conversation starter with a doctor.
• Less reliable for athletes and very muscular people, who may score high despite low body fat.
• Less reliable for older adults who have lost muscle, and for pregnant people.
• Not designed for children in the same way — they use age- and sex-specific percentiles.

Tips and common mistakes

Don't panic over a single number. BMI is a blunt instrument; waist circumference and body-fat percentage add useful context. Check your unit conversions — mixing pounds with centimetres is the most common error and throws the result off completely. Track the trend, not the snapshot: a falling or stable BMI alongside good fitness markers matters more than one reading. And see a professional for personalised advice — BMI flags a possible issue, it does not explain or solve it.

What BMI doesn't capture

BMI treats every kilogram the same, whether it is muscle, fat, bone or water, and it ignores where weight sits on the body. That matters because fat stored around the abdomen carries a higher health risk than the same amount on the hips and thighs. Two people with an identical BMI of 27 can have very different risk profiles: a powerlifter carrying dense muscle versus someone with excess abdominal fat. For this reason health professionals increasingly pair BMI with waist circumference (a waist over about 94 cm for men or 80 cm for women signals raised risk) and the waist-to-height ratio, a simple guideline being to keep your waist under half your height. These take seconds to measure and add the context that BMI alone misses.

BMI across populations

The standard BMI thresholds were derived largely from European populations, and they don't fit everyone equally. Research shows that people of South Asian descent tend to develop weight-related health risks at lower BMI values, which is why some health bodies suggest a lower overweight threshold (around 23 rather than 25) for these groups. Older adults, who naturally lose muscle, and very tall or very short people can also be misclassified. Use BMI as a universal first screen, but interpret borderline results in the light of your age, ethnicity, build and overall fitness rather than treating the cut-offs as absolute.