BMI Calculator (Body Mass Index)
Calculate your Body Mass Index (BMI) instantly. Free BMI calculator for adults in metric and imperial units, with WHO category breakdown.
What this calculates
Body Mass Index (BMI) is a quick, widely used screen for whether your weight is appropriate for your height. It works the same for men and women, and the WHO publishes the standard adult categories used worldwide. This calculator supports both metric (kg, cm) and imperial (lb, ft/in) units and shows your category instantly.
Formula & how it works
The metric formula is BMI = weight (kg) ÷ height (m)². The imperial formula is BMI = 703 × weight (lb) ÷ height (in)². The 703 factor converts pounds-over-inches-squared into the same numeric range as the metric version. Adult categories (WHO): under 18.5 = underweight, 18.5–24.9 = normal, 25–29.9 = overweight, 30+ = obese. BMI is a screening number, not a diagnosis, and does not separate fat from lean mass.
Worked example
A person who is 175 cm tall and weighs 70 kg has BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86, which is in the normal range (18.5–24.9). In imperial units, someone who is 5'9" (69 inches) and 154 lb has BMI = 703 × 154 ÷ (69 × 69) = 108 262 ÷ 4761 = 22.74, also normal. The two answers differ slightly because of unit-conversion rounding.
Frequently asked questions
Is BMI accurate for athletes or muscular people?
Not really. BMI cannot distinguish muscle from fat, so athletes with high lean mass often score in the overweight range despite being lean and healthy. For those cases, body-fat percentage or waist-to-height ratio is more informative.
Does BMI work for children?
No. Children use age-and-sex-specific BMI-for-age percentiles published by the CDC and WHO, not the adult cut-offs above.
Is BMI the same for men and women?
The formula and the WHO adult categories are identical for men and women. Body-composition differences mean that at the same BMI, women typically carry slightly more body fat than men, which is why BMI is best used alongside other measures.
What is a healthy BMI?
For adults, the WHO defines 18.5–24.9 as the normal range. Some research suggests slightly higher targets (up to ~27) for adults over 65, but always discuss your individual situation with a clinician.
Sources
Disclaimer: BMI is a screening tool, not a medical diagnosis. Consult a qualified healthcare professional before making decisions about your health.
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