How BMI is calculated
BMI is your weight in kilograms divided by your height in metres squared. If you use imperial units, the calculator converts feet and inches to metres and stone and pounds to kilograms first.
The result is placed in one of these adult categories: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, 30 to 39.9 is obese and 40 or above is severely obese.
The healthy weight range shown is the weight that would give you a BMI between 18.5 and 24.9 at your height.
Worked examples
Someone 175 cm tall who weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which is a healthy weight. The healthy weight range for that height is 56.7 to 76.3 kg.
Someone 5 ft 9 in tall who weighs 11 st 0 lb has a BMI of 22.7, also a healthy weight. Their healthy range is 8 st 13 lb to 12 st 1 lb.
Limits of BMI
BMI can't tell the difference between muscle and fat, so a muscular person can come out as overweight or obese without carrying much body fat. It also doesn't show where fat is stored, which matters for health.
The adult categories don't apply to children and teenagers, whose BMI is compared with others of the same age and sex, or during pregnancy. The NHS uses lower thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds: 23 to 27.4 is overweight and 27.5 or above is obese. This calculator uses the standard thresholds.
This calculator is not medical advice. If you're worried about your weight, talk to your GP.
Inputs for the BMI calculator
Height
Your height without shoes. Choose centimetres, feet and inches, or inches from the dropdown. For feet and inches, enter the whole feet and the remaining inches, for example 5 and 9 for 5 ft 9 in.
Weight
Your weight. Choose kilograms, stone and pounds, or pounds from the dropdown. For stone and pounds, enter the whole stone and the remaining pounds, for example 11 and 4 for 11 st 4 lb. The healthy range is shown in the unit you choose.
CALCULATOR QUESTIONS
BMI calculator questions
What is a healthy BMI?
For most adults, a BMI from 18.5 to 24.9 is in the healthy weight range. Some ethnic groups have a higher risk at a lower BMI, so lower thresholds are used for them.
Is BMI accurate for muscular people?
No. BMI only uses height and weight, so muscle counts the same as fat. Athletes and people who do a lot of strength training can be classed as overweight when they aren't carrying excess fat.
Can I use this calculator for my child?
No. Children's BMI has to be compared with growth charts for their age and sex. Use a calculator designed for children or ask your GP or health visitor.
Can I use BMI during pregnancy?
Not during pregnancy itself, because your weight changes as your baby grows. Your midwife may use your BMI from before or early in pregnancy instead.
What should I do if my BMI is outside the healthy range?
BMI is a starting point, not a diagnosis. Talk to your GP, who can look at other measures such as your waist size and your general health.