Limitations of BMI
BMI compares weight with height. It doesn’t measure body fat directly, can’t tell muscle from fat and doesn’t show where fat is stored, so it works best as a first screening step, read alongside other measures.
What BMI measures
BMI is your weight divided by your height squared. Across large groups of people, a higher BMI goes with more body fat and higher health risk, which is why it’s used for screening. For one person, though, the same BMI can describe very different bodies.
Muscle and athletes
Muscle is dense. Athletes and people who do a lot of strength training can have a BMI in the overweight range while carrying little fat.
Older adults
People tend to lose muscle and bone as they age. An older adult can have more body fat than a younger person with the same BMI, so BMI can underestimate body fat later in life.
Women and men
The categories are the same for both, but at the same BMI women tend to have more body fat than men.
Ethnic background
Health risks such as type 2 diabetes start at a lower BMI in some groups. For them, the WHO and the UK’s National Institute for Health and Care Excellence (NICE) recommend lower cut-offs: overweight from 23 and obesity from 27.5. NICE applies them to people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background. Asian BMI calculator →
Pregnancy
BMI isn’t meaningful during pregnancy. Pregnancy weight guidance is based on your BMI before pregnancy.
Children and teens
From 2 to 19, BMI is compared with children of the same age and sex and read as a percentile, because body fat changes as children grow. Kids & Teens BMI Calculator →
Where fat is stored
Fat around the middle, inside the abdomen, is more closely linked to type 2 diabetes and heart disease than fat elsewhere. BMI can’t see it; a waist measurement can. NICE advises keeping your waist to less than half your height, and US guidance links higher risk to a waist above 35 inches (88 cm) for women and 40 inches (102 cm) for men. Waist-to-height ratio calculator →
What experts recommend now
NICE recommends using waist-to-height ratio alongside BMI for adults with a BMI under 35. In 2025, a Lancet Diabetes & Endocrinology commission went further: it advised against diagnosing obesity from BMI alone. Instead, it recommended confirming excess body fat with another measure, such as waist size, or by measuring body fat directly. The exception is a very high BMI (above 40), where excess fat can reasonably be assumed.
BMI is still a useful first step. If your result worries you, a doctor can look at it together with your waist, blood pressure, blood tests and history.