For more than 30 years we have been wedded to the Body Mass Index (BMI) as the sole criterion for deciding whether someone is obese or not. It is used by most health authorities to classify both adults and ­children as overweight and obese in order to qualify for dieting advice, activity ­programmes, even bariatric surgery and to assess the risk of diabetes.

But some experts have criticised the BMI as a flawed system, which doesn't take into account whether a person is actually carrying more fat, or just more weight. Here's how to calculate your BMI, as well as other indicators that you might want to watch your weight.

Your BMI is calculated by dividing your weight in kilos by your height squared (H x H) in metres.

So a person who is 70kg (around 11st) and 1.68m tall (around 5'5) would have have a BMI of 24.8.

This is because 70 ÷ (1.68 x 1.68) = 24.8.

You’re judged to have a healthy weight if your BMI is between 18.5 and 24.9 for both men and women.

According to NHS Choices:

• If your BMI is less than 18.5 you are classed as underweight.
• If your BMI is between 18.5 and 24.9, you are in a healthy weight range.
• If your BMI is between 25 and 29.9 you are classed as overweight.
• If your BMI is 30 or above, you are classed as obese.

Criticisms of BMI

Using BMI as an indicator of obesity is imperfect. This is because it only ­measures weight, which is the sum of not only the weights of fat, but also muscle, bone, blood, brain and all body fluids. So your BMI can tell you if you're carrying more weight, but not if you're carrying more fat.

Just think of an athlete whose muscle mass is higher than average. Let’s exaggerate the case and make the athlete a lean, fast, tall, heavy rugby forward. His muscle will take his weight and therefore his BMI into the obese rage. Wrong. He’s not obese and that’s because BMI takes no account of the weight of your muscles.

Measuring waist size

Some experts are keen on waist ­measurement, not just as a measure of obesity but as a predictor of diabetes, heart disease and stroke.