31st January, 20213 min read

What is my BMI?

Medical reviewer:
Dr Adiele Hoffman
Dr Adiele Hoffman
Daniel Piggott
Daniel Piggott
Last reviewed: 20/01/2021
Medically reviewed

All of Healthily's articles undergo medical safety checks to verify that the information is medically safe. View more details in our safety page, or read our editorial policy.

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.

Infographic showing formula for working out BMI

Infographics showing BMI categories

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:

  • gender
  • age
  • muscle mass – the amount of muscle in your body
  • ethnicity

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.

Person unpacking workout equipment

Key points

  • 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

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