BMI calculator
Enter your weight and height to see your body mass index, the adult WHO category it falls in, and the weight range that matches your height.
Runs entirely in your browser
Enter your weight and height.
Your BMI
- Healthy weight range for your height
- Height used
| Category | BMI | Your result |
|---|---|---|
| Underweight | below 18.5 | |
| Healthy weight | 18.5 to 24.9 | |
| Overweight | 25 to 29.9 | |
| Obesity, class I | 30 to 34.9 | |
| Obesity, class II | 35 to 39.9 | |
| Obesity, class III | 40 and above |
This is for adults aged 18 and over. It does not apply to children and teenagers, who are assessed with age-and-sex growth charts, or to pregnancy, and it can overstate the BMI of very muscular people. BMI is a screening number, not a diagnosis. A doctor, nurse or dietitian can say what it means for you.
For adults aged 18 and over. BMI is a screening number, not a diagnosis, and it is not suited to children and teenagers, pregnancy or very muscular people. Nothing you type is stored or sent.
How to calculate your BMI
- Choose Metric (kg and cm) or Imperial (lb, feet and inches). Switching converts the numbers you have already typed.
- Enter your weight and height. The BMI is shown to one decimal place, with its category written out and marked on the scale.
- Read the weight range for your height if you want to see which weights fall within the healthy-weight band, then talk to a health professional about what any result means for you.
What BMI measures, and what it leaves out
Body mass index is a ratio: weight divided by the square of height. It was devised in the 19th century to describe populations, and it has stayed popular because it needs only a scale and a tape measure. The square matters. Doubling height does not double the healthy weight; it roughly quadruples it, which is why tall and short people can be compared on the same scale.
The scale has limits you should keep in mind. Muscle is denser than fat, so someone who trains heavily can land in the overweight band with a low body-fat percentage. The reverse also happens: a person with little muscle can sit in the healthy band while carrying more fat than is ideal. BMI also says nothing about age-related changes, ethnic differences in body composition, or where weight is distributed. Doctors usually combine it with other measurements.
When you switch units, the numbers you typed are converted and rounded to one decimal, and the BMI itself is always computed from kilograms and metres. As a check, 5 ft 9 in and 160 lb gives 23.6, and 70 kg at 175 cm gives 22.9. The healthy range at the top of the result is simply the BMI band 18.5 to 24.9 turned back into weights for the height you entered.
Tips
- Need to convert a weight or height into other units for a form? The unit converter covers kilograms, pounds, centimetres, feet and inches.
- To work out how a figure changed over time as a percentage, use the percentage calculator.
- Curious about exact age in days for a date-based record? Try the age calculator.
Questions
How is BMI calculated?
BMI is weight in kilograms divided by height in metres squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ 3.0625 = 22.9. For pounds and inches the tool first converts to kilograms and metres (1 lb = 0.45359237 kg, 1 in = 2.54 cm), so both unit systems give the same answer for the same person.
What are the adult BMI categories?
The World Health Organization groups adults as: under 18.5 underweight; 18.5 to 24.9 healthy weight; 25 to 29.9 overweight; 30 and above obesity, divided into class I (30 to 34.9), class II (35 to 39.9) and class III (40 and above). The category is taken from the BMI rounded to one decimal, as it is shown, so 24.96 is displayed as 25.0 and counts as overweight.
Can I use it for a child or teenager?
No. For people under 18, BMI is read against percentile charts for their age and sex, and the adult cut-offs do not apply. A paediatrician or school nurse can interpret a child's growth measurements properly.
Why might BMI be misleading?
BMI uses only weight and height. It cannot tell muscle from fat, where weight is carried, or differences in build between people and populations, and it does not take pregnancy into account. A muscular person can have a high BMI with little body fat. It is a quick screening number, not a diagnosis, and a doctor or dietitian can look at the wider picture.
What is the healthy weight range shown?
It is the weights that give a BMI from 18.5 to 24.9 at your height, rounded inwards to one decimal so that each end of the range really does fall inside the band. It is a description of the BMI scale, not a target or advice, and it is not meant to replace guidance from a health professional.
Are my measurements saved or sent anywhere?
No. The sum is done in your browser as you type, and only your choice of units is remembered on this device. Reloading the page clears the weight and height. For other conversions, try the unit converter.