For adults, BMI is a measure of whether you're a healthy weight for your height.
For children aged two and over, BMI centile is used. This is a measure of whether the child is a healthy weight for their height, age and sex.
If you have a BMI above the healthy range you are at raised risk of the serious health problems linked to being overweight, such as type 2 diabetes, heart disease and certain cancers. In children, BMI centile indicates whether the child is a healthy weight.
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Who can use BMI and BMI centile?
BMI is the best assessment of weight in adults, and BMI centile is the best assessment for children aged two and over.
Some adults who have a lot of muscle may have a BMI above the healthy range. For example, professional rugby players can have an "obese" BMI result despite having very little body fat. However, this will not apply to most people.
BMI for adults
BMI takes into account that people come in different shapes and sizes. That's why a range of BMIs is considered healthy for an adult of any given height.
A BMI above the healthy range indicates that you're heavier than is healthy for your height.
The ranges below only apply to adults. BMI results are interpreted differently for children.
- BMI below 18.5: a score this low means that you may be underweight. There are a number of possible reasons for this. Your GP can help you find out more, and achieve a healthy weight.You can learn more by reading Nutrition for underweight adults.
- BMI between 18.5-24.9: this is a healthy range. It shows that you're a healthy weight for your height. However, it's still important to eat a healthy, balanced diet and include physical activity in your daily life if you want to maintain a healthy weight.
- BMI score of 25 or more: your BMI is above the ideal range and this score means you may be overweight. This means that you're heavier than is healthy for someone of your height. Excess weight can put you at increased risk of heart disease, stroke and type 2 diabetes. It’s time to take action. See the section below for the next step, and learn more in our Lose weight section.
- BMI of 30 or more: a BMI above 30 is classified as obese. Being obese puts you at a raised risk of health problems such as heart disease, stroke and type 2 diabetes. Losing weight will bring significant health improvements, and your GP can help. See the section below and learn more in Lose weight.
Ethnicity, BMI and diabetes risk
New BMI advice was issued in July 2013 by the National Institute for Health and Care Excellence (NICE) to south Asian and Chinese adults, who have a higher risk of developing type 2 diabetes than white populations. These groups are advised to maintain a BMI lower than the standard 25.
The advice is:
- BMI of 23: Asians with a BMI score of 23 or more are at increased risk of developing type 2 diabetes.
- BMI of 27.5: Asians with a BMI of 27.5 or more are at high risk of developing type 2 diabetes.
Although the evidence is less clear-cut, black people and other minority groups are also advised to maintain a BMI below 25 to reduce their risk of type 2 diabetes.
If you're overweight
If your BMI shows that you're overweight or obese it's time to take action. There’s lots of information, advice and support on NHS Choices that can help you.
- Lose weight has information and advice on achieving a healthy weight
- Food and diet contains information and advice on healthy eating
- Health and fitness is full of fun and practical ideas to help you get into shape
Your GP or practice nurse can also offer advice on lifestyle changes, and may refer you to a weight loss group or discuss other treatments. Find out more in How your GP can help.
They may also measure your waist circumference. This can provide further information on your risk of certain health conditions, such as type 2 diabetes and heart disease. You can learn more by reading Why body shape matters.
Why lose weight?
For adults who are overweight or obese, losing even a little excess weight has health benefits. You’ll lower your risk of serious health problems such as heart disease, stroke, high blood pressure and type 2 diabetes.
Weight loss can also improve back and joint pain. Most people feel better when they lose excess weight.
The key is to make small, long-lasting changes to your lifestyle. If you are overweight or obese, changing your lifestyle so that you eat fewer calories can help you to become a healthier weight. Combining these changes with increased physical activity is the best approach.
To start with, you can cut down on excess calories by swapping high-calorie meals and snacks for healthier alternatives. Read Healthy food swaps to learn more.
Physical activity is an important part of losing weight, as long as it is combined with eating fewer calories. The amount of physical activity that is recommended depends on your age. Adults aged between 19 and 64 should get at least 150 minutes of moderate-intensity aerobic physical activity – such as fast walking or cycling – a week. Adults who are overweight are likely to need to do more than this to lose weight. If it's been a while since you've done any activity you should aim to build up to this recommendation gradually. Find out more in Benefits of exercise.
For more ideas on how to get you and your family active, visit Change4Life.
Height and weight chart
You can also use the height and weight chart to check if you're a healthy weight for your height. The chart is only suitable for adult men and women.
BMI for children
BMI results are interpreted differently for children.
When interpreting BMI for a child, health professionals look at a child's weight in relation to their height, age and sex. The result is called the child’s BMI centile. BMI centile is a good way of telling whether a child is a healthy weight, and is used by healthcare professionals.
Using your child’s BMI centile, a healthcare professional can tell whether they're growing as expected. You may have done something similar when your child was a baby, using the growth charts in the Personal Child Health Record ("red book").
Once your child’s BMI centile has been calculated, they will be in one of four categories:
- underweight: below 2nd BMI centile
- healthy weight: between the 2nd and 90th BMI centile
- overweight: between 91st and to 97th BMI centile
- obese: at or above 98th BMI centile. This BMI centile category is called "very overweight" in letters that are sent by the National Child Measurement Programme.
Most children should fall in the healthy weight range. A BMI at or above the 91st centile is likely to indicate your child has an increased risk of obesity-related health problems.
Some medical conditions or treatments may mean that BMI centile is not the best way to measure whether your child is a healthy weight. Your GP or other health professional can discuss this with you.
If your child is overweight
Research shows that children who are overweight or obese have a higher risk of ill health during childhood and in later life. If your child is overweight, it’s time to take action.
A GP or practice nurse can give advice and support on helping your child achieve a healthy weight as they grow. Find out more in When your child is overweight.