What BMI is
Body mass index is weight in kilograms divided by height in metres squared. The World Health Organization uses it to sort adults into classes: under 18.5 is underweight, 18.5–24.9 a healthy weight, 25–29.9 overweight, and 30 and over obesity, in three classes from 30, 35 and 40.
It is popular because it needs only a scale and a tape measure. Across large groups of people, a higher BMI goes with a higher risk of type 2 diabetes, heart disease and some cancers.
What BMI cannot tell you
It does not see what the weight is made of. A muscular person can be "overweight" while lean, and an older person with little muscle can be in the healthy range and still carry a lot of fat. Where fat sits matters too: fat around the waist is the riskier kind. A waist under half your height is a good simple check.
For people of Asian descent risk starts at a lower BMI, so many countries use 23 and 27.5 as the cut-offs instead. BMI for children and teenagers is read on growth charts for their age and sex, not with these classes.
If you want to change your weight
Small, steady changes last: about 0.25–0.5 kg a week. Losing even 5–10% of your weight improves blood pressure, blood sugar and cholesterol for people with a high BMI. What you eat every day matters more than any number on a chart.
Questions
- What is a healthy BMI?
- For adults, 18.5 to 24.9 according to the WHO. Many Asian countries use 18.5 to 22.9, because risk rises at a lower BMI there.
- Is BMI accurate?
- It is a quick screening number, accurate for groups of people but not for every individual: it cannot tell muscle from fat. Waist size, body fat and health checks give a fuller picture.
- Is BMI different for women and men?
- The sum and the classes are the same for adult women and men. Women usually carry more fat at the same BMI, which is one of the reasons BMI is only a rough guide.
- Can I use this for my child?
- No. Children's BMI is compared with growth charts for their exact age and sex. Ask your paediatrician.