There are lots of tools doctors use to help them check how healthy you are. The body mass index, or BMI, is the most commonly used way to work out if you’re a healthy weight.
How to work out your BMI
Your BMI is worked out by using your weight and height. Your weight in kilograms is divided by your height in metres squared (your height multiplied by your height). An answer of 23kg/m2 would mean you have a BMI of 23.
This number allows your doctor to see which weight category you’re in:
- below 18.5 – underweight
- between 18.5 and 24.9 – healthy weight
- between 25 and 29.9 – overweight
- over 30 – obese
Being overweight or obese increases your chances of getting certain health conditions. So knowing your BMI can help you decide if you need to take steps to safely lose weight.
Are there any drawbacks of using BMI?
There are some limitations to what your BMI can tell you about your weight and health, because it doesn’t take into account your:
- muscle mass – the amount of muscle in your body
While your BMI can tell you if you’re carrying too much weight (25 or more is a high BMI), it can’t tell if that’s because you have too much fat, or if it's because you’re muscular.
Muscle is much denser than fat, so a muscular athlete such as a boxer may have a BMI that classes them as overweight or obese – even if they’re actually low in body fat. In the same way, an older person who has lost muscle mass may have a BMI that puts them in the healthy weight range – even if they have too much body fat.
Your ethnic group can also affect what your BMI can tell you about your health. For example, people of Black, Asian and certain other ethnic groups are at a greater risk of health conditions such as type 2 diabetes and cardiovascular disease at a lower BMI than white Europeans.
Finally, it might sound obvious, but BMI doesn’t apply during pregnancy, because pregnancy weight gain is normal and expected. You should use your pre-pregnancy weight when calculating your BMI.
- BMI is a commonly used tool to check if you’re a healthy weight
- however, BMI doesn’t take muscle mass, age, gender and ethnicity into account
- for these reasons, BMI doesn’t always accurately reflect your health or risk of certain health conditions