Guides · Updated November 2026
BMI and muscle mass: when BMI is not enough
BMI is the first number most people check, but it only measures the relationship between weight and height. Here is when it can mislead and what to add.
Calculate your BMI
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What BMI actually measures
BMI (body mass index) is your weight in kilograms divided by the square of your height in metres. For adults, WHO uses these categories:
| BMI | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 or more | Obesity |
It is useful because it is simple and allows large groups of people to be compared. Its limitation is that it does not know what the weight is made of.
When it can mislead
- High muscle mass: muscle is heavy. People who lift weights or play sport can have a BMI in the overweight range with little body fat. NICE advises interpreting BMI with caution in these cases.
- Age 65 and over: muscle is lost with age and fat distribution changes, so the same BMI can mean something different. NICE also advises caution here.
- Some ethnic backgrounds: for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE uses lower thresholds: overweight from 23 and obesity from 27.5.
- Pregnancy, children and teenagers: adult categories do not apply. For under-18s, growth charts by age and sex are used.
What to add to BMI
The simplest measurement to add is your waist circumference, used to calculate the waist-to-height ratio. NICE recommends it for adults with a BMI below 35 precisely because it shows where fat is stored, which BMI does not.
Other methods, such as body composition analysis with dedicated devices, require equipment and trained staff: if in doubt, talk to your doctor.
Frequently asked questions
Is BMI accurate for people who lift weights?
Not always. With a lot of muscle, BMI can be high even with little fat. In these cases the waist-to-height ratio gives more useful information.
Does BMI apply to older adults?
It is used, but it should be interpreted with caution from age 65, because muscle mass and fat distribution change with age.
Why are thresholds lower for some ethnic backgrounds?
Research in different populations shows that the risk of diabetes and cardiovascular disease can appear at a lower BMI. That is why NICE uses 23 and 27.5 as the overweight and obesity thresholds for these groups.
Sources
This guide is for information only and summarises public WHO and NICE references. It is not a diagnosis and does not replace advice from a health professional.