BMI Calculator NHS
Use this independent UK adult BMI calculator to estimate your body mass index from your height and weight, check your BMI category and understand what the result means. The interpretation below is based on current NHS and NICE guidance for adults aged 18 and over.
BMI calculator
What Is the BMI Calculator NHS?
Body mass index (BMI) relates your weight to your height. It is a screening measure rather than a diagnosis. For most adults, BMI is useful as a starting point, but it cannot distinguish body fat from muscle and it does not show where fat is stored.
How to Use the Adult BMI Calculator
- Choose metric or imperial measurements in the calculator.
- Enter your height accurately.
- Enter your current weight.
- Calculate your BMI and read the category shown with your result.
- Use the guidance below to understand the result and its limitations.
This adult page is for people aged 18 and over. Children and teenagers need age- and sex-specific assessment, so use the BMI Calculator NHS Child for ages 2 to 17.
BMI Results and Adult Weight Categories
| BMI | Standard adult category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity class 1 |
| 35.0 to 39.9 | Obesity class 2 |
| 40.0 or above | Obesity class 3 |
For adults from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, NHS and NICE guidance uses lower thresholds because weight-related health risks can occur at a lower BMI. In these groups, overweight starts at BMI 23 and obesity at BMI 27.5.
Underweight: BMI below 18.5
A BMI below 18.5 is in the underweight range. If you have lost weight unexpectedly, struggle to gain weight, or are worried about eating or your health, speak to an appropriate healthcare professional.
Healthy weight: BMI 18.5 to 24.9
For most adults, 18.5 to 24.9 is the standard healthy BMI range. BMI is only one measure, so waist measurement, physical activity, diet, medical history and other risk factors can still matter.
Overweight: BMI 25 to 29.9
A standard adult BMI from 25 to 29.9 is in the overweight range. If you are tracking a change in weight, the How Much Weight Have I Lost Calculator NHS covers that separate calculation.
Obesity: BMI 30 or above
In the standard adult classification, BMI 30 or above is within an obesity category. A higher BMI can be associated with a greater chance of weight-related health conditions, but BMI alone cannot determine your individual risk.
BMI Formula and Methodology
BMI = weight in kilograms ÷ (height in metres × height in metres)
In other words, divide weight in kilograms by height in metres squared. Imperial measurements can be converted before applying the same calculation.
Worked BMI Example
For an adult who weighs 70 kg and is 1.70 m tall: 1.70 × 1.70 = 2.89. Then 70 ÷ 2.89 is approximately 24.2. This falls within the standard healthy-weight BMI range for most adults.
Healthy Weight Range for Your Height
A calculator can reverse the BMI formula to estimate the weights corresponding to a BMI range at your height. Treat this as general context rather than a personal target. Energy needs are a separate calculation; see the TDEE Calculator NHS and Calorie Deficit Calculator NHS.
BMI and Ethnic Background
The BMI formula itself does not change with ethnicity, but interpretation can. Current NHS guidance identifies lower BMI thresholds for some ethnic backgrounds because cardiometabolic risk can develop at a lower BMI.
Why Waist Measurement Can Matter Too
BMI does not show where body fat is carried. NHS guidance recommends using waist-to-height ratio alongside BMI when adult BMI is below 35, with the public-health message of trying to keep waist size to less than half of height. Check it separately with the Waist to Height Ratio Calculator NHS.
When BMI May Be Less Useful
BMI cannot distinguish muscle from fat, so it may overestimate weight-related risk in people with high muscle mass. NICE also advises caution when interpreting BMI in adults aged 65 and over. It should not be treated as a diagnosis.
Do not rely on this adult calculator for someone under 18 or during pregnancy. The NHS adult BMI service also advises against using its calculator if you have been diagnosed with, or think you may have, an eating disorder, or if you have a condition that affects your height.
Authoritative UK Sources
- NHS adult BMI calculator and guidance.
- NHS: overweight and obesity in adults.
- NICE NG246: overweight and obesity management.
Related Calculators
BMI Calculator NHS Child · Waist to Height Ratio Calculator NHS · How Much Weight Have I Lost Calculator NHS · TDEE Calculator NHS · Calorie Deficit Calculator NHS · How Many Steps to Lose Weight Calculator NHS · NHS Calculator
Frequently Asked Questions
What is a healthy BMI for most adults?
The standard healthy BMI range for most adults is 18.5 to 24.9. Lower thresholds are used to identify increased risk for some ethnic backgrounds.
How is BMI calculated?
Divide weight in kilograms by height in metres squared: BMI = kg ÷ m².
Is the BMI formula different for men and women?
The standard adult BMI formula is the same. Individual health assessment can still depend on factors that BMI does not measure.
Can I use an adult BMI calculator during pregnancy?
No. The NHS adult BMI calculator says it should not be used during pregnancy.
Can BMI be misleading if I have a lot of muscle?
Yes. BMI cannot distinguish muscle from fat, so people with high muscle mass need cautious interpretation.
Should I check my waist as well as BMI?
If adult BMI is under 35, NHS guidance recommends waist-to-height ratio as an additional measure of central fat.
Important: This independent calculator is for general information only. It is not affiliated with or endorsed by the NHS, and it is not a diagnosis or substitute for personalised advice from a qualified healthcare professional.