How to use the BMI Calculator
- Choose metric (kilograms and centimetres) or imperial (pounds, feet and inches).
- Enter your weight.
- Enter your height. In imperial mode, enter feet and inches separately, for example 5 ft and 9 in.
- The result updates instantly as you type. There is no button to press.
- Use Copy result to copy the figures, or Copy link to share a link that reopens the BMI Calculator with the same inputs.
How it works
Body mass index (BMI) compares your weight with your height to give a single number that is widely used to screen adults for weight categories that may raise health risks. The World Health Organization (WHO) classes a BMI of 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight and 30 or more as obesity, with obesity split into three classes.
BMI is popular because it is quick, free and needs only a scale and a tape measure. But it is a screening tool, not a diagnosis. It cannot tell muscle from fat, so muscular athletes often score as "overweight". It does not show where fat is stored, and fat around the waist is linked to more risk than fat on the hips. Some health bodies use lower cut-offs for people of South Asian, Chinese and other Asian backgrounds, for example treating 23 or more as overweight. BMI is also not used in the same way for children, teenagers or pregnant women.
Alongside your BMI, the calculator shows the weight range that would give a healthy BMI at your height, and BMI Prime, your BMI divided by 25. A BMI Prime above 1.0 means your BMI is above the upper limit of the healthy range.
Formula
The imperial version gives the same result. The factor 703 converts pounds and inches into kilograms and metres. This calculator converts imperial inputs to metric first (1 lb = 0.45359237 kg, 1 in = 2.54 cm).
Example
An adult 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 healthy range. At that height a healthy BMI corresponds to roughly 56.7 kg to 76.3 kg. In imperial units, 154 lb at 5 ft 9 in (69 in) gives 703 × 154 ÷ 69² ≈ 22.7, almost the same figure.
Frequently asked questions
Is BMI accurate for athletes?
Not very. BMI cannot distinguish muscle from fat, so people with a lot of muscle can be classed as overweight while being lean. Waist measurement or a body fat estimate gives extra context.
Does BMI differ for men and women?
The adult BMI formula and categories are the same for men and women. Women tend to have more body fat than men at the same BMI, which is one of the measure's limitations.
Can children use this BMI calculator?
No. For children and teenagers, BMI has to be compared against growth charts for age and sex (BMI-for-age percentiles). Ask a paediatrician or use a child-specific tool.
What waist size is a health concern?
Common guidance flags a waist over 94 cm (37 in) for men and 80 cm (31.5 in) for women as increased risk, with higher thresholds for substantially increased risk. A waist-to-height ratio below 0.5 is a simple target.
Disclaimer: This calculator provides general estimates for education only and is not medical advice, diagnosis or treatment. Formulas are based on population averages and may not fit your body, age, pregnancy or health conditions. Talk to a doctor, midwife or registered dietitian before making decisions about your health.