BMI Calculator (Body Mass Index)
Enter your height and weight to calculate your Body Mass Index instantly, with healthy-range guidance and a clear interpretation.
TL;DR
BMI = weight (kg) / height² (m²). Healthy range is 18.5–24.9. Enter your numbers in the form, the result appears instantly. Your data never leaves your browser.
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What is BMI?
Body Mass Index (BMI) is a simple measure that compares your weight to your height. Developed by Belgian mathematician Adolphe Quetelet in 1832, it is used by the World Health Organization (WHO) and the NHS as a basic screening tool for adults.
How is BMI calculated?
BMI = weight (kg) ÷ height² (m²)
Example: a 70 kg person who is 1.75 m tall:
70 ÷ (1.75 × 1.75) = 22.9 → normal range
BMI categories (WHO / NHS)
- Below 18.5 — Underweight
- 18.5 – 24.9 — Healthy
- 25.0 – 29.9 — Overweight
- 30.0 – 34.9 — Obese class I
- 35.0 – 39.9 — Obese class II
- 40.0 and above — Obese class III (severe)
When BMI is misleading
BMI does not distinguish between muscle and fat. It can be inaccurate for:
- Athletes and bodybuilders — high muscle mass may push BMI into the overweight range despite low body fat.
- Older adults (65+) — age-related muscle loss (sarcopenia) means a "normal" BMI may still hide high body fat.
- Pregnant women — different BMI guidance applies during pregnancy.
- Children (under 18) — paediatric BMI percentiles are used, not adult ranges.
Better accuracy: combine BMI with waist circumference, waist-to-hip ratio (WHR), or body-fat percentage (DEXA, bioelectrical impedance).
Not medical advice. BMI is a screening tool, not a diagnosis. Consult a doctor for any health concerns.
Frequently asked questions
What is BMI?
BMI (Body Mass Index) is your weight in kilograms divided by your height in metres squared. The healthy range for adults is 18.5–24.9.
How is BMI calculated?
Formula: BMI = weight (kg) / height (m)². Example: a 70 kg, 1.75 m person has BMI 22.9 — normal range.
What do BMI values mean?
Below 18.5: underweight. 18.5–24.9: normal. 25–29.9: overweight. 30–34.9: obese class I. 35+: obese class II or higher. Used by the WHO.
Is BMI accurate for athletes?
BMI does not distinguish between muscle and fat. A muscular athlete may register as "overweight". Use waist-to-hip ratio or body-fat percentage for more accuracy.
Is BMI different for men and women?
The formula is the same. However, body composition differs: women typically have higher body-fat percentage at the same BMI.
BMI is a snapshot. Track the trend.
FitCoach AI tracks your BMI weekly, body composition trends, and workout progress in one place.
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