BMI Calculator.
Calculate your Body Mass Index (BMI) using metric or imperial units, with a clear interpretation against WHO categories. A quick screen for body composition — not a diagnosis.
Enter weight in kilograms (metric) or pounds (imperial).
Enter height in centimeters (metric) or inches (imperial).
Updates instantly as you type. No data is sent to a server.
How to Use This Calculator
1. Pick your preferred unit system. The image-select cards make it easy to switch between Metric (kg + cm) and Imperial (lbs + inches). 2. Enter your weight using the slider or by typing. 3. Enter your height the same way.
Your BMI updates immediately. The card next to it tells you the WHO category your value falls into, plus the healthy-weight range for your current height — so you know exactly how far you are from the next category boundary.
The Mathematical Foundation
Metric: BMI = weight(kg) / height(m)² Imperial: BMI = 703 × weight(lb) / height(in)² WHO categories: Under 18.5 → Underweight 18.5 – 24.9 → Normal 25.0 – 29.9 → Overweight 30.0 – 34.9 → Class I Obesity 35.0 – 39.9 → Class II Obesity 40.0 and above → Class III Obesity
Worked Examples
Example 1 — Metric: 70 kg, 170 cm. BMI = 70 ÷ 1.7² = 24.2 — Normal range.
Example 2 — Imperial: 180 lb, 68 in. BMI = 703 × 180 ÷ 68² ≈ 27.4 — Overweight range.
Example 3 — A 5'10" adult (≈ 178 cm) is in the Normal range between roughly 58 kg and 79 kg. The healthy weight band shown by the calculator updates the moment you change your height.
Background
Body Mass Index (BMI) is a screening number that compares your weight to your height. Developed by Adolphe Quetelet in the 19th century, it's the most widely used population-level metric for body composition because it requires only two inputs and zero specialized equipment. BMI does not directly measure body fat and isn't ideal for athletes with high muscle mass or for the elderly — but for the average adult it's a fast, free first signal.
The World Health Organization (WHO) uses the same cutoffs around the world, which is why BMI remains a useful common language across clinics, public-health studies and fitness platforms.
Frequently Asked Questions
Is BMI accurate for athletes?
Not directly. Athletes often have BMI values in the Overweight range simply because muscle is denser than fat. For trained populations, body-fat percentage or DEXA scans are far more meaningful than BMI alone.
Does BMI work for children?
BMI is calculated the same way for children and teens, but the interpretation uses age-and-sex-specific percentile charts rather than the adult cutoffs above. Use a pediatric BMI calculator for anyone under 20.
What if I'm pregnant?
Standard BMI is not appropriate during pregnancy. Healthcare providers track gestational weight gain instead, which depends on the BMI you started with. Talk to your provider for personalized guidance.
How often should I check my BMI?
For most adults, once a month is plenty. BMI is a slow-moving signal — daily fluctuations in weight (water, food, sleep) far exceed any meaningful month-to-month change in body composition.