How to use BMI Calculator
- 1Choose Metric or Imperial units
- 2Enter your height
- 3Enter your weight
- 4Read your BMI, category and healthy weight range
What BMI Calculator does
Body mass index divides weight in kilograms by the square of height in metres. It was devised in the nineteenth century to describe populations, and today doctors and health services use it as a quick first screen for whether an adult's weight may be affecting their health.
Its strength is simplicity: it needs only two measurements. That is also its weakness. BMI cannot tell muscle from fat, so athletes can score as overweight, and it says nothing about where fat is carried. Treat the number as a prompt for a conversation with a professional, not a verdict.
When to use it
- Setting a realistic target
- The healthy weight range for your height turns a vague goal into a concrete band of kilograms or pounds, which is easier to plan around than a single ideal number.
- Before a medical appointment
- Many GP and insurance forms ask for your BMI. Measuring height and weight at home and working it out beforehand saves time filling them in.
- Tracking change over months
- Recalculating every few weeks shows the direction of travel. Because height is fixed in adults, a change in BMI mirrors a change in weight.
BMI Calculator settings explained
- Metric (cm, kg)
- Height in centimetres and weight in kilograms, with decimals allowed. BMI is calculated directly as kg ÷ m².
- Imperial (ft, in, lb)
- Height in feet and inches and weight in pounds. Inputs are converted exactly (1 in = 2.54 cm, 1 lb = 0.45359237 kg) before the same formula is applied.
- Category
- The World Health Organization adult bands: below 18.5 underweight, 18.5 to under 25 healthy, 25 to under 30 overweight, and 30 or more obesity.
- Healthy weight for your height
- The weights that give a BMI between 18.5 and 24.9 at the height entered, shown in the unit system you picked.
- BMI Prime
- Your BMI divided by 25, the upper healthy limit. A value of 1.00 sits exactly on the limit; 0.74 to 0.99 is the healthy band.
WHO BMI categories for adults
| Category | BMI | BMI Prime |
|---|---|---|
| Underweight | Below 18.5 | Below 0.74 |
| Healthy weight | 18.5 – 24.9 | 0.74 – 0.99 |
| Overweight | 25 – 29.9 | 1.00 – 1.19 |
| Obesity | 30 and above | 1.20 and above |
Limits and what to expect
- Designed for adults aged 20 and over; children and teenagers need age- and sex-specific percentile charts.
- Does not distinguish muscle from fat, so very muscular people may read as overweight.
- Not valid during pregnancy, and less reliable for older adults who have lost muscle mass.
- A screening number only — it is not a diagnosis or medical advice.
Troubleshooting
- No result appears
- The calculator only answers for realistic values: height between 50 cm and 2.8 m, weight between 10 and 500 kg. Check you have not typed height in metres into the centimetres box.
- My BMI says overweight but I am very fit
- Muscle is denser than fat, so strength athletes routinely score high. A waist measurement — ideally less than half your height — is a better check of health risk in that case.
- The result differs from my doctor's
- Small differences usually come from measuring at different times of day or with clothes and shoes on. Weigh yourself first thing in the morning for the most consistent figure.
Your files and your privacy
Height and weight are health information. They are used only by the calculation running in your browser and are never transmitted, logged or stored. Closing the page discards them.
What to use next
FAQ
What is a healthy BMI?
For most adults, 18.5 to 24.9. Some health services recommend lower thresholds for people of Asian, Black African or Caribbean background, where health risks rise at a lower BMI.
How is BMI calculated?
Weight in kilograms divided by height in metres squared. For example, 70 kg at 1.75 m is 70 ÷ (1.75 × 1.75) = 22.9.
Is BMI accurate?
It is a useful population-level screen but a rough individual measure. It ignores body composition and fat distribution, so combine it with waist size and professional advice.
Can I use it for children?
No. Children's BMI has to be compared with growth charts for their age and sex. Use a paediatric BMI calculator or ask a health visitor or doctor.
What is BMI Prime?
BMI divided by 25. It shows how far you are from the upper healthy limit as a simple ratio, so 1.10 means 10% above it.