BMI Calculator
Calculate Body Mass Index (BMI) from your weight and height in metric or imperial units, with instant WHO category classification (Underweight, Normal, Overweight, Obese).
Input
Output
| Metric | Value |
|---|---|
| No data yet | |
Guides
What does this calculator do?
This tool calculates your Body Mass Index (BMI) — a quick screening measure that estimates whether your weight is healthy relative to your height. Enter your weight and height in either metric or imperial units and get your BMI value along with its WHO (World Health Organization) weight category, updated instantly as you type.
Formula:
BMI = weight(kg) / height(m)²
For imperial input, weight in pounds is converted to kilograms (× 0.45359237) and height in inches is converted to meters (× 0.0254) before the same formula is applied — mathematically equivalent to the commonly quoted shortcut BMI = 703 × weight(lb) / height(in)².
Worked example: a person weighing 70 kg at 175 cm (1.75 m) tall: 70 / (1.75 × 1.75) = 70 / 3.0625 ≈ 22.86 — squarely in the "Normal weight" range.
How to use it
- Enter your weight and choose kilograms or pounds.
- Enter your height and choose centimeters or inches.
- Your BMI and category appear instantly and update as you type. Copy or download the result as CSV.
BMI categories (WHO standard, adults)
| Category | BMI range |
|---|---|
| Underweight | below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obese | 30 and above |
FAQ
Is BMI an accurate measure of health? BMI is a useful, quick screening tool at a population level, but it has well-known limitations. It doesn't distinguish between muscle and fat mass, so muscular athletes can register as "overweight" or "obese" despite having low body fat. It also doesn't account for fat distribution (visceral vs. subcutaneous fat), bone density, age, sex, or ethnicity — all of which affect how much health risk a given BMI actually represents.
Who shouldn't rely on BMI alone? Athletes and heavily muscled individuals, pregnant women, older adults (who tend to lose muscle mass), children and teens (who need age- and sex-specific growth charts, not adult thresholds), and people with unusual body compositions. For these groups, BMI can be misleading and other measures — such as waist circumference, body fat percentage, or a clinical assessment — give a more complete picture.
Why do the imperial and metric calculations sometimes give slightly different results elsewhere?
Some calculators round the imperial conversion constant (703) or the unit conversions differently. This tool converts weight and height to metric using precise conversion factors (1 lb = 0.45359237 kg, 1 in = 0.0254 m) before computing BMI, so results match the metric formula exactly regardless of which units you enter.
What should I do if my BMI is outside the "Normal" range? Treat it as a starting point for a conversation, not a diagnosis. Consult a healthcare provider who can weigh BMI alongside other factors like body composition, lifestyle, and family history before recommending any changes.
Is a lower BMI always better? No — being underweight carries its own health risks, including nutritional deficiencies and reduced bone density. The "Normal weight" range represents the band associated with the lowest average health risk in large population studies, not a target to minimize.
Privacy
All calculations run entirely in your browser (or via the API if you call it programmatically) — your weight and height are never uploaded or stored. This tool provides a general screening estimate only, not medical advice.