BMI calculator

BMI chart

This calculator should only be used by adults. Pregnant or lactating women should not rely on these BMI resultsand 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 disorderthe 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 exampleif you’re 1.75m metres tall and weigh 70 kilogramsthe calculation to get your BMI is: 

• 1.75m x 1.75m = 3m².

• 70kg ÷ 3m² = 23.

• Soyour BMI is 23which 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.

Underweight

Being underweight might mean you’re not getting all the nutrientsvitaminsand minerals that your body needs to be healthy. It may affect your skinhairand 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 havehow much physical activity you door your body type.

Sokeep an eye on your general healthdietand 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 diseasegallstones and cancer.

For help losing weightread 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 obesitywith 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 rangeyou 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. 

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 fatmuscular 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

BlackAsian 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 pregnantyou 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 issoif you are worried you should speak to your midwife or GP.

Children

Because children are still growingand boys and girls develop at different timestheir 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.

Soa 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 olderyou tend to have more fat and less muscle. SoBMI may not be reliable if you're over 60.

Waist-to-height ratio

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

To calculate your waist to height ratiomeasure 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 middleindicating increased health risks.

• 0.6 or more - you have a high amount of fat around your middleindicating 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 systempeople 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–CaribbeanSouth AsianChineseand Japanese origin.

• 94cm (37 inches) or more for men of white EuropeanBlack AfricanMiddle Easternand mixed origin.

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

Whilst this can be usefulwaist-to-height ratio is thought to be more accurate.