How to calculate your BMI
- Choose Metric (cm, kg) or Imperial (ft, in, lb). If you switch, the numbers you have entered are converted for you.
- Enter your height and weight. In imperial units, height goes into two boxes: feet, then the remaining inches.
- Your BMI appears to one decimal place with its category, and a marker shows where it falls on the colored scale.
- Under the scale you’ll find the weight range that counts as a healthy weight for your height.
On a first visit, the calculator starts in imperial units if your device is set to a US time zone and in metric everywhere else. After that it remembers your choice.
The BMI formula
Body Mass Index is weight divided by height squared: BMI = weight (kg) ÷ height (m)². With pounds and inches the same formula becomes BMI = 703 × weight (lb) ÷ height (in)²; the 703 converts the units. Internally the calculator converts imperial values to metric with the exact definitions (1 inch = 2.54 cm, 1 pound = 0.45359237 kg), so both give the same result.
- 170 cm and 65 kg: 65 ÷ 1.70² = 65 ÷ 2.89 = 22.5, in the healthy weight range. The healthy range shown for this height is 53.5–72 kg.
- 5 ft 7 in and 143 lb: 67 inches in total, so 703 × 143 ÷ 67² = 22.4. The healthy range shown is 118–159 lb.
- 160 cm and 70 kg: 70 ÷ 1.60² = 27.3, which the WHO classes as overweight (pre-obesity).
WHO categories for adults
| BMI | WHO category | Shown here as |
|---|---|---|
| Below 18.5 | Underweight | Underweight |
| 18.5–24.9 | Normal weight | Healthy weight |
| 25.0–29.9 | Pre-obesity | Overweight |
| 30.0–34.9 | Obesity class I | Obesity (class 1) |
| 35.0–39.9 | Obesity class II | Obesity (class 2) |
| 40 and above | Obesity class III | Obesity (class 3) |
The WHO also uses “overweight” for any BMI of 25 or more. The healthy weight range under the scale is the weight that would give a BMI from 18.5 to 24.9 at your height. It is deliberately wide – about 18.5 kg for a height of 170 cm – because it is a population guideline, not a target for one person.
Some countries use lower cut-offs because, in Asian populations, weight-related health risks tend to appear at a lower BMI. A WHO expert consultation in 2004 suggested 23 and 27.5 as additional points for action in Asian populations. In practice, Japan’s obesity society treats a BMI of 25 or more as obesity, Korean guidelines classify 23–24.9 as pre-obesity and 25 or more as obesity, and China uses 24 for overweight and 28 for obesity. Under those criteria the 160 cm, 70 kg example above would count as obesity in Japan and Korea rather than overweight.
What BMI can’t tell you
- Muscle versus fat. BMI only knows your total weight. A muscular athlete who is 185 cm tall and weighs 95 kg has a BMI of 27.8 – “overweight” on paper even with little body fat.
- Where fat is stored. Fat around the waist is linked with different health risks than fat on the hips and thighs, and BMI can’t see the difference. This is why waist measurements are often used alongside it.
- Children and teenagers. Body composition changes as children grow, so their BMI is compared with growth charts for their age and sex and expressed as a percentile or score. The adult categories on this page don’t apply to them.
- Older adults. People tend to lose muscle and some height with age, so the same BMI can mean a different amount of body fat than in a younger adult.
- Pregnancy. BMI is not meaningful during pregnancy.
- Ethnic background. As explained above, the same BMI can carry different risks in different populations.
BMI is a quick screening number, useful for spotting trends and comparing groups, but it is not a diagnosis. If your result worries you, talk it through with a doctor or another health professional, who can look at your health as a whole.
FAQ
Why does my BMI change slightly when I switch units?
The converted values are rounded to one decimal place. For example, 170 cm and 65 kg become 5 ft 6.9 in and 143.3 lb, which can move the last digit of the BMI by 0.1.
Are the categories different for men and women?
Not for adults: the formula and the cut-offs are the same for both. Sex only matters for children and teenagers, whose BMI is compared with separate growth charts for boys and girls.
How many calories should I eat to reach a different weight?
The Calorie Calculator estimates your daily energy needs from your sex, age, height, weight and activity level.
How do I convert my weight between kilograms and pounds?
Use the kilograms to pounds converter, or the Unit Converter for heights in feet and centimeters.