BMI calculator:
work out your body mass index

What is BMI and what does it measure?

Body mass index (BMI) is a single number that relates your weight to your height. Doctors and public-health researchers use it as a quick screen for weight-related health risk, much as a thermometer reading flags a possible fever.

The idea dates back to the nineteenth-century Belgian statistician Adolphe Quetelet, who noticed that weight in adults tends to rise roughly with the square of height. That is why height is squared in the formula rather than used as it is.

It is a screening tool, not a diagnosis. BMI does not measure body fat directly, and it says nothing about fitness, diet or medical history.

The adult categories in this guide apply from age 20. Children and teenagers are assessed against BMI-for-age percentiles, which depend on age and sex, so an adult chart can mislead for them.

BMI formula in kg and cm, and in pounds and inches

Divide your height in centimetres by 100 to get metres, square it, then divide your weight by that square.

If you know your height in feet and inches, convert to total inches first (5 ft 8 in is 5 × 12 + 8 = 68 in), then multiply by 0.0254 to get metres. That gives 1.727 m, so the imperial and metric routes meet in the same place.

Both forms give the same answer. A person weighing 160 lb at 5 ft 8 in (68 in) gets 703 × 160 ÷ 68² = 24.3, the same as the metric calculation to one decimal place.

How to calculate BMI: a worked example

1. 1Take your weight in kilograms: 72 kg.
2. 2Convert your height to metres: 170 cm = 1.70 m.
3. 3Square the height: 1.70 × 1.70 = 2.89.
4. 4Divide weight by that figure: 72 ÷ 2.89.

That figure falls in the healthy range, but only just. The next section shows how to read it against the standard adult categories.

Round to one decimal place, which is how categories are normally reported. Rounding to a whole number can push a result across a boundary that it does not really cross: 24.9 would become 25.

Finding the weight that matches a BMI

You can also run the formula backwards. If you know your height and have a BMI in mind, multiply that BMI by your height in metres squared.

For someone 160 cm tall, a BMI of 22 corresponds to 22 × 1.60² = 56.3 kg. Trying the lower and upper edges of the healthy band, 18.5 and 24.9, gives 47.4 kg and 63.7 kg.

That range is wide on purpose. Frame size, muscle mass and age all legitimately move where a healthy weight sits inside it.

BMI chart for adults: what each range means

These cut-offs are broad bands, not cliffs. Moving from 24.9 to 25.0 does not change your health overnight, and a reading near a boundary deserves a calmer interpretation than one far inside a band.

At 170 cm, the healthy band corresponds to roughly 53.5 kg to 72.0 kg, using 18.5 and 24.9 as the limits. A 0.5 kg change in weight moves BMI by only about 0.2, which is why daily fluctuations should not worry you.

What BMI cannot tell you

• Whether your weight comes from muscle, fat, bone or water.
• Where fat is stored. Fat around the waist carries different risks from fat on the hips.
• Your blood pressure, blood sugar, cholesterol or fitness.
• Whether a weight is appropriate during pregnancy, in childhood or in older age.

Two people with the same BMI can have very different body composition. A trained athlete and an inactive adult may share a number but not a risk profile.

For people of South Asian origin, metabolic risk can appear at lower BMI values than the standard international cut-offs. Several Indian medical guidelines use lower thresholds, with overweight starting from 23 and obesity from 25, so ask your doctor which scale applies to you.

Age matters too. Muscle tends to decline and fat tends to increase with age even when weight stays steady, so the same BMI can mean a different body composition at 60 than at 25. Pregnancy changes weight for good reason, and standard categories are not used to judge it.

Waist measurement adds the missing context. The waist-to-height ratio calculator on this page compares your waist with your height, and a ratio below 0.5 is a commonly used benchmark.

BMI mistakes to avoid and how to use the result

• Using centimetres instead of metres. 72 ÷ 170² gives 0.0025, which is clearly wrong.
• Forgetting to square the height, which turns BMI into a much larger number.
• Mixing units, for example pounds with centimetres, without converting first.
• Rounding early. Keep two or three decimals until the end, then round once.
• Applying adult categories to children or teenagers.

Another trap is chasing the decimal. Trying to move from 25.1 to 24.9 is not a health goal in itself; sustained changes in food, movement and sleep matter more than the third digit.

BMI also pairs well with other measures. The body fat percentage and lean body mass guides on this site look at composition, which BMI cannot do.

Treat BMI as a starting point for a conversation, not as a goal to chase. Track it alongside waist size, energy levels and routine health checks.

Recalculate every few months rather than every few days, using the same scale and similar clothing, so that the trend means something.

If your reading is outside the healthy band, or you are unsure how it applies to your age, build, or a medical condition, speak to a doctor or registered dietitian. They can interpret it with the full picture.