What Is BMI?
BMI definition explained in plain English. What the number means, the standard ranges, the formula, and the limits of using BMI on its own.
Reviewed Jan 15, 2025
Quick answer
BMI (Body Mass Index) is your weight in kilograms divided by your height in meters squared. It's a screening number that compares your weight to a standard healthy range for your height.
BMI = weight (kg) ÷ height (m)²
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Find your Body Mass Index and see where it sits on the standard healthy-range scale.
Result
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How it works
BMI is a screening tool that estimates whether your weight is in a healthy range relative to your height. It doesn't measure body fat directly.
Formula
BMI = weight (kg) ÷ height (m)²
Common mistakes
- Mixing units — entering height in cm but weight in pounds.
- Treating BMI as a diagnosis. It's a screening signal, not a verdict.
- Using adult ranges for children or teens.
- Ignoring muscle mass — athletes often score 'overweight' but have low body fat.
Source & methodology
Healthy ranges follow WHO adult cut-offs: underweight <18.5, healthy 18.5–24.9, overweight 25–29.9, obese ≥30.
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Last updated
Reviewed Apr 15, 2026.
Calcxo gives you the answer plus the reasoning behind it.
The standard ranges
- Under 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25 to 29.9 — overweight
- 30 and above — obese
Where BMI falls short
- Doesn't separate fat from muscle.
- Misclassifies very tall, very short, and highly muscular people.
- Not appropriate for children, teens, or pregnant people without adjusted charts.