BMI Calculator
BMI (body mass index) is your weight in kilograms divided by your height in metres squared. A result between 18.5 and 24.9 is classed as a healthy weight, below 18.5 as underweight, 25 to 29.9 as overweight, and 30 or above as obese. Enter your height and weight below to see your number and category instantly.
What is bmi calculator?
Body mass index is a screening tool, not a diagnosis. It was popularised in the 1970s as a cheap way to compare body weight across populations, and it is still used because it needs nothing more than a scale and a tape measure.
What BMI does well is flag weight ranges that statistically carry higher health risk at a population level. What it does badly is describe any individual body, because it cannot tell the difference between muscle, fat, bone and water.
That is why a muscular rugby player and a sedentary person of the same height and weight can share a BMI of 29, yet have completely different body composition and health profiles.
How BMI is calculated
BMI equals weight in kilograms divided by height in metres squared. If you use imperial units, the result is the same but the constant changes. The World Health Organization defines the standard categories: underweight below 18.5, healthy 18.5 to 24.9, overweight 25 to 29.9, and obese 30 and above.
BMI = weight (kg) ÷ height (m)²
BMI = 703 × weight (lb) ÷ height (in)²Source: World Health Organization body mass index classification.
How to use this calculator
- Pick metric (kg and cm) or imperial (lb and inches).
- Enter your height and your current weight.
- Read your BMI and the category it falls into.
- Check the healthy weight range shown for your height — that is the weight span that would put you between 18.5 and 24.9.
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above falls in the obese range.
Is BMI accurate for muscular people?
Not really. BMI treats all body mass the same, so muscle counts as if it were fat. Many athletes register as overweight despite low body fat. For them, body fat percentage or waist-to-height ratio gives a clearer picture.
Does BMI apply to children?
Children use age- and sex-specific BMI percentiles rather than the fixed adult thresholds. This calculator uses adult categories and is intended for people aged 18 and over.
Why does BMI differ between ethnic groups?
Body composition and health risk at a given BMI vary between populations. Some groups face elevated risk at lower BMI values, which is why several countries use adjusted thresholds. Your doctor can interpret your number in that context.
How do I lower my BMI?
BMI falls when weight falls relative to height. A sustained calorie deficit of about 500 kcal per day typically produces roughly 0.5 kg of loss per week. Combine it with resistance training to keep muscle while losing fat.
Related calculators
- Body Fat Calculator — Estimate your body fat percentage from waist, neck and hip measurements using the US Navy circumference method. Works in metric and imperial.
- Ideal Weight Calculator — See your estimated ideal weight using the Devine, Robinson and Hamwi formulas, plus the BMI-based healthy weight range for your height.
- Calorie Calculator — Free calorie calculator using the Mifflin-St Jeor equation. See how many calories you should eat to maintain, lose or gain weight, plus your BMR.
These figures are estimates from population formulas, not medical advice. Individual needs vary. If you are pregnant, managing a medical condition, or under 18, talk to a doctor before changing your diet.