Body mass index: Calculate your BMI

Last updated:

Reviewed by:

Dr Rhianna McClymont

, Lead GP at Livi

Medically reviewed

Body mass index (BMI) is a simple way to find out if you’re a healthy weight. It takes your age, height, and weight to give you a score on a chart – ranging from underweight to extremely obese. An ideal BMI is somewhere in between. Enter your height and weight into the BMI calculator to find out if you have a healthy BMI.

What is body mass index (BMI)?

BMI is calculated by taking your weight in kilogrammes and then dividing this number by your height in metres squared.

Being a healthy weight is important. If you have an overweight BMI, you’re more likely to develop heart disease, type 2 diabetes, stroke, and some types of cancer. If you’re underweight, this can also have a serious impact on your health, as your body will miss out on important nutrients it needs to stay healthy. A low BMI could also be a sign of an underlying illness.

BMI is a useful way to understand whether you’re at a generally healthy weight, but it’s a measurement that doesn’t always tell the whole story.

For example, it doesn’t take into account your gender and age, or how much fat you have and where it’s distributed. Having too much weight around your tummy isn’t good for your health, whatever your BMI.

Calculate your BMI

Calculate your BMI by filling in your weight in kilogrammes and your height in centimetres in our BMI calculator for men and women.

What is a normal BMI?

The BMI calculator shows what weight range your BMI is in. The table below explains what your BMI result means:

  • Below 18.5 – underweight range

  • Between 18.5 and 24.9 – healthy weight range

  • Between 25 and 29.9 – overweight range

  • Between 30 and 39.9 – obese range

For most adults, a normal BMI or ideal weight range is to have a BMI between 18.5 to 24.9. Having a healthy BMI means you probably already have an active lifestyle and a good, balanced diet, so you’re less likely to develop several serious health conditions.

Benefits of having a healthy BMI

  • Reduced risk of Covid-19 complications

  • Reduced risk of type 2 diabetes

  • Reduced risk of cardiovascular disease and heart attacks

  • Fewer joint and muscle problems

  • Improved fertility

  • Better mobility and self-confidence

Health risks of an overweight BMI

If you have a BMI of over 25, you’re in the overweight or obese range. This means you may be more likely to suffer from:

You’re also at a higher risk of developing more severe Covid-19 symptoms.

What you can do

If you’re regularly eating more calories than you use up through exercise, you’re likely to be gaining weight. The best way to reduce BMI is to combine changes to the way you eat with increased physical activity.

You should eat a healthy, balanced diet, and keep an eye on your calories. The recommended intake for an adult woman is 2,000 calories a day, for men it’s 2,500 calories a day.

Exercise is also important. You should aim to do at least 150 minutes of moderate intensity exercise a week (for example a brisk walk, hiking or cycling), or 75 minutes’ vigorous intensity. You should try to mix up cardio and strength training to tone and strengthen your muscles and lose more weight.

If you’re obese, you may need further specialist help - talk to the GP to find out more information.

General advice for a healthy weight

Health risks of a low BMI

If you have a BMI below 18.5, you may be underweight. There might be a medical reason that needs treating, like an over-active thyroid, for example.

Being underweight isn’t good for your health. If you aren’t eating enough, you could become malnourished. This can lead to:

  • Poor immunity, so you’re more likely to get an infection

  • Osteoporosis, which weakens bones and makes them more likely to break

  • Anaemia, which is when you don’t have enough iron in your body. Anaemia means you have fewer red blood cells to store and carry oxygen. It can make you feel tired and breathless

  • Heart problems

  • Slow-to-heal wounds

  • If you’re a woman, you may find your periods stop which can cause problems with fertility

How to increase your BMI

If you have a low BMI because of your diet, aim to reach a healthy weight gradually. Your diet should be healthy and balanced, and your calorie intake should take into consideration your gender, age, build and the amount of exercise you do.

If you’re concerned about being underweight, or you’re worried that you may have an eating disorder, talk to a GP for advice.

Limitations of BMI

The BMI calculator is a useful guide to help understand weight, but there are some limitations to BMI. You should keep some other things in mind:

  • The BMI calculator doesn’t consider if you’re male or female or your age. If you’re concerned about your child’s weight, it’s a good idea to talk to the GP.

  • BMI measures weight, but not the amount of fat you have on your body or where it’s distributed. If you do weight training, you may have heavier muscles. That may put your BMI in an overweight range even though you may be a healthy weight. In the same way, older people who have lost muscle may be carrying an unhealthy amount of fat that’s not picked up by measuring BMI.

  • Being pregnant will affect your BMI result, so you should use your pre-pregnancy weight when calculating your BMI.

It’s important to avoid excess weight around your tummy. This can make you more likely to develop type 2 diabetes, stroke, and heart disease. Whatever your height and BMI, you should try to lose weight if your waist measures:

  • Over 94cm for men

  • Over 80cm for women

How to work out your BMI yourself

To calculate your BMI without our handy BMI calculator, divide your body weight (kilogrammes) by your height (metres squared).

Here’s an example: If you weigh 85 kilogrammes and your height is 185 centimetres, divide 85 by 1.85 x 1.85 – that’s 85 divided by 3.4225. This will give you a BMI of 24.8 which is in the healthy weight range.

Last updated:
Reviewed by:
Dr Rhianna McClymont, Lead GP at Livi