How to calculate BMI

BMI, short for Body Mass Index, is a number that relates a person’s weight to their height. In other words, your BMI is meant to give you a hint as to whether your weight is right for your height.

Calculator

If you just want to work out your BMI, use this calculator. The method is explained below. Enter your height in cm and your weight in kg.

How BMI is calculated

Working out your BMI is fairly simple. We take our weight in kilograms and divide it by the square of our height in metres. For example:

The author of this article weighs 74 kg (well, he doesn’t – he would just like to) and is 177 cm tall. We convert the height to metres: 177 cm is 1.77 metres. We need to square the height, so we work out

$${1.77}^2=1.77\cdot 1.77=3.1329$$

Now we take the weight in kilograms and divide it by our result, 3.1329:

$$BMI=\frac{74}{3.1329}=23.62$$

So the author’s BMI is 23.62 (and that is after he has already shaved a good few kilos off). The whole formula looks like this:

$$\text{BMI}=\frac{\text{kg}}{\text{m}\cdot \text{m}}$$

where kg is your weight in kilograms and m is your height in metres.

How to read your BMI

The calculated BMI gives us a rough idea of whether a person is underweight, overweight or at a healthy weight. However, BMI is not a very accurate measure: it ignores many other factors, for example it does not distinguish fat from muscle. Because it only uses height and weight, it really is just a very approximate indicator. The values are usually interpreted like this:

• BMI below 18.5: underweight,
• BMI between 18.5 and 25: healthy weight,
• BMI between 25 and 30: overweight,
• BMI above 30: obese.