BMI Calculator

How to use the BMI calculator

1. Choose Metric (cm, kg) or Imperial (ft, in, lb). Values you have already typed are converted when you switch.
2. Enter your Height (centimetres, or feet and inches) and your Weight (kilograms or pounds). A decimal comma or point both work.
3. Read Your BMI, the WHO category and the weight range that corresponds to a BMI of 18.5 to 24.9 at your height. The matching row of the category table is highlighted.
4. The calculation steps under the results show exactly how the number was obtained. Clear empties the fields.

Everything is calculated in your browser as you type. Nothing you enter is sent anywhere.

The BMI formula

Body mass index divides weight by the square of height:

BMI = weight (kg) ÷ height (m)²

For example, 70 kg at 1.75 m gives 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. In imperial units the usual shortcut is BMI = 703 × weight (lb) ÷ height (in)². This tool converts pounds and inches exactly (1 lb = 0.45359237 kg, 1 in = 2.54 cm) and then applies the metric formula, so both unit systems give the same answer. The worked examples published by the US Centers for Disease Control and Prevention (CDC) are reproduced: 68 kg at 165 cm gives 24.98, and 150 lb at 5 ft 5 in gives 24.96.

The result is shown with one decimal, and the category is chosen from that rounded value so that the number and the label always agree.

WHO categories for adults

This tool uses the adult classification of the World Health Organization (WHO Technical Report Series 894, 2000):

• below 18.5: underweight;
• 18.5 to 24.9: normal weight;
• 25 to 29.9: overweight (pre-obesity);
• 30 to 34.9: obesity class I;
• 35 to 39.9: obesity class II;
• 40 and above: obesity class III.

These categories apply to adults aged 18 and over. Children and teenagers are not assessed with them. Their BMI is compared with BMI-for-age percentile charts (WHO or CDC growth charts) because body composition changes with age and puberty. Ask a paediatrician or school nurse to interpret a child’s BMI.

What BMI does not tell you

BMI is a quick screening number for populations, not a diagnosis. It cannot tell muscle from fat, so muscular people can show a “high” BMI while being lean, and older adults who have lost muscle can show a “normal” BMI with more body fat. It says nothing about where fat is stored; waist circumference adds useful information on that point. The same cut-offs may also fit some groups less well: WHO has noted that health risks can rise at lower BMI values in several Asian populations, and BMI is not meant for pregnancy.

The weight range shown is simply what a BMI of 18.5 to 24.9 means at your height. It is not a personal target. If your result worries you, or you are thinking about changing your weight, talk to a doctor, a registered dietitian or another qualified health professional, who can take your history, measurements and goals into account. Gradual, sustainable habits are safer than rapid changes.