How to use the BMI calculator
- Choose Metric (cm, kg) or Imperial (ft, st, lb). Switching keeps your numbers and converts them.
- Enter your Height and Weight. In imperial you can give stones and pounds, or leave stones empty and type your whole weight in pounds.
- Optionally add your Age. Under 18, the page shows why the adult categories do not apply.
- Tick the lower cut-offs box if you are of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background (see below).
- Read your BMI, its category, the scale and the healthy weight range for your height. Everything updates as you type.
How BMI is calculated
Body mass index is your weight divided by the square of your height: BMI = weight (kg) ÷ height (m)². In imperial units the same result is 703 × weight (lb) ÷ height (in)². Two worked examples:
- 180 cm and 75 kg: 75 ÷ (1.80 × 1.80) = 75 ÷ 3.24 = 23.1.
- 5 ft 10 in (70 in) and 165 lb: 703 × 165 ÷ (70 × 70) = 115,995 ÷ 4,900 = 23.7.
The calculator converts imperial entries with the exact definitions (1 inch = 2.54 cm, 1 pound = 0.45359237 kg) and rounds the result to one decimal place, which is also the value used to choose the category. BMI Prime is your BMI divided by 25, the upper limit of the healthy range: 1.00 sits right on the boundary, 0.92 is 8% below it and 1.20 is 20% above it.
Adult BMI categories
The World Health Organization (WHO) classifies BMI in adults aged 18 and over the same way for men and women:
| BMI | Category (WHO) | Lower cut-offs (NHS/NICE)* |
|---|---|---|
| Below 18.5 | Underweight | Below 18.5 |
| 18.5 to 24.9 | Healthy weight | 18.5 to 22.9 |
| 25.0 to 29.9 | Overweight | 23.0 to 27.4 |
| 30.0 to 34.9 | Obesity class I | 27.5 and above: obesity |
| 35.0 to 39.9 | Obesity class II | |
| 40.0 and above | Obesity class III |
* People of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background tend to have a higher risk of type 2 diabetes and heart disease at a lower BMI. In the UK, NICE guidance and the NHS therefore use lower thresholds for them: overweight from 23 and obesity from 27.5. Tick the box in the calculator to apply them.
The healthy weight range shown is simply the weight that would give a BMI of 18.5 to 24.9 at your height (18.5 to 22.9 with the lower cut-offs). For 180 cm that is about 59.9 to 80.7 kg.
What BMI cannot tell you
BMI is quick, free and the same everywhere, which is why doctors use it for screening. But it only knows your height and weight, not what that weight is made of or where it sits:
- Muscle: athletes and very muscular people can score as overweight with little body fat.
- Older adults: people lose muscle and height with age, so BMI can look fine while body fat has risen. A doctor may suggest a different target range.
- Pregnancy: BMI is not meaningful during pregnancy; midwives use your weight from before or early in pregnancy.
- Fat distribution: fat around the waist carries more health risk. NICE suggests also checking your waist-to-height ratio: keep your waist measurement to less than half your height.
Children and teenagers: under 18, BMI is calculated the same way but read differently. Because children's bodies change as they grow, their BMI is compared with children of the same age and sex on BMI-for-age charts (percentiles or centiles). In the CDC charts used in the US, for example, overweight starts at the 85th percentile and obesity at the 95th. Use a children's BMI tool or ask a doctor, school nurse or health visitor.
Want to know how many calories you need to maintain or change your weight? Use the calorie calculator (TDEE). To switch units, try the kg to lbs converter and the cm to inches converter. Our guide What Is BMI? explains the history, the evidence and the limits in more detail, and the health tools page lists our eye and hearing screenings.
Privacy: the calculation runs in your browser and your height, weight and age are not sent anywhere or stored. Only your choice of metric or imperial units is remembered in this browser.
Frequently asked questions
What is a healthy BMI for adults?
For most adults a BMI of 18.5 to 24.9 is classed as a healthy weight. If you are of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, the NHS uses 18.5 to 22.9, with overweight starting at 23.
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. At 1.80 m and 75 kg: 75 ÷ 3.24 = 23.1. In pounds and inches, multiply your weight in pounds by 703 and divide by your height in inches squared.
Is BMI different for men and women?
No, adult BMI uses the same formula and the same categories for men and women. Women tend to have more body fat than men at the same BMI, which is one reason BMI is a screening figure rather than a measure of body fat. For children, the BMI-for-age charts are different for boys and girls.
Why does BMI say I'm overweight when I'm muscular?
Muscle is denser than fat, and BMI cannot tell them apart. Very muscular people can have a BMI over 25 with a healthy amount of body fat. A waist measurement or a body composition test gives a better picture.
Can I use this BMI calculator for children?
Not for the category. Under 18, BMI has to be compared with others of the same age and sex using BMI-for-age percentiles, so the adult cut-offs do not apply. The calculator still shows the number if you enter an age under 18, but not an adult category.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value under 1.00 means you are below the overweight threshold; 1.20 means your BMI is 20% above it.
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