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Health & Fitness

BMI Calculator

Work out your body mass index from your weight and height, see the WHO weight category it falls into and the healthy weight range for your height.

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For adults aged 18 and over. The result is an estimate, not medical advice.

How to use the BMI calculator

Choose US or Metric units, enter your weight and height and press Calculate. In US units you can type your height as feet and inches; leave the inches box empty if your height is a whole number of feet. The result shows your BMI to one decimal place, the WHO category for adults and the weight range that corresponds to a healthy BMI at your height.

The formula

Body mass index is weight divided by height squared, using kilograms and meters:

BMI = weight (kg) / height (m)²

For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 70 / 3.0625 ≈ 22.9. In US units the calculator first converts pounds to kilograms (1 lb = 0.4536 kg) and inches to centimeters (1 in = 2.54 cm), so you get exactly the same answer.

WHO categories for adults

  • Underweight: below 18.5
  • Normal weight: 18.5 to 24.9
  • Overweight: 25.0 to 29.9
  • Obesity: 30.0 and above, split into class I (30–34.9), class II (35–39.9) and class III (40 and above)

The healthy weight range is found by turning the formula around: weight = BMI × height². For a height of 1.75 m, a BMI of 18.5 to 24.9 means roughly 56.7 kg to 76.3 kg (about 125 lb to 168 lb).

Limitations of BMI

BMI is a quick screening number, not a diagnosis. It only looks at weight and height, so it cannot tell muscle from fat or show where fat is stored.

  • Muscular people, such as many athletes, can have a high BMI without having excess body fat.
  • Older adults often lose muscle, so a normal BMI can hide a high body fat percentage.
  • The same BMI can carry different health risks in different ethnic groups, and some health bodies use lower cut-offs for people of Asian descent.
  • The adult categories do not apply to children and teens, who are assessed with age- and sex-specific growth charts, or to pregnant women.

For a fuller picture, BMI is often combined with waist measurement, body fat percentage and other health checks. Results from this calculator are estimates and are not medical advice; talk to a doctor or other qualified health professional about your weight and health.

Frequently asked questions

What is a healthy BMI for adults?

The World Health Organization treats a BMI from 18.5 to 24.9 as the normal range for adults aged 18 and over. Values below 18.5 are classed as underweight, 25 to 29.9 as overweight and 30 or more as obesity.

Is BMI accurate for athletes and very muscular people?

Not always. Muscle is denser than fat, so people with a lot of muscle can have a BMI in the overweight range while having a healthy amount of body fat. A body fat measurement or waist size gives more useful information in that case.

Can I use this calculator for children?

No. The categories used here are for adults. Children and teens are assessed with BMI-for-age percentiles that take their age and sex into account, so their results should be checked with a pediatric growth chart or a health professional.

How is the healthy weight range calculated?

The calculator multiplies the lower and upper limits of the normal BMI range, 18.5 and 24.9, by your height in meters squared. The two results are the lightest and heaviest weights that still give a BMI in the normal range at your height.