How to calculate your Body Mass Index, and why BMI might not be the best indicator of obesity

By Mirror | 4yrs ago | 3 min read

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.

How to calculate your BMI

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.

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BMI can indicate whether you are overweight.

What your BMI means

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.

ALSO READ: People with larger thighs and hips live longer, study suggests

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.

To measure your waist:

Find the bottom of your ribs and the top of your hips Measure around your waist midway between these points Breathe out naturally before taking the measurement

Your waist shouldn’t measure any more than 80cm (32”) for women, and 94cm (37”) for men. You are at high risk if your waist is 88cm (34") or more for women, and 102cm (40") or more for men.

The waist and height ratio is better than BMI in assessing cardiovascular disease in men and diabetes in women. The beauty of waist measurement is that it takes account of abdominal fat and that particular fat is toxic and causes many health problems.