Health & Fitness 2026-05-03 5 min read By Konvertibly Team

BMI Calculator: What Your Body Mass Index Actually Means

Understand Body Mass Index — the formula, BMI categories, limitations, and how to use it alongside other health metrics.

BMI Calculator: What Your Number Means

Body Mass Index (BMI) is a simple screening tool that estimates body fatness using height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is now used worldwide by doctors, researchers, and public health organisations.

The BMI Formula

BMI (metric) = weight (kg) ÷ height² (m)
BMI (imperial) = 703 × weight (lb) ÷ height² (in)

BMI Categories (WHO)

Below 18.5 — Underweight
18.5 – 24.9 — Normal weight
25.0 – 29.9 — Overweight
30.0 – 34.9 — Obese (Class I)
35.0 – 39.9 — Obese (Class II)
40.0 and above — Severely obese (Class III)

Quick Reference Examples

170 cm / 65 kg → BMI 22.5 (Normal)
175 cm / 85 kg → BMI 27.8 (Overweight)
160 cm / 50 kg → BMI 19.5 (Normal)
180 cm / 100 kg → BMI 30.9 (Obese Class I)

Limitations of BMI

BMI does not distinguish between muscle and fat mass. Athletes and bodybuilders frequently fall into the "overweight" category despite low body fat. Conversely, older adults can have a normal BMI but carry excess visceral fat. Always interpret BMI alongside waist circumference, body fat percentage, and clinical assessment.

BMI for Children

For individuals under 20, BMI is interpreted using age- and sex-specific percentile charts, not fixed cut-offs. A child at the 95th percentile or above is classified as obese.

Calculate your BMI instantly: Free BMI Calculator