BMI Calculator

Calculate your Body Mass Index instantly. Supports both metric (kg/cm) and imperial (lbs/in) units with WHO category classification.

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For Waist-to-Height Ratio (WtHR) — a more accurate health risk indicator

What is Body Mass Index (BMI)?

Body Mass Index (BMI) is a simple, widely used screening measure for categorizing individuals based on their body weight relative to height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it provides a quick estimate of whether a person has a healthy body weight.

BMI is used by the World Health Organization (WHO), healthcare providers, and insurance companies as an initial screening tool. However, it does not measure body fat directly, nor does it distinguish between fat mass and lean muscle mass. Athletes, elderly individuals, and pregnant women may need alternative assessments.

WHO BMI Categories

  • Underweight: BMI below 18.5
  • Normal weight: BMI 18.5 – 24.9
  • Overweight: BMI 25.0 – 29.9
  • Obese: BMI 30.0 and above

BMI Formula

Metric Formula

BMI = Weight (kg) ÷ Height (m)²

Imperial Formula

BMI = [Weight (lbs) ÷ Height (in)²] × 703

Both formulas yield the same result — the unit conversion factor of 703 bridges the difference between metric and imperial systems.

Step-by-Step BMI Calculation

Scenario: A person weighing 70 kg and standing 175 cm tall.

Step 1: Convert Height to Meters

175 cm ÷ 100 = 1.75 m

Step 2: Square the Height

1.75 × 1.75 = 3.0625 m²

Step 3: Divide Weight by Height²

70 ÷ 3.0625 = 22.9

Result

A BMI of 22.9 falls in the Normal weight range (18.5 – 24.9). This suggests a healthy body weight for this height, though individual health assessments should also consider factors like body composition, fitness level, and medical history.

Related Calculators

If you are checking health metrics and eligibility, use our Age Calculator for exact age in years, months, and days.

People Also Ask

What is BMI and how is it calculated?
BMI (Body Mass Index) is a screening measure calculated as weight in kilograms divided by height in meters squared: BMI = kg / m². It provides a quick estimate of body fat based on height and weight, used by the WHO and medical professionals globally.
What are the BMI categories?
The WHO classifies BMI as: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). These thresholds help assess health risk, though BMI does not distinguish between muscle and fat mass.
Is BMI accurate for athletes or muscular individuals?
BMI can overestimate body fat in athletes or muscular individuals because it does not differentiate between lean muscle mass and fat. If you have a high BMI but are highly active, consider complementary measures like body fat percentage or waist-to-hip ratio.
What is a healthy BMI for my age?
For adults (18+), a healthy BMI is generally between 18.5 and 24.9 regardless of age. However, older adults may be advised to maintain a slightly higher BMI (25-27) to protect against bone density loss and illness reserve. Always consult a healthcare provider for personalized advice.