On this page
What BMI shows and what it doesn't
BMI does some jobs well and others poorly.
| What BMI shows | What it doesn't show |
|---|---|
| Weight for height in large numbers of people, quickly and cheaply | Whether your weight is muscle or fat |
| Change for the same person over time | Where fat is stored: fat around the waist carries more risk than fat on the hips |
| Very low or very high weight that's worth a closer look | Your health directly, such as blood pressure, blood sugar and cholesterol |
| Differences between populations in research | Differences by age, sex and ethnicity |
Waist size fills one of the biggest gaps, because fat around the stomach adds health risk.1
Who BMI can misjudge
- Muscular people can have a high BMI with little fat.2 BMI for athletes.
- Older adults lose muscle, so a "healthy" BMI can hide more body fat.2 BMI for older adults.
- Women carry more body fat than men at the same BMI.2
- Some ethnic groups face health risks at lower BMIs. Asian BMI.
- Very short or very tall people. Squaring height doesn't perfectly match how weight scales with height. The "new BMI".
- Pregnant women. BMI isn't used during pregnancy. Pre-pregnancy BMI.
- Children. Children need percentiles for age and sex.2 BMI for kids.
What to measure alongside BMI
| Measure | Why it helps |
|---|---|
| Waist size | Shows fat around the stomach, linked with heart disease and diabetes |
| Waist-to-height ratio | Works for all heights; under 0.5 is the common target |
| Body fat percentage | Separates fat from muscle |
| Blood pressure, blood sugar, cholesterol | Show health directly, not just size |
Compare BMI with other measures, using your own numbers.
Is BMI the right measure for you?
Tick what applies to you to see whether BMI fits, and what to use alongside it.
Standard adult BMI is a reasonable place to start.
It’s a quick screen, not a diagnosis. Adding your waist size gives a fuller picture.
Use BMI-for-age percentiles, not adult categories.
Children’s and teens’ BMI is compared with others of the same age and sex.
Use your BMI from before pregnancy.
BMI during pregnancy isn’t interpreted with adult categories; your pre-pregnancy BMI sets your weight-gain range.
BMI may overstate body fat for you.
Muscle is dense, so a high BMI can be mostly muscle. Add a body-fat or waist measure.
The healthiest BMI range may be a little higher.
In people over 65, a BMI in the mid-to-high 20s is linked with the lowest risk, and weight loss matters more.
Lower cutoffs may apply to you.
Health risks start at a lower BMI for many people of Asian descent and some other backgrounds.
Adjust your weight before calculating.
BMI needs your estimated weight with the missing limb included.
Frequently asked questions
Is BMI accurate?
For most adults, BMI tracks body fat reasonably well.2 It's least accurate for very muscular people, older adults, and people at the edges of the height range.
Why do doctors still use BMI?
It's quick, free and needs no equipment, and at a population level it's strongly linked with health risks. Doctors use it as a first check, not a diagnosis.
Sources
This page is for education and is not medical advice. Talk with a healthcare professional about your weight and health.
