BMI is your weight in kg divided by your height in metres squared; 18.5 to 24.9 is the healthy range for adults. It is a quick screen, but it cannot tell muscle from fat, so check your waist and body fat as well.
Body mass index, or BMI, is probably the most used health number in the world. Doctors use it, insurers use it, and most fitness apps show it on day one. It is quick and it needs only two things you already know. But it is also easy to misread.
How BMI is calculated
BMI is your weight in kilograms divided by your height in metres, squared.
For someone who weighs 82 kg and is 1.78 m tall: 1.78 × 1.78 = 3.17, and 82 ÷ 3.17 = a BMI of about 25.9.
The ranges
The World Health Organization uses these bands for adults:
- Below 18.5: underweight
- 18.5 to 24.9: healthy range
- 25 to 29.9: overweight
- 30 and above: obesity
Some health bodies use lower cut-offs for people of Asian background, because health risks start to rise at a lower BMI.
What BMI does well
Across large groups of people, a higher BMI goes along with a higher risk of type 2 diabetes, high blood pressure and heart disease. As a first, free screening tool, it does its job: if your BMI is 33 and you rarely exercise, it is a fair signal to take a closer look.
Where BMI gets it wrong
BMI only knows your total weight. It cannot see what that weight is made of.
- Muscular people often land in the overweight range. A man who is 1.78 m, 88 kg and lifts weights may have a BMI of 27.8 and very little body fat.
- People with little muscle can sit in the healthy range and still carry a lot of fat around the belly, which is the kind linked most closely to health problems.
- Older adults lose muscle with age, so the same BMI can hide more fat.
- Where the fat sits matters as much as how much there is. BMI does not know.
BMI tells you how heavy you are for your height. It does not tell you how healthy you are.
What to look at alongside it
- Waist size. Measure around your belly at the level of your belly button, relaxed. For many adults, a waist over about half your height is a sign worth acting on.
- Body fat percentage. A tape-measure method like the US Navy formula gives a much better picture than BMI alone. Our body fat calculator does it in a minute, and our guide to body fat percentage explains the ranges.
- FFMI (fat-free mass index). It works like BMI but uses only your lean mass, so it shows whether your weight is mostly muscle. The BMI calculator on this page shows it next to your BMI.
- How you feel and perform. Energy, sleep, blood pressure and blood tests from your doctor all say more than one ratio.
What to do with your number
If you are in the healthy range and active, keep doing what you do. If you are above it, check your waist and body fat before you decide anything. If those are high too, a small calorie deficit, more protein, strength training and more daily steps are the proven way down. And if your BMI is below 18.5, or very high, talk to a doctor first.
Use the free calculators on this site for BMI, FFMI and body fat, and the Shulah app to track your weight over time, with your goal broken into small milestones.
Sources
- World Health Organization, obesity and overweight: who.int
- BMI cut-offs for Asian populations: WHO expert consultation, 2004
- Waist-to-height ratio compared with BMI: Ashwell et al., 2012