BMI Calculator
Calculate your Body Mass Index and category.
kg (metric) or lb (imperial)
cm (metric) or inches (imperial)
Results update as you type.
This calculator provides estimates based on population research. Results are not medical advice. Consult a qualified healthcare provider before making health or nutrition decisions.
How it works
Body Mass Index is a screening figure that relates a person's weight to their height, expressed as a single number in kilograms per square metre. Adolphe Quetelet devised it in the 1830s as a way to describe populations, and it later became the standard first-pass measure of weight status because it needs only two easily taken measurements and no equipment beyond scales and a stadiometer. This bmi calculator applies the World Health Organization thresholds: underweight below 18.5, normal from 18.5 to 24.9, overweight from 25 to 29.9, and obese at 30 and above. Those cut points matter because BMI tracks, at the group level, with the risk of conditions such as type 2 diabetes, hypertension and cardiovascular disease, which is why clinics use it to flag people for closer assessment. The WHO adopted these ranges after reviewing large cohort data linking body mass to mortality and morbidity. Doctors, dietitians, insurers and public-health researchers all reach for a bmi calculator as a fast, standardised starting point. It has a real limitation: because it measures total mass, not composition, this bmi calculator can misclassify very muscular athletes as overweight and can understate fat in older adults who have lost lean tissue.
BMI = weight (kg) ÷ height (m)² · Imperial: BMI = 703 × lb ÷ in²
Weight is divided by height squared because, across adults of similar build, body mass scales roughly with the square of height rather than height itself. Squaring the height therefore normalises weight so that tall and short people can be compared on one scale. The imperial version multiplies pounds over inches squared by 703 to convert the result into the same metric kg/m² units.
Source:World Health Organization (WHO), Technical Report Series 854 (1995)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter weight and height.
- 2
Adjust the options
Adjust units if they apply — sensible defaults are already set.
- 3
Read your result
BMI updates instantly as you type — no button to press.
Frequently asked questions
What is a normal BMI for men and women?
The healthy band is 18.5 to 24.9 for both sexes, since the WHO applies the same adult cut points to men and women. Sex differences in muscle and fat distribution are not built into the number itself. Use it only as a broad screen, and pair it with a waist measurement or body-fat testing for a fuller picture of an individual's overall health.
How accurate is BMI?
As a population screen it correlates well with body fat and health risk, but for one person it can mislead. It reads only total mass, so a muscular athlete may land in the overweight range while carrying little fat, and a sedentary older adult may sit in the normal range despite high fat. Treat it as a flag, not a verdict.
Does BMI apply to children?
Not with these adult categories. Children and teenagers grow at different rates, so they use age- and sex-specific BMI-for-age percentiles from growth charts instead. A child in the 85th to 95th percentile is considered overweight. This tool is built for adults aged 20 and over; use a paediatric percentile chart for anyone younger. A doctor or paediatrician can plot the correct percentile and interpret it against the child's growth history.
How often should I recalculate my BMI?
For most adults, checking every few months is plenty, since height is stable and weight moves slowly. Recalculate after a deliberate change in weight, a new training programme, or a shift in health goals. Frequent daily checks add little, because normal fluctuations in food and water can move the scale by a kilogram or two without changing your real status.
Can I use BMI if I am very athletic?
You can calculate it, but interpret it carefully. Muscle is denser than fat, so athletes and heavy lifters often score as overweight or obese while being lean and healthy. If that describes you, a body-fat percentage estimate or a waist-to-height ratio gives a more meaningful reading than BMI alone. The number is still worth tracking for trends over time, even when the category label itself misjudges your build.