Free BMI Calculator Online

How to calculate BMI

BMI is calculated by dividing your weight in kilograms by the square of your height in metres. The formula is:

BMI = weight (kg) / height (m)²

For example, a person weighing 70 kg at 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 22.9. In imperial units, the equivalent formula is BMI = (weight in lbs × 703) / height in inches². The calculation is identical for men and women. This tool handles the arithmetic instantly so you do not need to calculate your BMI manually.

How to use the BMI calculator online

1. Select your unit system: metric (kg, cm) or imperial (lbs, ft, in).
2. Enter your height and weight in the input fields provided.
3. Your BMI calculates instantly and the gauge updates to show your category.
4. Read your BMI classification and healthy weight range in the results panel.
5. Use the WHO reference table to compare your result against all BMI categories.

BMI categories and what they mean

The World Health Organization defines four standard BMI categories for adults. A BMI below 18.5 is classified as underweight and may indicate inadequate calorie intake or an underlying condition. A BMI between 18.5 and 24.9 is the normal weight range, associated with the lowest risk of weight-related chronic disease in large population studies. A BMI between 25.0 and 29.9 falls in the overweight range, associated with moderately elevated risk of type 2 diabetes, cardiovascular disease, and hypertension. A BMI of 30 or above is classified as obese, with substantially increased risk across multiple conditions.

These thresholds are the same for all adults regardless of sex. To understand your ideal target weight, pair this result with the ideal weight calculator, which provides healthy weight ranges from four clinical formulas specific to your height and sex.

BMI calculator for men vs women

The BMI formula and WHO category thresholds are identical for men and women, but the result has a different meaning by sex. Women naturally carry a higher proportion of body fat than men at the same BMI because of physiological differences in muscle-to-fat ratio. A BMI of 23 in a female typically corresponds to a higher body fat percentage than a BMI of 23 in a male, even though both fall in the normal weight category. This is why BMI alone is not a complete picture of body composition for either sex.

For a sex-specific body composition estimate, use the body fat percentage calculator, which applies the US Navy circumference method and accounts for biological sex in its formula. This gives a more accurate picture of fat mass versus lean mass than the BMI index calculator alone.

BMI by age: adults, children, and teenagers

For adults aged 20 and over, BMI categories are fixed at the WHO thresholds (18.5, 25, 30) regardless of age. Some research suggests that adults over 65 may have better health outcomes at a slightly higher BMI (22 to 27) than younger adults, but clinical guidelines have not formally revised the thresholds for older adults.

For children and teenagers (ages 2 to 20), BMI is calculated with the same formula but interpreted using age- and sex-specific growth percentiles rather than fixed cut-off points. A child BMI calculator compares the score against a reference population of the same age and sex. A result at or above the 95th percentile is classified as obese; 85th to 94th percentile is overweight; 5th to 84th percentile is healthy weight; and below the 5th percentile is underweight. This percentile approach is used because children's body fatness changes naturally as they grow.

Asian BMI thresholds and NHS guidelines

Standard BMI thresholds were developed largely from data on European populations. Research has since shown that people of Asian descent accumulate visceral fat and develop metabolic risk at lower BMI values. The WHO and NHS both recognise alternative action points for Asian adults: overweight is typically defined from a BMI of 23.0 rather than 25.0, and obese from 27.5 rather than 30.0. If you are of South Asian, East Asian, or Southeast Asian descent, your healthcare provider may use these adjusted thresholds when interpreting your BMI index result.

NHS BMI guidelines for the general UK population follow the standard WHO thresholds for adults, with a healthy range of 18.5 to 24.9. The NHS also advises that for adults from Black, Asian, and minority ethnic backgrounds a healthy BMI is 18.5 to 22.9, with overweight defined from 23.0. This makes accurate body mass index BMI calculation particularly important for people in these groups who may face elevated risk at BMI levels that fall within the standard normal range.

BMI limitations and when to use other tools

BMI is a useful population-level screening tool, but it has well-documented limitations when applied to individuals. It cannot distinguish fat mass from lean mass. A highly muscular person with very low body fat may score in the overweight range. Conversely, someone with a BMI in the normal range can still carry dangerous levels of visceral fat, sometimes called normal-weight obesity or skinny fat, which carries similar metabolic risks to a higher BMI.

For a complete picture of your health, combine your BMI result with the body fat percentage calculator and the TDEE calculator to understand both your body composition and daily energy needs. If your BMI suggests action is needed, the calorie calculator can help you set a realistic daily calorie target to reach a healthier weight. All four tools work together to give you an accurate and actionable health snapshot.