BMI calculator

BMI (body mass index) is a measure for adults to check what category their height and weight puts them in - underweight, healthy, or overweight. 

The calculator will give you an idea of how your weight compares to common values. Body Mass Index (BMI) is calculated as your weight divided by the square of your height or BMI = weight/height².

BMI chart

This calculator should only be used by adults. Pregnant or lactating women should not rely on these BMI results, and no action should be taken based on its values other than to consult a qualified person such as a doctor.

If you think you have an eating disorder, the BMI calculator may not be suitable for you.

How to calculate your BMI

Your BMI is calculated by dividing your weight by your height squared. 

For example, if you’re 1.75m metres tall and weigh 70 kilograms, the calculation to get your BMI is: 

• 1.75m x 1.75m = 3m².

• 70kg ÷ 3m² = 23.

• So, your BMI is 23, which falls within the healthy weight range. (Figures are rounded up).

You can also do this in imperial measurements. But rather than working it out yourself you can use the BMI calculator above to find out which range you’re in.

What your BMI score means

Underweight

Being underweight might mean you’re not getting all the nutrients, vitamins, and minerals that your body needs to be healthy. It may affect your skin, hair, and teeth or you may feel more tired than you should. Women may have irregular periods and have a greater chance of osteoporosis.

You should speak to your doctor to find out why you have a low BMI and about how to gain weight healthily.

Healthy weight

Having a BMI score within the healthy weight range is a good sign that you are the right weight for your height. But always remember that the BMI does not tell you anything about the make-up of your body – such as how much muscle or fat you have, how much physical activity you do, or your body type.

So, keep an eye on your general health, diet, and exercise and track any changes in your weight.

Overweight

If your BMI is in the overweight range you may be more at risk of developing health conditions such as type 2 diabetes, heart disease, gallstones and cancer.

For help losing weight, read our weight loss information leaflet.

Obesity - Class 1-2

If your BMI score is between 30 to 35 then there is a high chance you have class 1 obesity, with a range between 35 and 40 being Class 2. Your chances of developing health issues associated with being overweight can increase if you are obese.

If you’re BMI is in the obese range, you should see your GP and talk about a plan to lower your BMI in a healthy way.

Severely Obese – Class 3

A BMI of 40 or above means you’re in the range of severe class 3 obesity – which can have a high chance of affecting your health.

The National Cancer Institute found that severe obesity can reduce life expectancy by 6 to 13 years. You should speak to your doctor about how to lose weight and bring down your BMI to a healthy level. 

Is BMI accurate?

Your BMI result is a useful starting point for talking with a GP about your weight and general health. But a healthcare professional will consider many other factors when assessing if you are a healthy weight.

Groups of people where a BMI result is less helpful include: 

• Muscular people.

• Black and Asian ethnic groups.

• Pregnant women.

• Children.

• People over 60.

Muscular people

Because muscle weighs more than fat, muscular people - such as weight trainers and athletes - may be a healthy weight even though their BMI is classed as obese.

Black or Asian ethnic groups

Black, Asian and some other minority ethnic groups have a higher risk of developing some long-term conditions such as type 2 diabetes despite having a lower BMI.

The BMI ranges for Black and Asian people are:

• Underweight - below 18.5.

• Normal range - 18.5 to 22.

• Overweight - 23 to 24.

• Obesity - 25 to 30.

• Severe obesity – above 30.

Pregnant women

If you are pregnant, you will gain weight as your baby grows and from the fluid that cushions your baby in your womb. It is hard to know what your true weight is, so, if you are worried you should speak to your midwife or GP.

Children

Because children are still growing, and boys and girls develop at different times, their BMI is measured differently.

For children and young people - between 2 to 18 years old – the children’s BMI calculator considers age and gender as well as height and weight. A child’s BMI is given as a ‘centile’ number which shows how their BMI compares with other children of the same age and gender.

So, a boy on the 60th centile is heavier than 60 out of 100 other boys of their age. The BMI calculator works out if a child is:

• Underweight – on the 2nd centile or below.

• A healthy weight – between the 2nd and 91st centiles.

• Overweight – 91st centile or above.

• Very overweight – 98th centile or above.

If you're worried about your child's weight you should speak to your doctor – especially if they fall outside the healthy centile range. 

People over 60

As you get older, you tend to have more fat and less muscle. So, BMI may not be reliable if you're over 60.

Alternatives to BMI

Waist-to-height ratio

Your waist to height ratio can tell you if you have too much fat around your stomach, even if your BMI is healthy.

To calculate your waist to height ratio, measure your waist and divide it by your height. Use measurements in the same units – so centimetres (cms) or inches.

To measure your waist:

• Find the halfway point between the bottom of your ribs and the top of your hips - just above your belly button.

• Wrap a tape measure around this halfway point.

• Breathe out and then take the measurement.

What your score means:

• 0.4 to 0.49 - you have a healthy amount of fat around your middle indicating you have no increased health risks.

• 0.5 to 0.59 – you have increased fat around your middle, indicating increased health risks.

• 0.6 or more - you have a high amount of fat around your middle, indicating further increased health risks.

You should try to keep your waist to half your height – which is a waist-to-height ratio below 0.5. These categories are the same no matter what gender and ethnicity you are and if you are muscular.

Anybody with a BMI under 35 can use this calculation.

Waist circumference

Some guidelines just look at waist circumference to measure tummy fat. Using this system, people are classed as having a higher risk of health problems if they have a waist circumference of:

• 90cm (35 inches) or more for men of African–Caribbean, South Asian, Chinese, and Japanese origin.

• 94cm (37 inches) or more for men of white European, Black African, Middle Eastern, and mixed origin.

• 80cm (31.5 inches) or more for women of any background.

Whilst this can be useful, waist-to-height ratio is thought to be more accurate.