What BMI is
Body mass index (BMI) is a simple number that relates weight to height. Health services use it as a quick first check of whether an adult’s weight is in a typical range for their height. It is easy to calculate, and this BMI calculator only needs your height and weight.
For adults 18+. Not medical advice. Children and teens use age-based percentiles.
The formula
BMI = weight (kg) ÷ height (m)²
Worked example: a person who is 170 cm tall (1.70 m) and weighs 65 kg.
- Square the height: 1.70 × 1.70 = 2.89.
- Divide the weight by that number: 65 ÷ 2.89 = 22.5.
If you enter imperial units, the calculator first converts inches to centimeters (1 inch = 2.54 cm) and pounds to kilograms (1 lb = 0.4536 kg), then applies the same formula. The working is shown under every result.
Adult BMI categories
The calculator uses the widely used World Health Organization categories for adults:
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25 – 29.9 | Overweight |
| 30 and above | Obese |
A colored scale shows where your result falls. The calculator also shows the weight range that corresponds to a BMI of 18.5 to 24.9 at your height, which some people find easier to relate to than the BMI number itself.
An imperial example
Suppose someone is 5 feet 7 inches tall and weighs 150 pounds.
- Height: 5 × 12 + 7 = 67 inches, and 67 × 2.54 = 170.18 cm, or 1.7018 m.
- Weight: 150 × 0.4536 = 68.04 kg.
- BMI: 68.04 ÷ (1.7018 × 1.7018) = 68.04 ÷ 2.896 = 23.5.
A BMI of 23.5 falls in the normal weight category for adults. You do not need to do these conversions yourself: choose Imperial, and the calculator shows the result and its working.
Tracking changes over time
If you are following a plan agreed with a health professional, calculating your BMI every few weeks, at the same time of day, can help you see a trend. Small day-to-day changes in weight are normal and are often caused by water, food and time of day rather than real change.
What BMI does not tell you
BMI is a screening tool, not a diagnosis. Keep its limits in mind:
- It cannot distinguish muscle from fat. Athletes and very muscular people may be classed as overweight while being lean.
- It does not show where fat is carried. Health professionals may also look at waist measurement and other checks.
- It is not designed for children, teens or pregnancy. Young people are assessed with age-based percentiles instead.
- Cut-off points can differ. Some health bodies use different thresholds for certain groups, so your doctor may interpret your result differently.
If your result concerns you, or you are planning to change your weight, speak to a doctor or a qualified health professional.
Measuring accurately
- Measure height without shoes, standing straight against a wall.
- Weigh yourself at a similar time of day, ideally in the morning, in light clothing.
- Use the same units throughout, and double-check feet versus inches when using imperial.
Privacy
Your height and weight are processed in your browser and are not sent to a server. The values appear in the page address only so that you can bookmark or share the calculation if you choose.
Works on phones and computers
The calculator works in any up-to-date browser, on phones, tablets and computers, with a numeric keypad on mobile and no sign-up.
Other calculators you may find useful: the Age Calculator, the Percentage Calculator for working out percentage changes, and the GPA Calculator for students.