How to use this BMI calculator
- Select metric or imperial units.
- Enter body weight and standing height without shoes. Use consistent measurement conditions when comparing changes.
- Select Calculate to see the ratio and adult screening category.
The formula and method
BMI = weight in kilograms ÷ height in metres squared. The tool converts pounds and inches to metric before calculating, so changing the unit system preserves the same ratio.
The displayed adult categories are underweight below 18.5, healthy-weight range from 18.5 to below 25, overweight from 25 to below 30, and obesity range at 30 or higher. These are screening categories for adults aged 20 and over.
BMI describes weight relative to height. It does not measure body fat, locate fat, or distinguish muscle, bone and water. Interpret the category alongside other information rather than treating it as a personal health verdict.
Worked example
For an adult weighing 70 kg at a height of 175 cm (1.75 m): BMI = 70 / (1.75)² = 70 / 3.0625 = 22.86 kg/m². This falls squarely in the normal weight category (18.5–24.9 kg/m²).
Understanding the estimate
- The adult categories shown here are for ages 20 and over; children and teenagers require an age-specific assessment.
- Muscularity, pregnancy and fluid changes can make the category less informative for an individual.
- BMI alone cannot diagnose a condition or determine a suitable training or nutrition plan.
Frequently asked questions
Does BMI measure body fat?
No. Two people can have the same BMI with different amounts of muscle and fat. The result is a weight-to-height ratio.
Why does my BMI stay the same when I change units?
The unit switch converts the actual weight and height. The calculator then evaluates the same physical measurements.
Can I use this to track muscle gain?
BMI changes with total weight, so it cannot tell whether a change came from muscle, fat or water. Pair it with training records and other measurements.