Health & Fitness
BMI Calculator
Estimate BMI and see the broad weight category for adults.
Your details
Enter your details below
Your result
Based on the values you enter
Useful next checks
- Treat the result as general guidance, not a diagnosis.
- Check unusual results with a qualified health professional.
- Retest with accurate measurements for a better estimate.
Intro
Use this BMI calculator to estimate body mass index from height and weight for adults aged 18 and over. It shows a broad screening category, not a diagnosis or a personal assessment of health.
What this BMI calculator does
Enter weight in kilograms and height in centimetres. The calculator converts height to metres, calculates BMI, and displays the result to one decimal place.
It uses only height and weight. It does not collect age, ethnic background, waist measurement or medical history, and cannot provide the personalised interpretation offered by the NHS adult BMI service.
How it works
BMI = weight in kilograms / (height in metres × height in metres), measured in kg/m². This is the formula described by the NHS.
The labels used by this calculator are:
| Unrounded BMI | Displayed category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to below 25 | Healthy weight |
| 25 to below 30 | Overweight |
| 30 and above | Obesity range |
These are general adult thresholds. NICE guidance NG246, recommendations 1.9.10–1.9.13 describes lower thresholds for people with South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean backgrounds: overweight starts at 23 and obesity at 27.5. This calculator does not apply those adjustments or distinguish obesity classes. Its label can therefore understate risk for those groups.
Example calculation
For a weight of 70 kg and height of 175 cm:
- Convert height: 175 / 100 = 1.75 m.
- Square height: 1.75 × 1.75 = 3.0625 m².
- Divide: 70 / 3.0625 = 22.857142… kg/m², displayed as 22.9.
The calculator labels this healthy weight using its general thresholds. That label does not establish that someone is healthy. Categories use the unrounded BMI, so rounding can make a value near a threshold appear to fall in the next band.
How to use the result
Treat BMI as one piece of information. The NHS explains that it cannot distinguish muscle from fat; NICE also advises caution for adults with high muscle mass and people aged 65 and over. Waist measurement and a person's wider health context can affect interpretation.
If you are concerned about your weight or symptoms, seek guidance from a qualified health professional. For other estimates, see the Calorie Calculator and Body Fat Percentage Calculator, which have their own limitations.
Assumptions and limitations
- Adults aged 18 and over only. Children and teenagers need age- and sex-specific assessment; use the NHS BMI services for adults and children to find the appropriate tool.
- Following the NHS adult service's exclusions, do not use this screening tool during pregnancy, if you have or suspect an eating disorder, or if a condition affects your height. Ask a health professional for suitable assessment.
- This form cannot check those exclusions. It does not measure body fat, fitness or individual medical risk, or adjust its labels for ethnicity or older age.
- General health information only; the result is not medical advice.
Sources and checks
Sources checked: 14 October 2026. The formula, adult scope and exclusions were compared with the NHS adult BMI guidance; interpretation limits were compared with NICE NG246. The worked example and category boundaries were also checked against this calculator's implementation.
This is a dated source and calculation check. It is not a clinical review, professional endorsement or assessment of any person's health. Source guidance may change.
Frequently asked questions
Is BMI a diagnosis?
No. BMI is a screening estimate. It cannot diagnose symptoms or establish health from height and weight alone.
Why can muscular people have a high BMI?
BMI uses total weight, so it cannot separate muscle from fat. Its category can be less informative for people with high muscle mass.
Can children use this calculator?
No. This tool is for adults aged 18 and over. Children and teenagers need age- and sex-specific BMI assessment.
What should I do with an unexpected result?
Recheck the measurements. If you are concerned about your weight or symptoms, seek guidance from a qualified health professional.
Does BMI apply to children?
Children and teens need age-specific BMI charts, so this adult calculator is not suitable for them.