What is BMI?
Body Mass Index (BMI) is a single number that compares your weight to your height. It was developed in the 19th century as a way to study populations and was later adopted by the World Health Organization as a quick screening tool for healthy weight in adults.
BMI doesn't measure body fat directly — it estimates whether your weight is in a sensible range for your height. That makes it useful as a starting point, not a verdict.
How BMI is calculated
In imperial units the equivalent formula is BMI = weight (lbs) × 703 ÷ height (in)². Both produce the same number — only the units differ. A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9, which falls in the Normal range.
Healthy BMI ranges
The WHO classifies adult BMI into the following categories. The Normal range is associated with the lowest average long-term health risk.
- Under 16Severe thinness
- 16 – 18.4Underweight
- 18.5 – 24.9Normal weight
- 25 – 29.9Overweight
- 30 – 34.9Obesity class I
- 35 – 39.9Obesity class II
- 40 and aboveObesity class III
BMI limitations
BMI is intentionally simple, and that simplicity is its weakness. It cannot tell muscle from fat, doesn't see where fat is stored, and is calibrated on adult European populations — so it under- or over-estimates risk for some ethnic groups.
For a fuller picture, combine BMI with waist circumference, body-fat percentage, blood pressure, and routine lab work from a healthcare professional.
BMI for athletes
Strength athletes, rowers, sprinters, and most contact-sport players tend to carry far more lean mass than the average adult. They often land in the overweight (25–29.9) or even Class I obese (30–34.9) range while having single- digit body-fat percentages. If you train seriously, lean on body-fat measurement and performance metrics rather than BMI alone.
BMI and age
Adult BMI categories apply from age 20. Children and teens use age- and sex-specific BMI percentiles instead of fixed cutoffs. Older adults tend to lose muscle and gain fat at the same BMI, so a slightly higher BMI in later life is not automatically a problem — and a very low BMI may signal frailty.
Metric vs imperial BMI
Metric (kg + cm) and imperial (lbs + ft/in) formulas give the same BMI — only the units differ. This calculator converts between them automatically, so feel free to enter whichever measurements you have at hand.