WHO BMI classification for adults

BMI (kg/m²) Category Risk of related illness What usually helps
Below 18.5 Underweight Raised (low reserves, weak bones, infections) Check for an underlying cause; a dietitian can help with healthy weight gain
18.5 to 24.9 Healthy weight Lowest Keep active and eat a balanced diet
25.0 to 29.9 Overweight (pre-obesity) Increased Diet, activity, sleep and stress changes; check blood pressure and sugar
30.0 to 34.9 Obesity class I Moderate A structured weight programme, sometimes with medicines
35.0 to 39.9 Obesity class II Severe Specialist weight management; surgery may be discussed, especially with diabetes or other conditions
40.0 and above Obesity class III Very severe Specialist care; weight loss surgery is often an option

How BMI is worked out

BMI is your weight in kilograms divided by your height in metres squared: BMI = weight (kg) ÷ height (m)². Someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

When BMI is useful, and when it is not

BMI is a quick screening number for adults. It is good at spotting weight-related risk across large groups of people, and it is a sensible starting point for a conversation with your doctor.

  • It does not measure body fat directly. Muscular people can have a high BMI without extra fat, and older adults can have a "normal" BMI while carrying too little muscle.
  • Where fat sits matters. Fat around the waist carries more risk. A waist measurement of more than half your height is a warning sign at any BMI.
  • Pregnancy: BMI is not used to judge weight during pregnancy. Use your pre-pregnancy BMI with your midwife or doctor.
  • Children and teenagers need BMI-for-age charts, because healthy body shape changes as they grow.

Is BMI different for men and women?

The adult formula and categories are the same for everyone. At the same BMI, women usually carry more body fat than men, which is one reason a waist measurement and a clinical check add useful detail.

Frequently asked questions

What is a healthy BMI?

For most adults, 18.5 to 24.9. For many people of Asian background, risk starts to rise from about 23.

How accurate is BMI?

It is a good screening tool but not a diagnosis. Your waist size, blood pressure, blood sugar, cholesterol, fitness and family history give a fuller picture.

My BMI is high. What should I do first?

Book a check with a health worker, who can measure your blood pressure and blood sugar and help you plan realistic changes. Losing even 5 to 10 percent of your weight brings real health benefits.

My BMI is low. Should I worry?

If you have lost weight without trying, feel tired, or your periods have stopped, see a doctor. Unplanned weight loss can be a sign of an illness that needs checking.

For students & clinicians (technical detail)
  • Categories follow the WHO adult classification. The WHO Expert Consultation (2004) suggested public health action points of 23 and 27.5 kg/m² for many Asian populations; national guidance varies.
  • Healthy weight range shown = 18.5 × height² to 24.9 × height² (or 18.5 to 22.9 with the Asian option).
  • Imperial conversion: 1 in = 2.54 cm, 1 lb = 0.45359237 kg.
  • Bariatric surgery is commonly considered at BMI ≥ 40, or ≥ 35 with obesity-related disease; many guidelines now include BMI 30 to 34.9 with poorly controlled type 2 diabetes, and lower thresholds for Asian patients.