BMI and body fat percentage: what they measure and their limits
By Johannes Nieuwboer · 6 min read
Body Mass Index (BMI) is the most widely used quick check of whether weight is in a healthy range for a given height. It is useful for populations, but has clear limits for individuals. This guide explains both.
How BMI is calculated
BMI = weight in kilograms divided by height in metres squared. Someone weighing 70 kg at 1.75 m has a BMI of 70 / (1.75 × 1.75) ≈ 22.9. In pounds and inches the formula is 703 × lb / in².
The World Health Organization uses these adult categories: below 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 and above obesity.
Where BMI falls short
BMI does not distinguish muscle from fat. A muscular athlete can have a BMI above 25 with low body fat, while an inactive person with little muscle can have a 'healthy' BMI but a high fat percentage.
It also ignores where fat is stored, and the categories are not designed for children, pregnant women or, without adjustment, some ethnic groups for whom health risks start at lower values.
Body fat percentage
Body fat percentage estimates how much of your weight is fat tissue. The U.S. Navy method used in many calculators needs height, neck and waist measurements (plus hips for women). It is an estimate, typically within a few percentage points of lab methods when measured carefully.
Measure at the same time of day, with a flexible tape held level and snug but not tight, and repeat each measurement twice.
Waist size as extra context
Fat around the abdomen is linked more strongly to health risks than fat elsewhere. A waist that is more than half your height is often used as a simple warning sign.
Use numbers as a starting point
None of these figures is a diagnosis. They are prompts for a conversation with a doctor, who can consider blood pressure, blood tests, family history and lifestyle.