Body mass index (BMI) compares your weight with your height to give one number that's used worldwide to screen adults for underweight, overweight and obesity. This calculator works in metric or imperial units, shows which World Health Organization (WHO) category your result falls into, and gives the healthy weight range for your height.
BMI is a screening measure, not a diagnosis. It can't tell muscle from fat or show where fat is stored, so read your result alongside the other checks described below.
How to use the BMI calculator
- Choose metric (centimetres and kilograms) or imperial (feet, inches and pounds).
- Enter your height.
- Enter your weight.
- Read your BMI, its category, and the healthy weight range for your height.
The calculator is for adults aged 18 and over. Children and teenagers need a different method, described below.
How BMI is calculated
BMI is your weight in kilograms divided by the square of your height in metres:
BMI = weight (kg) ÷ height (m)²
In imperial units, the same result comes from multiplying by 703:
BMI = 703 × weight (lb) ÷ height (in)²
Worked example
A person who is 170 cm tall and weighs 70 kg:
- Height in metres squared: 1.70 × 1.70 = 2.89
- BMI: 70 ÷ 2.89 = 24.2, in the healthy weight range
The same person in imperial units is 5 ft 7 in (67 inches) and 154 lb:
- 703 × 154 ÷ (67 × 67) = 108,262 ÷ 4,489 = 24.1
The small difference comes from rounding the unit conversions.
Healthy weight range for your height
The calculator also turns the healthy BMI range (18.5 to 24.9) back into weights for your height. At 170 cm that is 18.5 × 2.89 = 53.5 kg to 24.9 × 2.89 = 72.0 kg. At 5 ft 7 in it is about 118 to 159 lb.
BMI categories for adults
These are the WHO categories used by the calculator:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
Lower thresholds for some ethnic groups
People from some ethnic backgrounds have a higher risk of type 2 diabetes and heart disease at a lower BMI. The UK's National Institute for Health and Care Excellence (NICE) recommends lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background: 23 for overweight and 27.5 for obesity. If this applies to you, compare your result with those numbers rather than the table above.
What BMI doesn't tell you
BMI is quick and useful across large groups of people, but it has known blind spots for individuals:
- Muscle. Muscle is denser than fat, so athletes and very muscular people can have a high BMI without excess fat.
- Where fat is stored. Fat around the waist carries more health risk than fat on the hips and thighs, and BMI can't see the difference.
- Age. Older adults often lose muscle, so a "healthy" BMI can hide a higher proportion of body fat.
- Pregnancy. BMI isn't meaningful during pregnancy. Pre-pregnancy BMI is what midwives and doctors use.
Check your waist too
NICE recommends measuring your waist as well as your BMI. A simple rule is to keep your waist measurement to less than half your height. For someone who is 170 cm tall, that means a waist under 85 cm. Measure around your middle, halfway between your lowest rib and the top of your hip bone.
BMI for children and teenagers
For people under 18, BMI changes naturally with age and differs between boys and girls. Doctors compare a child's BMI with growth charts for their age and sex and express it as a percentile. The adult categories above don't apply, so use a child BMI tool or ask a health professional.
BMI calculator FAQ
What is a healthy BMI?
For most adults, 18.5 to 24.9. For people from the ethnic backgrounds listed above, NICE suggests aiming for below 23.
Is BMI accurate?
It's accurate as a measure of weight relative to height, and it's a reasonable screening tool for most adults. It's less reliable for very muscular people, older adults and some ethnic groups. Waist measurement or a body fat assessment adds useful detail.
Why does my BMI show a different category from another calculator?
Some calculators round differently or use different thresholds, such as the lower ones for some ethnic groups. This calculator uses the standard WHO cut-offs and assigns the category from the BMI shown, rounded to one decimal place.
Who invented BMI?
The Belgian mathematician Adolphe Quetelet devised the formula in the 1830s. The name "body mass index" was introduced by the physiologist Ancel Keys in 1972.
Is my information stored?
No. The calculation happens in your browser and nothing you enter is sent anywhere.
Should I lose or gain weight based on my BMI?
Your BMI is one piece of information, not a treatment plan. If your result is outside the healthy range, or you're worried about your weight, talk to a doctor or registered dietitian, who can take your overall health into account.