BMI (Body Mass Index) is a measurement that uses your height and weight to estimate whether you’re at a healthy weight. It’s calculated by dividing your weight in kilograms by your height in metres squared.

It is used by the NHS and healthcare professionals as a screening tool to identify potential weight-related health risks. It is also one of the key measures used to determine eligibility for weight loss treatment. While it has limitations, it is a useful starting point for understanding your weight status.

BMI = weight (kg) / height (m)²

For example, if you weigh 70kg and are 1.70m tall:

• 1.70 x 1.70 = 2.89
• 70 / 2.89 = 24.2 BMI

The calculator above does this automatically. Just enter your height and weight to get your result.

Your result places you into one of these categories. Thresholds vary depending on your ethnic background, as research shows some groups develop weight-related health conditions at lower BMIs.

For white ethnic backgrounds:

• Underweight: below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obesity Class 1: 30 to 34.9
• Obesity Class 2: 35 to 39.9
• Obesity Class 3: 40 and above

For Asian, Black, Middle Eastern or Mixed backgrounds:

• Underweight: below 18.5
• Healthy weight: 18.5 to 22.9
• Overweight: 23 to 27.4
• Obesity Class 1: 27.5 to 32.4
• Obesity Class 2: 32.5 to 37.4
• Obesity Class 3: 37.5 and above

Underweight

Being underweight can increase your risk of malnutrition, weakened immunity, and bone density issues. Speaking with a healthcare professional can help you reach a healthy weight safely.

Healthy weight

This range is associated with the lowest risk of weight-related health conditions. Maintaining a healthy weight through balanced nutrition and regular activity supports long-term health and reduces your risk of chronic diseases.

Overweight

Excess weight at this level is associated with an increased risk of type 2 diabetes, high blood pressure, and cardiovascular disease. Losing weight can help reduce these risks and improve your overall health.

Obesity

At this level, health risks are significantly higher, including type 2 diabetes, heart disease, and certain cancers. Losing weight can make a meaningful difference to your health and quality of life.

BMI is a useful starting point, but it has real limitations.

Muscle vs fat

It cannot distinguish between muscle and fat. Because muscle is denser than fat, people with high muscle mass, such as athletes or regular weightlifters, often weigh more despite having low body fat. BMI only measures total weight relative to height, so it has no way of knowing what that weight is made up of.

Fat distribution

Where you store fat matters. Visceral fat (around your organs) carries more health risk than fat stored under the skin, and two people with the same BMI can have very different risk profiles depending on fat distribution. Waist circumference (increased risk above 94cm for men and 80cm for women) and waist-to-height ratio (ideally between 0.4 and 0.5) can give you a more complete picture.

Age and sex differences

BMI uses the same thresholds for everyone, regardless of age or sex. But body composition changes naturally as we get older, and biological women typically carry more body fat than men at the same BMI, meaning the same number can mean different things for different people.

Metabolic health

BMI tells you nothing about blood pressure, cholesterol, or blood sugar levels, all of which are important markers of health.

If your BMI is 25 or higher, you may be eligible for prescription weight loss medication, and losing weight does more than change a number on the scale.

What can weight loss treatment do for your health?

Research shows that weight loss is associated with meaningful improvements in blood sugar, cholesterol, and blood pressure, with greater improvements seen as more weight is lost.

For women with PCOS, losing as little as 5% of body weight can help restore hormonal balance and improve fertility.

Wegovy and Mounjaro also have benefits beyond weight loss. For people with existing heart disease, Wegovy is approved to reduce the risk of heart attack and stroke, with clinical trials showing a 20% reduction in risk. Mounjaro has also shown heart protective benefits in large clinical trials.

How effective is treatment?

Voy members lose an average of 21.5% of their body weight in 10 months. This is based on a clinical study of 57,975 participants, conducted in partnership with Imperial College London.

See if you are eligible.

There are some situations where BMI needs to be interpreted with extra care, or where this calculator may not be the right tool.

Pregnancy

BMI calculations are not appropriate during pregnancy. Speak with your midwife or GP for appropriate guidance.

Age 65 and older

A slightly higher BMI may be protective in later life, and muscle loss becomes a greater concern as we age. Healthcare professionals consider overall health, mobility, and other factors alongside BMI for older adults.

Athletes and high muscle mass

BMI may overestimate health risks in people with high muscle mass. Waist circumference or body composition analysis can give a more accurate picture.

Children and teenagers

This calculator is for adults only. For under 18s, BMI is calculated and interpreted differently. Speak with your child's GP or school nurse for an appropriate assessment.

Conditions that affect your height

If you have a condition that affects your height, BMI may not give an accurate result. Speak with your GP for an appropriate assessment.

Eating disorders

If you have a history of an eating disorder, please speak with a specialist before using this tool or pursuing weight loss.