How BMI is calculated
Body mass index is your weight in kilos divided by your height in metres squared: BMI = weight / height². A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 22.9.
The WHO categories
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class I |
| 35 to 39.9 | Obesity class II |
| 40 or more | Obesity class III |
With those cut-offs, the calculator gives you the weight range that matches a BMI of 18.5 to 24.9 for your height: for 1.75 m, from 56.7 to 76.3 kg.
What BMI cannot see
BMI does not tell muscle from fat. In the study by Romero-Corral and colleagues (2008), with 13,601 adults from the US NHANES III survey, a BMI of 30 or more was almost never wrong when it flagged obesity (specificity of 95% in men and 99% in women), but it missed more than half of the people with excess fat: its sensitivity was 36% in men and 49% in women. It failed mostly in the intermediate range, between 25 and 29.9.
If you do strength training and carry a lot of muscle, you can show as overweight without having excess fat. To find out how much fat you carry, use the body fat percentage calculator.
The waist-to-height ratio
The systematic review by Ashwell, Gunn and Gibson (2012), with studies of more than 300,000 adults, found that the waist-to-height ratio detects cardiometabolic risk (high blood pressure, type 2 diabetes, abnormal cholesterol) better than waist circumference and BMI. The practical cut-off is 0.5: keep your waist to less than half your height.
In the analysis by Ashwell and Gibson (2016), 35% of the people that a combined BMI and waist classification placed at no increased risk had a ratio of 0.5 or more. And those with a healthy BMI and a ratio of 0.5 or more had some significantly higher cardiometabolic risk factors.
How to measure your waist
Stand relaxed, at the end of a normal breath out, with the tape level at your navel and without pulling it tight. If you want to bring it down, we explain how in how to lose belly fat and in body fat percentage: chart by age.
