Adult BMI Categories Explained: From Underweight to Obesity Class 3
A BMI result is usually given with a label: underweight, healthy weight, overweight or obesity. Those labels come from cut-offs agreed by the World Health Organization and used, with small differences in wording, by the CDC in the United States and the NHS and NICE in England. This guide sets them out, explains the obesity classes and the lower thresholds for some ethnic groups, and suggests how to read a result calmly.
The standard adult categories
BMI is weight in kilograms divided by height in metres squared, usually shown to one decimal place. The CDC lists the adult categories as follows, and the NHS and NICE use the same numbers:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity, class 1 |
| 35 to 39.9 | Obesity, class 2 |
| 40 or more | Obesity, class 3 (also called severe obesity) |
The CDC writes the boundaries as "18.5 to less than 25" and so on; since results are rounded to one decimal place, that is the same as 18.5 to 24.9. Our calculator classifies the rounded figure, so the label always matches the number on screen.
The WHO's own definition for adults is short: overweight is a BMI greater than or equal to 25, and obesity a BMI greater than or equal to 30. The finer classes are used in clinical practice to describe the degree of obesity and to decide which kinds of support may be offered.
Lower thresholds for some ethnic groups
Research has shown for years that the link between BMI and health risk is not the same in every population. NICE's guideline on overweight and obesity management (NG246) states that people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background are prone to central adiposity, meaning fat stored around the abdomen, and that their cardiometabolic risk occurs at a lower BMI. It therefore recommends these thresholds:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 22.9 | Healthy weight |
| 23 to 27.4 | Overweight |
| 27.5 or more | Obesity (classes 2 and 3 usually from 32.5 and 37.5) |
The NHS uses the same lower figures on its overweight and obesity page, and its online BMI calculator asks about ethnic background for this reason. The WHO also discussed lower "public health action points" for Asian populations in an expert consultation published in 2004, which suggested 23 and 27.5 as additional cut-offs for countries to consider.
Our calculator offers these thresholds as an option you can tick. Ethnic background is a rough guide here, not a precise rule; NICE itself lists research questions about immigrants of different generations and single cut-off points. If this applies to you, a waist-to-height ratio is a useful extra check, because NICE says it can be used for adults of all ethnicities.
Reading your result
Healthy weight
A BMI between 18.5 and 24.9 (or 22.9 with the lower thresholds) is reassuring but not a clean bill of health. NICE reminds clinicians that someone in the healthy range may still carry excess fat around the middle, so a waist measurement is worth adding.
Underweight
A BMI under 18.5 can reflect a naturally slim build, but it can also be linked to not eating enough, illness, or a problem absorbing nutrients. If you have lost weight without trying, feel unwell or tired, or are worried about eating, speak to a GP or another health professional.
Overweight and obesity
A higher BMI is associated with a greater chance of conditions such as type 2 diabetes, high blood pressure, heart disease, some cancers and joint problems. The NHS notes that losing even 5 to 10% of body weight can improve health for people living with obesity. But the number alone does not tell a professional what, if anything, should be done. They will look at your waist, blood pressure, blood tests, other conditions and what matters to you. NICE asks health professionals to seek permission before discussing weight and to do so in a non-judgemental way.
One reading, or a trend?
Categories have fixed edges, but bodies do not. A BMI of 24.9 and one of 25.1 describe almost the same person, even though the labels differ. Weight also moves from day to day with food, fluids and clothing, and home scales and tape measures vary. That is why clinicians tend to look at a trend over months rather than a single figure, and at other measures alongside it. If you track your own BMI, measure in the same way each time, at the same time of day, and treat small changes across a boundary as noise rather than news. A steady change over several months, up or down, is more informative than any single number, and is worth discussing with a professional if it was not expected.
Who the categories do not fit
- Children and teenagers: their BMI is compared with others of the same age and sex. See BMI for children and teens.
- People with a lot of muscle: NICE advises interpreting BMI with caution.
- Adults aged 65 and over: NICE advises caution and mentions the possible protective effect of a slightly higher BMI when older.
- Pregnancy: adult BMI tools are not designed for it.
Where to go next
If you want to see the weights behind these numbers for your height, use the healthy weight calculator or the table of ranges by height. To add a measure of central fat, see waist-to-height ratio. And for the broader picture of what BMI can and cannot show, read what is a healthy weight?
Sources
- Centers for Disease Control and Prevention, "Adult BMI Categories".
- NICE guideline NG246, "Overweight and obesity management", recommendations 1.9.10 to 1.9.14 (nice.org.uk).
- NHS, "Overweight and obesity in adults": lower BMI figures, 5 to 10% weight loss, when to see a GP.
- World Health Organization, "Obesity and overweight" fact sheet (who.int).
- WHO Expert Consultation, "Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies", The Lancet, 2004.
Healthy weight & BMIWhat is a healthy weight?
Why health services use BMI to describe a healthy weight, what it misses, which other measures help and what affects weight beyond food and exercise.
Healthy weight & BMIWaist-to-height ratio
Keep your waist to less than half your height: how to measure, how to calculate the ratio and what the NICE bands mean.
Healthy weight & BMIHealthy weight ranges by height
A table of healthy weight ranges for adults from 4 ft 10 in to 6 ft 6 in and 150 to 195 cm, with how the range is worked out and when it does not apply.