Step-by-step guide: How Is BMI Calculated? Formula, Categories and Limits
How to use the BMI Calculator
- Choose metric or imperial units.
- Enter your height and weight. In imperial mode you can give weight in stone and pounds or in pounds only.
- Optionally enter your age. Adult BMI categories apply from age 20 (age 18 in some countries); for younger people the calculator explains why the categories do not apply.
- Read your BMI, its category and the weight range for a BMI of 18.5 to 24.9 at your height.
Example calculation
For a height of 175 cm and a weight of 70 kg:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
In imperial units, 5 ft 9 in and 154 lb give 703 × 154 ÷ 69² = 22.7 — slightly different because 5 ft 9 in (175.3 cm) and 154 lb (69.9 kg) are not exactly the same measurements.
What BMI measures
BMI divides weight in kilograms by the square of height in metres. Public-health organisations use it because it is quick and correlates with body fat at the population level. The World Health Organization classifies adult BMI as follows:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity (subdivided into classes I, II and III at 30, 35 and 40) |
What BMI does not tell you
BMI does not distinguish muscle from fat or show where fat is carried, so it can overstate the body fat of muscular people and understate it in older adults who have lost muscle. Some health bodies recommend lower thresholds for people of South Asian, Chinese and some other backgrounds, because health risks rise at lower BMI values in these groups. It is not designed for pregnant people.
For children and teenagers, BMI is interpreted with age- and sex-specific percentile charts rather than the adult categories above. A BMI result is a starting point for a conversation with a healthcare professional, not a diagnosis.
Formulas used
metric: BMI = kg ÷ m²
imperial: BMI = 703 × lb ÷ in²
weight range = 18.5 × m² to 24.9 × m²
Limitations to keep in mind
- This calculator applies adult categories only and is not suitable for children, teenagers or pregnant people.
- BMI is a screening measure and does not account for muscle mass, body composition, ethnicity or age-related changes.
- The result is general information and is not medical advice. Speak to a doctor or other qualified professional about your health.
How your data is handled
Height, weight and age are health-related information. They are processed only in your browser and are never sent to a server or stored.
Frequently asked questions
Is BMI accurate?
It is a useful, simple indicator for populations, but for individuals it can be misleading, particularly for athletes, older adults and some ethnic groups. Measurements such as waist circumference add useful context.
Why is the result different from another calculator’s?
Small differences usually come from rounding or unit conversion. This calculator uses the exact formulas and rounds only the displayed result to one decimal place.
Can I use this for my child?
No. Children’s BMI must be compared with growth charts for their age and sex. Ask your child’s doctor or use a paediatric BMI tool from a health authority.