Metric BMI formula
BMI = weightKg / (heightM * heightM)
Divide weight in kilograms by height in metres squared. For example, 70 kg and 1.75 m gives 70 / (1.75 * 1.75), which is about 22.9.
US BMI formula
BMI = 703 * weightLb / (heightIn * heightIn)
Multiply weight in pounds by 703, then divide by height in inches squared. For example, 165 lb and 69 inches gives about 24.4.
Why the US formula uses 703
BMI was originally expressed using kilograms and metres. Pounds and inches are different units, so the US formula includes 703 as a conversion factor. The goal is to produce the same type of BMI score no matter which unit system you use.
A calculator helps avoid conversion mistakes, especially when height is entered as feet and inches. The final BMI number is usually rounded to one decimal place for easier reading.
After calculating BMI
Once you have a BMI score, compare it with adult BMI categories. Below 18.5 is usually underweight, 18.5 to 24.9 is the healthy BMI range, 25 to 29.9 is overweight, and 30 or higher is obesity. These categories are screening ranges and should be interpreted with context.
Medical disclaimer
This calculator is for general informational purposes only and does not replace professional medical advice. Always consult a qualified healthcare provider for personal health concerns.