BMI Calculator
Calculate your body mass index from weight and height and see where you sit on the World Health Organization adult BMI scale.
BMI — Normal weight
24.5
- WHO category
- Normal weight
- Healthy weight — lower (kg)
- 56.7
- Healthy weight — upper (kg)
- 76.3
BMI = weight (kg) ÷ height (m)². WHO defines a healthy adult BMI as 18.5–24.9. Categories are population-level guidance and do not replace a clinical assessment. Not validated for children, pregnancy, or highly muscular athletes — muscle is denser than fat and can lift BMI without indicating excess body fat.
How to use this calculator
Pick metric or imperial, then enter your weight and height. The calculator returns your BMI to one decimal place, the World Health Organization category it falls into, and the weight range that would put you in the healthy band for your height. Switching units recalculates instantly — type your numbers in the chosen system without manual conversion.
How the calculation works
BMI is body mass divided by height squared, with weight in kilograms and height in metres: BMI = kg ÷ m². Imperial inputs are converted using the exact NIST factors of 1 lb = 0.45359237 kg and 1 inch = 0.0254 m, then the metric formula is applied — so the answer is the same whichever system you choose. The WHO adult BMI cut-offs are: under 18.5 underweight, 18.5–24.9 normal, 25.0–29.9 overweight, 30.0–34.9 obesity class I, 35.0–39.9 obesity class II, and 40 or above obesity class III.
Worked example
A 70 kg adult who is 1.75 m tall. Height squared is 1.75 × 1.75 = 3.0625 m². BMI = 70 ÷ 3.0625 = 22.857, which rounds to 22.9 — comfortably inside the WHO normal-weight band (18.5–24.9). The healthy weight range at 1.75 m is 18.5 × 3.0625 = 56.7 kg at the lower end and 24.9 × 3.0625 = 76.3 kg at the upper end.
Frequently asked questions
What is a healthy BMI?
The World Health Organization defines a healthy adult BMI as 18.5 to 24.9. Below 18.5 is classed as underweight, 25.0–29.9 as overweight, and 30.0 or above as obese. These thresholds apply to adults aged 18 and over and are based on population-level risk of chronic disease — they are not diagnostic on their own.
Is BMI accurate for athletes or very muscular people?
No. Muscle is denser than fat, so a lean, heavily-trained person can have a BMI in the overweight or obese band without carrying excess body fat. Use a body-composition measurement (DEXA, hydrostatic weighing, skinfolds) instead, or pair BMI with a waist-to-height ratio, which captures abdominal fat regardless of muscle mass.
Does BMI work the same for men and women?
The WHO adult thresholds are sex-neutral. Women tend to carry slightly more essential body fat than men at the same BMI, but the cut-offs do not adjust for that — the formula and the categories are identical. For body-composition targets specifically, use a calculator that does distinguish by sex (lean body mass, body fat percentage).
Should I use BMI for children?
No. Children and adolescents are assessed against age- and sex-specific BMI percentile charts (CDC in the US, WHO growth standards internationally), not the adult cut-offs. A child with an adult-formula BMI of 22 might be perfectly healthy or notably overweight depending on age. Use a paediatric BMI-for-age tool instead.
What about during pregnancy?
BMI is not valid in pregnancy — weight gain during gestation includes the foetus, placenta, amniotic fluid and increased blood volume, none of which are body composition. Clinicians track gestational weight gain against pre-pregnancy BMI guidelines (e.g., the Institute of Medicine ranges in the US) rather than computing BMI on current weight.
Why are imperial and metric results sometimes off by 0.1?
The calculator converts pounds and inches to kilograms and metres using NIST-exact factors before dividing, so the underlying maths is identical. Tiny differences come from rounding the displayed inputs — if you enter 154 lb and the equivalent 69.85 kg you should get the same BMI to one decimal place, but rounding 154 lb to the nearest kilogram (70 kg) will shift it by a fraction.