One number from two measurements, with an honest account of what it can and cannot tell you.
BMI is a screening measurement, not a diagnosis. It compares weight with height and nothing else. It cannot see the difference between muscle and fat, and it was never designed to assess an individual. Use it as one rough signal and talk to a doctor about anything that concerns you.
What the bmi calculator does
Body mass index divides weight in kilograms by height in metres squared. Public health bodies use it to describe populations, and clinics use it as a quick first look before better measurements. It is genuinely useful at that scale and genuinely limited for any individual.
This calculator takes metric or imperial measurements, gives the number, names the standard category, and shows what the 18.5 to 24.9 band works out to in weight for your height.
How to use it
- Choose your units. Metric takes centimetres and kilograms. Imperial takes feet, inches and pounds.
- Enter height and weight. Measure without shoes, and weigh yourself at a consistent time of day for comparable figures.
- Read the number and the range. You get the BMI figure, the category it falls in, and the weight range that corresponds to the 18.5 to 24.9 band at your height.
Features
- Metric and imperial input
- BMI to one decimal place
- Standard World Health Organization categories, including the obesity classes
- Healthy weight range in your own units
- Input validation with plain-language errors
- Nothing stored and nothing transmitted
Common uses
- A quick check before a doctor's appointment
- Understanding a BMI figure that appeared in a health report
- Seeing what weight range corresponds to the healthy band at your height
- Converting between metric and imperial while comparing measurements
How your data is handled
Height and weight are used only to produce the number on screen. Nothing is transmitted or stored.
Frequently asked questions
What is the formula for BMI?
Weight in kilograms divided by height in metres squared. With pounds and inches, multiply weight by 703 and divide by height in inches squared. This tool converts imperial input to metric and uses the same calculation.
What are the standard BMI categories?
Below 18.5 is under the healthy range, 18.5 to 24.9 is the healthy range, 25.0 to 29.9 is the overweight range, and 30.0 and above is obesity, split into class I at 30.0 to 34.9, class II at 35.0 to 39.9 and class III at 40.0 and above.
Is BMI accurate for everyone?
No. It ignores body composition, so muscular people often score high without excess fat, and it can understate risk in older adults who have lost muscle. It is not used the same way for children, who are assessed on age and sex percentile charts, and it is not appropriate during pregnancy.
Do the thresholds differ between populations?
Yes. Several health authorities use lower cut-offs for people of South Asian, Chinese and some other ancestries, because health risks appear at a lower BMI. Ask a local clinician which thresholds apply to you.
What measurements work better than BMI?
Waist circumference and waist-to-height ratio say more about where fat is carried, which matters more than total weight. A clinician can also use body composition measurement, blood pressure and blood tests.
Should I change my diet based on this number?
Not on this number alone. BMI is a starting point for a conversation, not a plan. A doctor or registered dietitian can look at the whole picture before anything changes.