Body mass index (BMI) compares your weight with your height to give a single number that is widely used to screen adults for underweight, overweight and obesity. Enter your weight and height in metric (kilograms and centimetres) or imperial (pounds, feet and inches) units and the calculator shows your BMI, the World Health Organization category it falls into and where it sits on the scale.
It also shows the weight range that gives a BMI between 18.5 and 24.9 for your height, and how far you are from that range. BMI is a quick, useful starting point, but it does not measure body fat directly, so athletes, older adults and some ethnic groups may need other measures such as waist size.
How to use the BMI Calculator tool
- Choose Metric or Imperial units.
- Enter your weight (kg or lb).
- Enter your height (cm, or feet and inches).
- Read your BMI, its WHO category and the marker on the colour scale.
- Check the healthy weight range for your height and the formula used.
Why use ToolOrca BMI Calculator
- Metric and imperial units, values carry over when switching
- WHO adult categories including thinness and obesity classes
- Healthy weight range for your height
- How much to gain or lose to reach the range
- BMI Prime and the full formula shown
- Private: nothing is stored or sent
BMI formula with a worked example
BMI = weight in kilograms ÷ (height in metres)². In imperial units, BMI = 703 × weight in pounds ÷ (height in inches)².
Worked example: a person who weighs 70 kg and is 175 cm tall has a BMI of 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9, which is in the normal range. In imperial units, 154 lb at 5 ft 9 in (69 in) gives 703 × 154 ÷ 69² = 22.7. For a height of 175 cm, a BMI of 18.5–24.9 corresponds to about 56.7–76.3 kg.
WHO BMI categories for adults
Below 18.5: underweight (below 16 severe, 16–16.9 moderate, 17–18.4 mild thinness). 18.5–24.9: normal weight. 25–29.9: overweight (pre-obese). 30–34.9: obesity class I. 35–39.9: obesity class II. 40 and above: obesity class III. Some Asian health authorities use lower cut-offs, such as 23 for overweight, because health risks rise at a lower BMI.
BMI is for adults aged 20 and over; children and teenagers are assessed with age- and sex-specific percentiles. This calculator is for information only and is not medical advice. Talk to a doctor about your own weight and health.
Frequently asked questions
What is a healthy BMI?
For adults, the WHO defines a BMI of 18.5 to 24.9 as normal weight. A BMI of 25 to 29.9 is overweight and 30 or more is obesity.
What is the BMI for 70 kg and 175 cm?
70 ÷ 1.75² = 22.9, which is in the normal range.
What should I weigh for my height?
A healthy BMI range of 18.5–24.9 gives about 47.4–63.7 kg at 160 cm, 56.7–76.3 kg at 175 cm and 59.9–80.7 kg at 180 cm. At 5 ft 9 in it is about 125–169 lb.
Is BMI accurate for muscular people?
Not always. Muscle is denser than fat, so athletes can have a BMI over 25 with little body fat. Waist circumference or a body fat measurement gives a better picture in that case.
Can I use this BMI calculator for children?
No. For people under 20, BMI must be compared with growth charts for the same age and sex. Ask a paediatrician or use a child BMI percentile tool.