Your weight and height can tell you a lot about your health, and that’s where body mass index, or BMI, comes in. Your BMI helps you understand if your weight is within a healthy range for your height.

Use our handy tool to quickly calculate your BMI to see whether, for your height, you are underweight, at a healthy weight, overweight, or obese. You can also use this tool to see if you are eligible for weight loss treatment. We’ll also explain what your results mean, the differences between sexes, and why BMI isn’t always the most accurate tool to use.

BMI is a simple number that helps you to understand if your weight is right for your height. It determines whether you are underweight, of correct weight, overweight, or obese. The higher your BMI is, the higher the chances are that you will develop certain health conditions, like:

• heart disease
• high blood pressure
• type 2 diabetes
• cancer

BMI is a popular tool used by doctors, nurses, and researchers because it doesn’t require any complicated equipment or calculations, it only requires your height and weight.

Calculating your BMI is simple with our BMI calculator. All you need to do is input your weight in kilograms (kg) and height in centimetres (cm), and the calculator will do the rest. You’ll be given a number – this is your BMI – and a category explaining whether you are underweight, normal weight, overweight, or obese.

Once you have been given your BMI number, you’ll also fall into one of the following categories:

Underweight (less than 18.5)

If your BMI is below 18.5, it suggests that your weight is lower than what is considered healthy for your height. Being underweight can lead to nutritional deficiencies, a weakened immune system, and other health issues. You may experience tiredness, getting sick more often, and have low energy levels. If you are underweight, you should contact your doctor to discuss a plan to help you get within a normal weight range.

Healthy weight (18.5 to 24.9)

This is the range where everyone should aim to be, as it suggests a healthy weight for your height. It doesn’t necessarily mean you are perfectly healthy, but it is a good sign that you have a lower chance of developing health complications related to your weight. Even if you are within a healthy weight category, it is still important to get regular exercise and follow a healthy, balanced diet.

Overweight (25 to 29.9)

If your BMI is within this range, it suggests that you are overweight for your height and that there is a higher chance that you could develop conditions, like diabetes, heart disease, and even some cancers. However, there are things you can do to help you lose weight and bring you within a healthy range, such as:

• following a diet plan consisting of healthy, balanced meals
• increasing the amount of exercise you do each week
• getting help from your doctor

Obese (over 30)

A BMI of 30 or above indicates obesity and can have a significant impact on your health. Obesity increases the likelihood of severe health problems, including heart attacks, strokes, joint and mobility issues, and diabetes. If your BMI is in this category, you may be eligible for weight loss treatment.

What are the health risks of being underweight?

Although obesity gets a lot of attention for its effects on health, being underweight can also cause problems, including:

• nutrient deficiencies
• hormonal imbalances
• fatigue and weakness
• mental health issues
• reduced strength
• weakened immunity
• poor growth and development

Your doctor can help if you are underweight by providing nutrition support, exercise recommendations, and mental health support, if required.

What are the health risks of being overweight?

Being overweight can have a detrimental impact on your health, especially over time. Although not everyone with an increased BMI will experience health problems, being overweight increases your risk of several medical conditions, including:

• high blood pressure
• type 2 diabetes
• stroke
• coronary heart disease
• non-alcoholic fatty liver disease (NAFLD)
• sleep apnoea
• mental health problems
• cancer

If you are overweight, it’s crucial that you take steps towards a healthier lifestyle and weight through proper nutrition, regular exercise, counselling, and getting help from your doctor.

By managing your weight, you will reduce your chance of developing adverse health effects and experience a better quality of life.

BMI results in women vs men

BMI is calculated in the same way for both men and women, but the numbers can mean slightly different things because of the differences in body composition. For example, women tend to have a higher body fat percentage than men, while men often have more muscle mass. So, because muscle is heavier than fat, men may have a higher BMI even though they have less body fat.

One study found that the range of BMI values that included 90% of healthy people was between:

• 19.5 and 30 for men
• 18 and 30 for women

The average BMI for men was 24.5 and 21.5 for women. While BMI is a useful tool, it is important to consider the differences between male and female bodies to get a more accurate picture of health. It should also be used alongside other signs and tools that monitor how healthy you are.

BMI is a helpful starting point when trying to assess your weight-related health risks, but it isn’t always perfect. One reason is that it doesn’t differentiate between muscle mass and body fat. That means anyone with a higher muscle mass, including athletes, may have a higher BMI even though they are healthy and not overweight.

BMI also doesn’t take into account your:

• age – as you get older, you are more likely to lose muscle and gain fat and BMI charts are different for children
• ethnicity – some ethnic groups may have a higher chance of certain health issues, even at lower BMI levels
• body shape – BMI doesn’t tell you where fat is stored, so fat around the stomach is a greater health issue than fat on the hips, for example

It’s important to note that BMI is also not used during pregnancy.

So, while a BMI calculator is a quick and easy way to determine which weight category you fall into, it should be used with other health information and interpreted by a doctor for a better understanding of your overall health.

One measurement you can use alongside your BMI is the size of your waist. Your waist circumference can tell you how much fat is sitting around your abdomen (belly), also called visceral fat.

Visceral fat can increase the chance of poor health outcomes. For example, research shows that visceral fat contributes to insulin resistance and can quickly raise your blood pressure, but it also increases the likelihood of:

• heart disease
• heart attacks
• stroke
• cancer
• Alzheimer’s disease

You can work out if you are carrying too much fat around your stomach by calculating your waist-to-height ratio.

Calculating your waist-to-height ratio

Here’s how you can calculate your waist-to-height ratio:

1. Locate the bottom of your ribs and the top of your hips.
2. Wrap a tape measure around your stomach at the halfway point between the bottom of your ribs and the top of your hips.
3. Pull the tape measure tight, but not so tight that it is digging into your skin.
4. Breathe out and make a note of where the tape measure reaches all the way around, either in inches or centimetres.
5. Now, measure your height using the same unit you used to measure your waist.
6. Divide your waist measurement by your height measurement to find out your waist-to-height ratio.

What does it mean?

The more visceral fat you have around your abdomen, the greater your waist-to-height ratio will be, and the greater the chance that you will develop health problems. You can use this calculation with your BMI to give you a better idea of your weight and overall health.

Here’s what your waist-to-height ratio means:

A ratio of 0.5 or higher means that your waist circumference is equal to or more than half of your height, and that you are at an increased risk of health problems.

Now that you understand the importance of tools, such as the BMI calculator and waist-to-height ratio, for determining your health status, calculate your BMI with our BMI calculator. It is a good first step to helping you better understand your health and body. You can use it to:

• track changes with your weight and body composition
• get a better understanding of your health by combining it with other measurements
• help you focus on healthy habits
• start a conversation with your doctor about weight management

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• Body mass index (BMI), NHS Inform [accessed 19 June 2025]
• Body mass index of healthy men compared with healthy women in the United States, International Journal of Obesity [accessed 19 June 2025]
• Calculate your waist to height ratio, NHS [accessed 19 June 2025]
• Obesity, NHS [accessed 19 June 2025]
• Serum retinol binding protein 4 is associated with visceral fat in human with nonalcoholic fatty liver disease without known diabetes: a cross-sectional study, Lipids in Health and Disease [accessed 19 June 2025]