The BMI formula
BMI = weight (kg) ÷ height (m)²
In pounds and inches the formula is BMI = 703 × weight (lb) ÷ height (in)². Both versions give the same result.
A worked example
A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. You can calculate your own with the BMI Calculator.
The standard adult categories
The World Health Organization groups adult BMI like this:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal range |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
Obesity is often split further into class I (30 to 34.9), class II (35 to 39.9) and class III (40 and above).
Why cut-offs can differ
Risk is not identical at the same BMI across all populations. A WHO expert consultation noted that some Asian populations have a higher risk of type 2 diabetes and heart disease at lower BMI values, and suggested additional action points around 23 and 27.5. Some national guidelines use lower thresholds for this reason. If you belong to a group where different cut-offs apply, ask your clinician which ones suit you.
Where BMI falls short
BMI uses only weight and height, so it cannot tell muscle from fat. A muscular person can have a high BMI with low body fat, while an older person who has lost muscle can have a normal BMI with high body fat. BMI also says nothing about where fat is stored; fat around the waist carries more health risk than fat on the hips. For children and teenagers, BMI is interpreted against age- and sex-specific growth charts, not the adult table, so the adult calculator should not be used for them.
Using BMI sensibly
Treat BMI as a starting point for a conversation, not a verdict. Combine it with waist measurement, blood pressure, blood tests, activity level and family history. If your result is outside the normal range, or if you are concerned about your weight, speak with a doctor or registered dietitian rather than relying on one number. This page is educational and is not medical advice.
Related tools
The BMR Calculator estimates energy needs at rest, and the BMI Calculator gives your BMI instantly in your browser.
Frequently asked questions
Is a normal BMI always healthy?
Not necessarily. BMI ignores body composition and fat distribution, so someone in the normal range can still have health risks.
Can I use the adult BMI table for children?
No. Children and teenagers are assessed with age- and sex-specific growth charts.
Why do some guidelines use lower BMI cut-offs?
Some populations show higher health risk at lower BMI values, so certain guidelines set lower thresholds for those groups.
Written and maintained by the CalculatorsOnline Editorial Team. Last updated . Educational content only; no professional credentials are claimed. Read our editorial policy.