What does BMI tell you?
BMI is a measure of body weight that can be used to estimate body fat. It’s used to assess whether your weight is within a ‘healthy’ range for your height.
If you have a high BMI, you may be overweight or obese, and this can affect your risk of certain health conditions. Your BMI alone cannot diagnose your risk of disease, but it can be used by a healthcare professional to help assess that risk.
How can I calculate my BMI?
Your BMI is calculated by dividing your weight in kilograms (kg) by your height in metres squared (m2). So, you’ll need to know your weight and your height to calculate your BMI. If you know these, you can try our BMI calculator to calculate your BMI.
A different calculation is used to assess weight in children. That’s because their age and sex will need to be considered as they grow. This means the result from a BMI calculator alone will not be an accurate measure of whether they are overweight or obese.
What should my BMI be?
There are four weight ranges of BMI, as follows:
- BMI less than 18.5 – underweight
- BMI between 18.5 and 24.9 - healthy weight
- BMI between 25 and 29.9 - overweight
- BMI of 30 or above – obese
It is important to note that BMI, body weight, and body fat can vary with age, gender, ethnicity, and bone and muscle mass. These factors need to be taken into account when interpreting the above ranges.
What is BMI used for?
If you have a BMI of 30 or higher, you’re at increased risk of developing several health problems. These include:
Carrying excess fat around your tummy (abdomen) can increase your risk of health problems such as hypertension, diabetes, and coronary heart disease. You might carry excess fat there even if your BMI falls within the ‘healthy’ range.
For this reason, health care professionals will also consider your waist-to-height ratio. See the ‘How else can I measure my weight’ section for more information.
What are the limitations of BMI?
Although BMI can be a useful reference, there are limitations to using it. Two people with the same BMI may have different amounts of body fat and have differing risks of disease.
For example, if you have a lot of muscle, your BMI might indicate you’re overweight because muscle weighs more than fat. An athlete will normally have less body fat than a non-athlete with the same BMI.
Older people tend to have more fat and less muscle than younger adults. And women generally have more body fat than men with the same BMI.
People with a south Asian, Chinese, Black African, or African-Caribbean background may also be at increased risk of health problems at a lower BMI. The threshold for these populations is therefore lower: overweight is a BMI of 23 to 27.4, and obese is a BMI of 27.5 or above.
How else can I measure my weight?
There are other calculations you can do to find out whether you might be overweight or obese. These could be useful if you don’t think your BMI accurately reflects your weight:
- waist circumference
- waist-to-height ratio
To measure your waist circumference, hold a tape measure just above your belly button, and measure as you breathe out. A waist circumference greater than 32 inches (for women) or 37 inches (for men) suggests you’re at increased risk of obesity-related health conditions.
If your BMI is less than 35, you can measure your waist-to-height ratio. To do this, divide your waist circumference by your height. If your waist is more than half your height – that is, the ratio is greater than 0.5 – you may be at increased risk.
If you have any questions about your disease risk or need help measuring your weight, speak to a GP or nurse. They’ll be able to advise you about your health and how to manage your weight.