Understanding BMI: What It Tells You and What It Does Not

Body mass index, or BMI, is one of the most quoted health numbers in the world. It is quick to calculate and easy to compare, but it is often misunderstood. This article explains what it measures and where it falls short. Try it yourself with our BMI calculator.

What BMI measures

BMI compares your weight with your height: weight in kilograms divided by height in metres squared. A person who weighs 70 kg and is 1.70 m tall has a BMI of about 24.2. The result is placed into broad categories used by the World Health Organization for adults.

The standard categories

• Below 18.5: underweight range
• 18.5 to 24.9: healthy range
• 25 to 29.9: overweight range
• 30 and above: obesity range

Why it is only a screening tool

BMI does not separate muscle from fat, so a strength athlete may appear overweight. It does not show where fat is stored, and fat around the waist carries more health risk than fat on the hips. It also does not account for age, which matters because muscle tends to fall and fat tends to rise as people get older.

Ethnicity and thresholds

Studies have found that people of South Asian background can develop conditions such as type 2 diabetes at lower BMI values than European populations. Some health bodies therefore suggest lower action thresholds for these groups. A doctor can tell you which guidance applies to you.

Better questions to ask

Instead of chasing a single number, look at waist measurement, blood pressure, blood sugar, cholesterol, fitness, sleep and diet. If your BMI is outside the healthy range, treat it as a reason to speak with a healthcare professional rather than as a verdict.

This article is for general information and is not medical advice.