BMI Calculator Guide: What Your Number Means
Learn how BMI is calculated, WHO weight categories, limitations for athletes, and when to use waist measurements—with a free BMI calculator.
Published May 28, 2026 · 2 min read
Body Mass Index (BMI) relates weight to height: a single number used in clinics and public health to flag underweight, normal weight, overweight, and obesity ranges. It is fast and standardized—but it does not measure body fat directly.
This guide explains the formula, WHO categories, who BMI misclassifies, and Toolsle’s free BMI calculator.
BMI formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Example: 70 kg, 1.75 m → 70 ÷ (1.75 × 1.75) ≈ 22.9 (normal range).
Calculate your BMI
Calculate your Body Mass Index in imperial or metric units. See your BMI category, healthy weight range, and body fat estimate.
Open tool →WHO categories (adults)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0+ | Obese (classes I–III) |
Some Asian populations use lower overweight thresholds—context matters.
Limitations
- Muscle — athletes may read “overweight” with low body fat
- Age — older adults may carry more fat at the same BMI
- Fat distribution — abdominal fat risk is not captured; consider waist circumference
- Pregnancy — BMI categories do not apply
For energy balance questions, pair results with the calorie calculator.
Common mistakes
- Mixing metric and imperial without converting
- Using BMI alone for fitness goals
- Comparing your number to teen charts (adult cutoffs differ)
Try the free BMI calculator
Enter height and weight in metric or imperial units on the BMI calculator. See your BMI, category, and healthy weight range for your height.
Try the free BMI Calculator
Calculate your Body Mass Index in imperial or metric units. See your BMI category, healthy weight range, and body fat estimate.
Open tool →