BMI Calculator: Check Your Body Mass Index

This free BMI calculator works out your body mass index from your height and weight, in metric or US units, and shows the matching World Health Organization adult category. BMI is a quick screening number, not a diagnosis, so read your result alongside waist size, fitness, diet and a conversation with your clinician.

Calculate your BMI

For adults aged 18 and over. Not for children, teens, or during pregnancy. This calculator is for general information and is not medical advice.

How does the BMI calculator work?

Body mass index is your weight in kilograms divided by your height in meters squared: BMI = kg / m2. If you choose US units, the tool converts inches to meters (1 inch = 0.0254 m) and pounds to kilograms (1 lb = 0.4536 kg) before doing the same sum. The healthy weight range shown is simply the weight that would give a BMI of 18.5 to 24.9 at your height.

How do I read my BMI result?

The tool uses the World Health Organization adult categories:

BMI (kg/m2)WHO category
Below 18.5Underweight
18.5 to 24.9Normal weight
25.0 to 29.9Pre-obesity (overweight)
30.0 to 34.9Obesity class I
35.0 to 39.9Obesity class II
40.0 and aboveObesity class III

A result outside the normal range is a prompt to look closer, not a verdict. If your number is high, small and steady changes usually work better than drastic ones, and our guide on why some body fat won't budge and what actually works explains realistic next steps. If your BMI is low, read how to gain weight healthily before simply eating more of anything.

What are the limits of BMI?

  • It cannot tell muscle from fat. Muscular people can score high while being lean, and older adults can score normal while carrying little muscle.
  • It ignores where fat sits. Fat around the waist matters more for health risk than fat elsewhere, so a tape measure adds useful context.
  • It is for adults only. Children and teens need age and sex specific charts, and BMI is not used to judge weight during pregnancy.
  • Cut-offs can differ by population. Some health services use lower thresholds for people of certain ethnic backgrounds, so ask your clinician which applies to you.

See a doctor if your BMI is below 18.5 or 30 and above, if you have lost or gained weight without trying, or if you have concerns such as tiredness, breathlessness or changes in appetite. Building strength also helps at any weight, and these longevity exercises are an easy place to start. For the bigger picture, our overview of what healthy living really means puts weight in context with sleep, food and stress.

More free wellness calculators

Frequently asked questions

Is BMI accurate for athletes and muscular people?

Not very. Muscle is denser than fat, so people who do a lot of strength training can land in the overweight range while having little body fat. In that case, waist measurement, body composition testing or a clinician's assessment gives a fairer picture. BMI works best as a quick screening tool across large groups of people rather than as a precise personal measure.

Can I use this BMI calculator for my child?

No. This tool uses adult cut-offs, which do not apply to children and teenagers because their body fat changes as they grow. Young people need BMI-for-age percentile charts that compare them with others of the same age and sex. Your child's doctor or a national health service child BMI tool is the right place to check a young person's weight.

What should I do if my BMI is in the obesity range?

Treat it as useful information, not a judgment. Book a check-up so a doctor can look at blood pressure, blood sugar, cholesterol and other markers that BMI cannot show. Small, sustainable changes to eating, activity and sleep are usually the safest start, and a clinician can tell you whether extra support or treatment would help in your situation.

This calculator is for general information and is not medical advice.

Sources

Last reviewed: October 2026.

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