Check if your BMI is correct for your age and what to do if it isn't

Body mass index (BMI) is a tool used to determine if your weight is appropriate for your height. The outcome provides insight into whether your weight is appropriate for your height, and if not, whether you are overweight or underweight.

BMI may not always be the most accurate measure as there are other factors to take into account when assessing weight. However, it can be a useful guide.

Your BMI is calculated by dividing your weight in kilograms by your height in metres squared. BMI is specifically designed for adults, as children and teenagers are in a continuously growing.

• A BMI of 18.4 and below is classed as underweight.

• A BMI of 18.5 to 24.9 is classed as a healthy weight.

• A BMI of 25 to 29.9 is classed as overweight.

• A BMI of 30 or more is classed as obese.

If you are overweight and physically inactive, you may develop cardiovascular (heart and blood circulation) disease, gallbladder disease, high blood pressure (hypertension), type 2 diabetes, osteoarthritis, certain types of cancer, such as colon and breast cancer, depression and other mental health disorders.

If you are underweight you may be malnourished and develop a compromised immune function, respiratory disease, digestive diseases cancer or osteoporosis.

The NHS has its own BMI calculator online you can use. "BMI is just one way to measure health. It cannot tell you if you're carrying too much fat, or if you've got lots of muscle," it states.

"It is advised that you measure your waist to find out if you're carrying too much fat around your tummy. You should try to keep your waist measurement to half your height."

Waist circumference can be used to indicate health risk for chronic diseases.

For men:

• 94 cm or more – increased risk

• 102 cm or more – substantially increased risk.

For women:

• 80 cm or more – increased risk

• 88 cm or more – substantially increased risk.

The NHS adds: "A higher BMI increases the chance of developing long-term conditions, such as type 2 diabetes and heart disease. The BMI calculation is just one measure of health. It cannot tell the difference between muscle and fat.

"For example, if you have a lot of muscle, you may be classed as overweight or obese despite having low body fat. This is why you may get a better idea of your overall health from measuring your waist."

You should not use a BMI tool to diagnose any symptoms. If you are worried about your weight, contact your local pharmacist or GP surgery to discuss why your BMI might be too high or too low for your height.

You should not use a BMI calculator if you are under 18 years old, pregnant, have been diagnosed with an eating disorder, or think you may have one or have a condition that affects your height.