How to Calculate BMI Online — Free BMI Calculator (Metric & Imperial)
Calculate Body Mass Index in kg/m² or lbs/ft. Understand what your BMI number means, its limits, and why athletes and children need different interpretations.
The BMI formula
BMI is weight divided by height squared: BMI = kg ÷ m² in metric, or BMI = (lbs ÷ in²) × 703 in imperial. The BMI calculator handles the conversion and the rounding so you can type in whichever units you think in.
What the categories mean
- Under 18.5 — Underweight
- 18.5 to 24.9 — Normal weight
- 25.0 to 29.9 — Overweight
- 30.0 to 34.9 — Obesity class I
- 35.0 to 39.9 — Obesity class II
- 40.0 or above — Obesity class III
Why BMI is a blunt instrument
BMI was designed in the 19th century as a population-level statistic. It has known limitations when applied to individuals:
- Muscle weighs more than fat. A lean weightlifter and a sedentary person can have the same BMI with dramatically different body compositions.
- Bone density varies. Heavy-boned people run higher BMIs at the same fat percentage.
- It ignores fat distribution. Visceral fat (around organs) is far more metabolically risky than peripheral fat — BMI doesn't distinguish.
- Ethnic differences matter. Asian populations often have higher body-fat percentages at lower BMIs; clinical thresholds are being revised.
- Age changes composition. Older adults lose muscle; BMI can look "normal" while fat percentage has climbed.
Better metrics (when you need them)
Waist-to-height ratio (waist cm ÷ height cm; below 0.5 is generally healthy) tracks cardiovascular risk better than BMI. Body-fat percentage measured via DEXA or bioimpedance is more accurate for athletes and anyone whose muscle mass is unusual. Use BMI as a rough screen, not a diagnosis.
Children and teenagers are different
Pediatric BMI uses percentile charts, not the adult 18.5-25 cutoff. A 13-year-old with a BMI of 22 might be in the 85th percentile for their age — the interpretation changes entirely. Adult categories only apply from age 20+.
Related calculators
For body-composition goals, pair BMI with the calorie calculator (daily maintenance calories) or age calculator for precise age-in-years/months/days. If you're tracking body-fat trends, record measurements weekly — single readings are noisy.
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