These measurements are screening tools, not a verdict on your health. A doctor looks at them together with blood pressure, blood tests and how you feel and function in daily life.
What is BMI and how do you calculate it?
BMI is a simple number that compares your weight with your height. You calculate it by dividing your weight in kilograms by your height in metres, multiplied by itself.
An example: a person who weighs 85 kg and is 1.70 m tall has a BMI of 85 ÷ (1.70 × 1.70) = 85 ÷ 2.89 = about 29.4. If you measure in pounds and inches, multiply your weight in pounds by 703 and divide by your height in inches squared.
BMI is quick, cheap and the same everywhere, which is why the World Health Organization (WHO) uses it to compare countries and track trends over time. For one person, it is a starting point for a conversation, not a diagnosis on its own.
What are the BMI categories for adults?
For adults, the WHO counts a BMI of 25 or more as overweight and 30 or more as obesity. Obesity is often split into three classes, because health risks tend to rise as BMI goes up.
| BMI (kg/m²) | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class I |
| 35 to 39.9 | Obesity class II |
| 40 or higher | Obesity class III (sometimes called severe obesity) |
These categories are for adults only. Children and teenagers are assessed with growth charts that take age and sex into account; read more on our page about obesity in children. BMI is also not used to judge weight during pregnancy.
Are BMI cut-offs different for different ethnic groups?
Yes. For people from some ethnic backgrounds, health risks such as type 2 diabetes start at a lower BMI, so lower cut-off points are often used.
A WHO expert consultation on Asian populations kept the international BMI categories but identified extra action points at 23, 27.5, 32.5 and 37.5. Countries were advised to decide which points to use for their own populations.
In the UK, NICE recommends lower thresholds for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background:
| Category | Standard threshold | Lower threshold |
|---|---|---|
| Overweight | 25 to 29.9 | 23 to 27.4 |
| Obesity | 30 or more | 27.5 or more |
| Obesity classes II and III | 35 and 40 | Usually 2.5 points lower |
Ethnicity is only one factor. Family history, blood sugar and where fat is stored also matter, so ask your doctor which thresholds apply to you.
Why does waist size matter?
Waist size matters because fat around the belly, especially deep around the organs, is more closely linked to type 2 diabetes and heart disease than fat elsewhere. Two people with the same BMI can have very different risks.
Commonly used waist thresholds for adults of European background were first proposed in the BMJ and are now used around the world:
| Risk | Men | Women |
|---|---|---|
| Increased | More than 94 cm | More than 80 cm |
| Substantially increased | More than 102 cm | More than 88 cm |
Lower waist thresholds are often used for people of Asian background. Another measure is the waist-to-hip ratio, which also shows how much fat is stored around the belly.
What is the waist-to-height ratio?
The waist-to-height ratio is your waist size divided by your height, and a result of 0.5 or more means increased health risk. The simple rule is: keep your waist to less than half your height. NICE says the same cut-offs can be used for adults of both sexes and all ethnicities.
| Ratio | Meaning |
|---|---|
| 0.4 to 0.49 | Healthy central fat, no increased risk |
| 0.5 to 0.59 | Increased central fat, increased risk |
| 0.6 or more | High central fat, further increased risk |
NICE uses this ratio for adults with a BMI under 35, and for children and young people aged 5 and over.
How to measure your waist
- Find the bottom of your ribs and the top of your hip bones.
- Wrap a tape measure around your waist midway between these points.
- Stand straight and breathe out naturally before you read the tape.
- Use the same unit (centimetres or inches) for waist and height, then divide waist by height.
What does BMI not tell you?
BMI does not measure body fat, where fat is stored, or how well your body is working. That leads to some known gaps:
- Muscle: people with a lot of muscle can have a high BMI without excess fat.
- Older adults: from about age 65, muscle is often lost and BMI should be read with care.
- Fat distribution: people with a BMI in the healthy range can still carry risky belly fat.
- Body shape and height: BMI is less reliable for people with conditions that affect height.
Direct measurements of body fat, such as a DEXA scan or bioelectrical impedance, give more detail but are not always available.
Because of these limits, the Lancet Diabetes & Endocrinology Commission on clinical obesity advised that BMI alone should not be used to diagnose obesity in an individual. It recommends confirming excess fat with at least one waist measurement as well as BMI, with two body measurements, or with a direct body-fat measurement.
Above a BMI of 40, excess fat can be assumed. Read more about this on our page on clinical and preclinical obesity.
This choice of measure makes a real difference. A recent study of US national survey data found that about 40 percent of adults had obesity based on BMI alone, but about 75 percent when a waist-to-height ratio above 0.5 was added. The authors noted that this cut-off may overestimate excess fat in some groups.
What should I do with my numbers?
Use your numbers as a starting point for a conversation with your doctor, not as a judgement about you. A good assessment also looks at blood pressure, blood sugar, cholesterol, sleep, mood, mobility and family history, which together show your personal health risks.
If you want to prepare, our doctor visit checklist can help. You have the right to be measured respectfully, and you can ask for your results to be written down rather than said aloud. If you have had a bad experience, our page on weight stigma in healthcare explains your options.
Frequently asked questions
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. For example, 85 kg at 1.70 m gives 85 ÷ 2.89, which is about 29.4. In pounds and inches, multiply your weight by 703 and divide by your height in inches squared.
What are obesity classes I, II and III?
They are BMI ranges for adults that show increasing levels of obesity: class I is 30 to 34.9, class II is 35 to 39.9 and class III is 40 or higher. Health risks generally rise from one class to the next. For some ethnic groups, lower thresholds are often used.
Is waist-to-height ratio better than BMI?
It adds information that BMI misses, because it shows how much fat is stored around the belly. A ratio of 0.5 or more is linked to increased health risk. Most guidelines suggest using it together with BMI rather than instead of it.
Why do people of Asian background have a lower BMI cut-off?
Research shows that, on average, people of Asian background develop conditions such as type 2 diabetes at a lower BMI. A WHO expert consultation therefore identified extra action points such as 23 and 27.5, and UK guidance uses 27.5 as the obesity threshold for several ethnic groups.
Sources
- NHLBI (NIH) – Classification of overweight and obesity by BMI, waist circumference and associated disease risks
- NICE – Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity
- WHO Expert Consultation – Appropriate body-mass index for Asian populations, The Lancet (2004)
- Lean MEJ, Han TS, Morrison CE – Waist circumference as a measure for indicating need for weight management, BMJ (1995)
- King's College London – Commission proposes major overhaul of obesity diagnosis (2025)
- Healio – About 75% of US adults considered to have obesity under new Lancet Commission criteria (2026)