What BMI does not tell you

Picture two people who are both 175 cm tall and weigh 80 kg. One has been lifting weights for years. The other does not exercise and carries extra weight around the belly. Both have the same BMI, 26.1. Once you know what BMI looks at and what it leaves out, you can read the calculator’s result properly.

BMI looks only at height and weight

BMI is your weight in kilograms divided by the square of your height in meters.

• 175 cm, 80 kg: 80 ÷ (1.75 × 1.75) = 80 ÷ 3.0625 = 26.12
• 170 cm, 65 kg: 65 ÷ (1.70 × 1.70) = 65 ÷ 2.89 = 22.49

Only two numbers go into the formula. What that weight is made of, how old you are and whether you are a man or a woman play no part. The US Centers for Disease Control and Prevention (CDC) describes BMI as a quick screening measure of weight relative to height, and says it should be considered together with other factors such as blood pressure, cholesterol levels and a physical exam.

What the same number can hide

What BMI cannot see	What happens as a result
Muscle mass	BMI cannot tell fat from muscle or bone. A muscular person can land in the overweight category without carrying much body fat.
Where the fat is	It says nothing about where on the body the fat sits. At the same BMI, a person with more fat around the belly is more likely to have weight-related health problems.
Age	As people get older they tend to lose bone and muscle and gain body fat. Health risk can rise even when BMI stays the same.
Sex	The adult BMI categories are the same for men and women. Waist circumference thresholds, by contrast, are set separately for each.

The two people at the start of this guide are the first and second rows. The adult categories are also not used for children and teenagers. While someone is still growing, their BMI is judged by comparing it with others of the same sex and age.

The cut-offs differ from country to country

A BMI value is the same wherever you calculate it, but the category it is given depends on the standard being used. In 2004 a World Health Organization (WHO) Expert Consultation reviewed evidence that the relationship between BMI, body fat percentage and health risk in Asian populations differs from that in European populations. It kept the WHO cut-off points as the international classification and proposed a way for each country to set risk thresholds suited to its own population. That is why one BMI can fall in the normal category under one country’s standard and in the category above it under another’s.

Why waist size is checked alongside it

WHO also explains that other measurements, such as waist circumference, can be used alongside BMI to identify overweight and obesity. The reason for choosing the waist is simple. A tape measure gives you a rough answer to a question BMI misses: where the fat is. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) in the United States says that where fat is carried matters, and that people with a lot of fat around the belly can be at higher health risk even when their BMI is not high.

The waist size at which abdominal obesity begins is defined differently from country to country.

Do not look only at whether one number has crossed a cut-off. Set it beside your waist measurement and check-up figures such as blood pressure and cholesterol. The result is for reference and does not replace a diagnosis from a doctor.