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Body Mass Index

The Body Mass Index is a useful tool for finding out if you have a healthy weight for your height. But remember it is only a guide.

You can also use our handy BMI chart to find out if you are underweight, overweight or just right. The chart has already done the calculations for you. It shows the underweight, healthy, overweight and obese ranges for various weights and heights. Just find the point where your weight (going across) meets your height (going up).

To calculate your BMI value exactly, divide your weight (in kilograms) by the square of your height (in metres). This number tells you how healthy your weight is:

• under 18.5 is underweight
• 18.5-25 is healthy weight
• 25-30 is overweight
• 30-35 is obese
• over 35 is very obese.

The Food Standards Agency have an electronic BMI calculator on their Eat well be well website.

What should I do if I am underweight, overweight or obese?

If you are not within the healthy weight region, you may be putting your health at risk. In particular, being overweight or obese could significantly increase your risk of cancer. You may be eating the wrong types or amounts of food.

Consult your GP for more advice. And have a look at our healthy eating and physical activity sections for tips on making healthy lifestyle choices.

Is BMI always a helpful tool?

In most cases, BMI is a very reliable tool. However, it does have limitations because it doesn’t exactly calculate the percentage of body fat. It is not accurate for the following groups:

• children and young people - doctors use gender and age specific charts to measure young people’s BMIs
• professional athletes and bodybuilders - BMI doesn’t distinguish between muscle and fat, so heavily-muscled people with low body fat could be classified as overweight or obese
• pregnant or breastfeeding women, who need higher fat reserves than usual.

There are also small variations in the BMI thresholds for the two sexes, and for different races. For example, research suggests that the range of normal BMI should be lower for people of Asian origin.

If you have any questions or worries about your BMI then visit your GP.