BMI Explained: How It's Calculated, What It Tells You and Its Limits

By the SAA Tool editor, an IT support engineer · Last reviewed 1 October 2026 · 7 min read

Body mass index (BMI) turns your height and weight into a single number that health services around the world use as a quick first check of weight. It takes seconds to work out and it is genuinely useful, but it can't see what your weight is made of, so it's easy to read too much into it. This guide shows the formula with worked examples, the WHO ranges for adults, the lower cut-offs some countries use for certain ethnic backgrounds, why children are measured differently, and where BMI falls short.

This is general information, not medical advice. Health guidance differs between countries and is updated over time, so check what your own health service recommends and talk to a doctor or nurse about your result.

How BMI is calculated

The World Health Organization (WHO) defines BMI as your weight in kilograms divided by your height in metres, squared:

BMI = weight (kg) ÷ height (m)²

Example: someone 175 cm tall who weighs 72 kg. Height in metres is 1.75, and 1.75 × 1.75 = 3.0625. Then 72 ÷ 3.0625 = 23.5.

If you work in pounds and inches, multiply by 703 so the units match:

BMI = 703 × weight (lb) ÷ height (in)²

Example: 5 ft 9 in is 69 inches, and 69 × 69 = 4,761. At 160 lb, 703 × 160 = 112,480, and 112,480 ÷ 4,761 = 23.6. The 703 is just a conversion factor: one pound per square inch is about 703 kg per square metre.

Because height is squared, small measuring errors matter. The same 72 kg person entered as 170 cm instead of 175 cm gets 24.9 instead of 23.5, right at the top of the healthy range. Measure your height without shoes, standing straight against a wall.

The WHO ranges for adults

WHO counts a BMI of 25 or more as overweight and 30 or more as obesity. Its full adult classification is below, next to the labels our calculator shows.

You can turn the healthy range round to find a healthy weight for your height: multiply your height in metres squared by 18.5 and by 24.9. At 1.75 m that's 3.0625 × 18.5 = 56.7 kg up to 3.0625 × 24.9 = 76.3 kg.

These ranges are for adults, and the starting age differs a little between countries. The NHS adult calculator in the UK is for people over 18, while the US Centers for Disease Control and Prevention (CDC) uses its adult BMI categories from age 20.

Lower cut-offs for some ethnic backgrounds

The same BMI doesn't carry the same health risk for everyone. A WHO expert consultation, published in The Lancet in 2004, concluded that the proportion of Asian people with a high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs below 25. The point where risk starts to rise varied from 22 to 25 across different Asian populations. The experts kept the WHO cut-offs as the international classification, but identified extra action points at 23, 27.5, 32.5 and 37.5 for countries to consider.

Some health services now use lower thresholds. The NHS says that for people from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background:

• a BMI of 23 to 27.4 is overweight;
• a BMI of 27.5 or higher is obese.

Example: someone 165 cm tall who weighs 76 kg has a BMI of 76 ÷ 2.7225 = 27.9. On the WHO scale that's overweight; with the lower thresholds it falls in the obese range. Other countries set their own national cut-offs, so if this applies to you, check which ones your health service uses.

What BMI can't tell you

BMI is a screening measure. The CDC says it should not be used as a diagnostic tool, and there are several reasons a single number can mislead.

• It can't tell muscle from fat. BMI doesn't distinguish fat, muscle and bone. A 183 cm rugby player who weighs 95 kg has a BMI of 28.4, which reads as overweight even if they carry little body fat.
• It ignores where fat sits. BMI can't show where in the body fat is carried. WHO notes that waist circumference can help in diagnosing obesity, and the NHS suggests keeping your waist to less than half your height: under 87.5 cm if you're 175 cm tall.
• Age and sex change the picture. According to the CDC, older adults tend to have more body fat than younger adults at the same BMI, and women tend to have more body fat than men.
• It isn't for pregnancy. The NHS says not to use its adult BMI calculator if you're pregnant.
• It's one piece of information. The CDC says a person's BMI should be considered alongside their medical history, habits, a physical examination and lab tests. That's what a doctor does with it.

Children and teenagers use percentiles

Children's bodies change as they grow, and boys and girls grow differently, so a child's BMI is compared with other children of the same age and sex rather than with fixed adult cut-offs. The result is a percentile, sometimes called a centile. A child on the 85th percentile has a higher BMI than 85% of children of the same age and sex in the reference data.

Countries use different reference charts and cut-offs:

• CDC (United States, ages 2 to 19): underweight below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, and obesity at the 95th percentile or above.
• WHO (ages 5 to 19): overweight means a BMI-for-age more than 1 standard deviation above the median of the WHO Growth Reference, and obesity more than 2 standard deviations above it.
• NHS (UK, ages 2 to 17): a separate children's calculator that uses the same data as the UK growth charts from the Royal College of Paediatrics and Child Health.

So never judge a child's BMI against an adult chart, including ours, which is for adults 18 and over. Use your country's children's calculator, or ask a doctor or school nurse.

Working out your BMI with our calculator

The BMI Calculator runs in your browser, and the figures you type aren't sent anywhere.

1. Choose Units: Metric (cm, kg) or US / imperial (ft, lb).
2. Enter Height (cm) and Weight (kg), or your height in feet and inches and your Weight (lb). There's no button to press: the result updates as you type.
3. Read your BMI, its category and the marker on the scale bar, which runs from 15 to 40.
4. Check the row Healthy weight for your height, and the row below it, which shows roughly how much you'd need to gain or lose to reach a healthy BMI.
5. For a daily calorie estimate, open the Calorie Calculator, enter your Age, Sex, height, weight and Activity, and read the calories to stay the same weight, lose 0.25 to 1 kg a week, or gain up to 0.5 kg a week.

Know the limits. The BMI Calculator uses the WHO adult ranges only. It doesn't ask about your background, so compare your number with the lower cut-offs yourself if they apply to you, and it doesn't do children's percentiles. The Calorie Calculator uses the Mifflin-St Jeor formula, so it gives an estimate rather than a measurement, and it flags any target below 1,200 calories a day for women or 1,500 for men as not advised without a doctor. If you need to switch units first, the kg to lbs and cm to inches converters help.

Quick answers

Is a BMI of exactly 25 overweight?

Yes, on the WHO scale. The healthy range ends at 24.9 and the overweight range starts at 25.0.

Why does my BMI say overweight when I'm fit?

If you carry a lot of muscle, BMI can put you in the overweight or even obese range with little body fat. A waist measurement gives a useful second opinion.

Checklist before you trust the number

• Measure your height without shoes and weigh yourself in light clothing.
• Check you picked the right units.
• Look at your waist size as well as your BMI.
• For anyone under 18, use a children's percentile calculator.
• Treat the result as a prompt for a conversation with a health professional, not a diagnosis.

Spotted a mistake or something out of date? Tell us and we'll fix it.