BMI Calculator
Calculate your Body Mass Index (BMI) using standard adult BMI categories and NHS-aligned guidance, in metric or imperial units. This is general information, not personalised medical advice.
About the BMI Calculator
Our BMI Calculator works out your Body Mass Index from your height and weight, in metric or imperial units, and shows which standard adult BMI category it falls into, along with the healthy weight range for your height. BMI is a widely used general screening measure, but it has known limitations, which are explained below.
What Is BMI?
Body Mass Index (BMI) is calculated by dividing weight in kilograms by height in metres squared (BMI = kg ÷ m²). It’s a simple, widely used measure that gives a general indication of whether someone’s weight falls within a range typically associated with good health, based on population data. It does not measure body fat directly, and doesn’t account for factors like muscle mass, bone density, or where fat is distributed in the body.
NHS-Aligned BMI Categories for Adults
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy Weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obese (Class 1) |
| 35 – 39.9 | Obese (Class 2) |
| 40 and above | Obese (Class 3) |
The Limitations of BMI
BMI is a useful population-level screening tool, but it has real limitations at the individual level. It can’t distinguish between muscle and fat, so very muscular people, including many athletes, can register as “overweight” or “obese” despite having low body fat. It may also be less reliable for older adults, who tend to have less muscle mass for a given weight, and it isn’t designed for use during pregnancy. The NHS also notes that healthy BMI thresholds can differ for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, who can face increased health risks at lower BMI values than the general adult thresholds shown above. BMI is not valid for children or teenagers, who require age- and sex-specific growth charts rather than the adult categories used here.
How to Use the BMI Calculator
- Choose Metric or Imperial units.
- Enter your height and weight.
- Your BMI, category, and the healthy weight range for your height are calculated automatically.
- Read the notes underneath your result, which explain what BMI does and doesn’t tell you.
Frequently Asked Questions
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by your height in metres squared (BMI = kg ÷ m²). For imperial units, the equivalent formula is weight in pounds divided by height in inches squared, multiplied by 703.
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 falls within the healthy weight range, according to NHS-aligned classifications. This is a general population guide, not an individual target, and healthy ranges can vary based on factors like ethnicity, muscle mass and age.
Is BMI accurate for everyone?
No. BMI doesn’t distinguish between muscle and fat, so it can be inaccurate for very muscular people and athletes. It may also be less reliable for older adults, isn’t designed for use during pregnancy, and doesn’t apply to children or teenagers, who need age- and sex-specific charts instead.
Does BMI mean different things for different ethnic groups?
Yes. The NHS notes that people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds can face increased health risks at lower BMI values than the general adult population thresholds. This calculator uses the standard adult thresholds only and does not adjust or personalise your category based on ethnicity, so standard categories may not fully reflect individual health risk for everyone.
What should I do if my BMI is outside the healthy range?
BMI is a screening tool, not a diagnosis. If you have concerns about your weight or health, it’s best to speak to a GP or another healthcare professional, who can consider your BMI alongside other factors relevant to you.
What’s the difference between BMI and body fat percentage?
BMI is based only on height and weight and doesn’t distinguish between fat and muscle. Body fat percentage measures the proportion of your body that is fat tissue directly, typically using methods like skinfold callipers, bioelectrical impedance scales, or DEXA scans, and generally gives a more precise picture of body composition than BMI alone.