Body Mass Index Calculator
Our human physiology calculator computes body mass index accurately. Enter measurements for results with formulas and error analysis.
Formula
BMI = weight (kg) / height (m)^2
BMI is calculated by dividing body weight in kilograms by the square of height in meters. WHO categories: Underweight < 18.5, Normal 18.5-24.9, Overweight 25-29.9, Obese I 30-34.9, Obese II 35-39.9, Obese III 40+. BMI Prime = BMI / 25 (values above 1.0 indicate overweight).
Worked Examples
Example 1: Normal Weight BMI Calculation
Problem: Calculate BMI for a 65 kg person who is 170 cm tall.
Solution: BMI = weight / height^2\nBMI = 65 / (1.70)^2\nBMI = 65 / 2.89\nBMI = 22.5\nCategory: Normal Weight (18.5-24.9)
Result: BMI = 22.5 (Normal Weight, low health risk)
Example 2: Overweight BMI with Ideal Weight
Problem: A 90 kg person is 175 cm tall. Calculate BMI and how much weight to reach normal range.
Solution: BMI = 90 / (1.75)^2 = 90 / 3.0625 = 29.4\nCategory: Overweight (25-29.9)\nIdeal weight upper limit: 24.9 x (1.75)^2 = 24.9 x 3.0625 = 76.3 kg\nWeight to lose: 90 - 76.3 = 13.7 kg
Result: BMI = 29.4 (Overweight), need to lose 13.7 kg to reach normal range
Frequently Asked Questions
What is Body Mass Index (BMI)?
Body Mass Index is a numerical value calculated from a person's weight and height using the formula BMI = weight(kg) / height(m)^2. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the WHO as a simple screening tool for categorizing weight status. BMI ranges are: underweight (below 18.5), normal weight (18.5-24.9), overweight (25-29.9), and obese (30+). While BMI is widely used due to its simplicity, it does not directly measure body fat and has known limitations for athletes, elderly individuals, and different ethnic populations.
How is body fat percentage estimated from BMI?
The Deurenberg formula estimates body fat from BMI: Body Fat % = (1.20 x BMI) + (0.23 x Age) - 16.2 for males, or - 5.4 for females. This formula explains about 80% of the variance in actual body fat as measured by underwater weighing or DEXA scans. However, it has limitations: it overestimates body fat in muscular individuals and underestimates it in the elderly. Healthy body fat ranges are typically 8-19% for men and 21-33% for women, though these vary by age. Direct measurement methods like DEXA, hydrostatic weighing, or bioelectrical impedance provide more accurate results.
What is the difference between mass and weight?
Mass measures the amount of matter in an object (kilograms) and is constant everywhere. Weight is the gravitational force on that mass (newtons) and varies by location. On Earth, a 1 kg object weighs about 9.8 newtons. In everyday use, the terms are often used interchangeably.
How accurate are the results from Body Mass Index Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.