Body Mass Index, or BMI, is one of the most widely used health numbers in the world. It is quick to calculate and easy to compare, but it is often misunderstood. This guide explains what BMI measures, how to work it out and where it falls short.
What BMI measures
BMI compares your weight to your height. It was designed as a simple screening tool for populations, not as a diagnosis for one person. It does not measure body fat directly, and it says nothing about where fat is stored or how healthy your habits are.
The formula
BMI = weight in kilograms ÷ (height in meters)2
For 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, which is about 22.9. If you use pounds and inches, multiply weight in pounds by 703 and divide by height in inches squared. Our BMI Calculator handles both unit systems for you.
The standard categories
The World Health Organization uses these adult ranges:
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25 to 29.9: overweight
- 30 and above: obesity
Some health authorities use lower cut-offs for people of South and East Asian descent, because health risks can rise at a lower BMI in those groups. In some guidelines, 23 is where extra risk begins. If this applies to you, ask your doctor which thresholds to use.
What BMI cannot tell you
- Muscle versus fat. A muscular athlete can have a high BMI with low body fat.
- Where fat is stored. Fat around the waist carries more health risk than fat elsewhere, and BMI does not show it.
- Age and sex. Older adults tend to carry more fat at the same BMI, and women generally carry more than men.
- Children and teens. They are assessed with growth charts that compare BMI against others of the same age and sex, not with adult ranges.
- Pregnancy. BMI is not used in the same way during pregnancy.
Better questions to ask alongside BMI
BMI works best as one piece of a bigger picture. Other useful measures include waist circumference, blood pressure, blood sugar, cholesterol, fitness level, sleep and family history. A health professional can interpret these together, which no single number can do.
Using your result sensibly
If your BMI is in the normal range, it is a good sign but not a guarantee of health. If it is outside the range, treat it as a prompt to look at your habits and talk to a doctor rather than as a judgment. Small, steady changes in food, activity and sleep matter far more than chasing a single number, and crash diets are usually counterproductive.
This article is general information, not medical advice. For personal health concerns, please speak with a qualified healthcare professional.
Frequently asked questions
What is a healthy BMI for adults?
The WHO normal range is 18.5 to 24.9. Some guidelines for South Asian populations use lower thresholds, so ask your doctor which ones apply to you.
Is BMI accurate for athletes?
Often not. Muscle weighs more than fat, so a muscular person can have a high BMI without excess body fat. Waist measurement or a body-composition test gives a better picture.
How often should I check my BMI?
There is no need to check often. Once every few months is enough to see a trend, because body weight naturally varies from day to day.