What is the BMI formula?

The BMI formula is: BMI = weight (kg) / height (m)². For imperial units: BMI = (weight in pounds × 703) / height in inches².

Can I calculate BMI without a calculator?

Yes, you can calculate BMI manually using the formula. You just need your weight and height measurements and basic math skills.

Why is 703 used in the imperial BMI formula?

The number 703 is a conversion factor used to make the imperial formula equivalent to the metric formula, ensuring consistent BMI values regardless of the measurement system used.

How Do You Calculate BMI Calculator

Published on January 1, 2025•4 min read•Tutorial

Understanding how to calculate BMI manually is valuable knowledge that helps you comprehend what BMI calculators are actually doing behind the scenes. This guide will teach you the mathematical formula and walk you through step-by-step calculations.

The BMI Formula Explained

BMI (Body Mass Index) is calculated using a simple mathematical formula that relates your weight to your height. There are two versions of the formula depending on your measurement system:

BMI Formulas

Metric System:

BMI = weight (kg) ÷ height (m)²

Imperial System:

BMI = (weight (lbs) × 703) ÷ height (inches)²

Step-by-Step Calculation: Metric System

Let's calculate BMI for someone who weighs 70 kg and is 1.75 meters tall:

Example: Metric Calculation

Given: Weight = 70 kg, Height = 1.75 m

Step 1: Square the height: 1.75² = 1.75 × 1.75 = 3.0625

Step 2: Divide weight by height squared: 70 ÷ 3.0625 = 22.86

Result: BMI = 22.86

Step-by-Step Calculation: Imperial System

Now let's calculate BMI for someone who weighs 154 pounds and is 5 feet 9 inches tall:

Example: Imperial Calculation

Given: Weight = 154 lbs, Height = 5'9" (69 inches)

Step 1: Convert height to inches: 5 × 12 + 9 = 69 inches

Step 2: Square the height: 69² = 69 × 69 = 4,761

Step 3: Multiply weight by 703: 154 × 703 = 108,262

Step 4: Divide by height squared: 108,262 ÷ 4,761 = 22.73

Result: BMI = 22.73

Why 703 in the Imperial Formula?

The number 703 is a conversion factor that ensures the imperial formula produces the same BMI value as the metric formula. It's derived from the conversion between pounds and kilograms, and inches and meters, making the results consistent regardless of which measurement system you use.

Common Calculation Mistakes to Avoid

Forgetting to square the height: Height must be multiplied by itself, not just doubled
Using wrong units: Make sure weight and height units match the formula you're using
Decimal errors: Be careful with decimal places, especially when converting measurements
Mixing measurement systems: Don't use pounds with meters or kilograms with inches
Rounding too early: Keep more decimal places during calculation and round only the final result

Converting Between Measurement Systems

If you need to convert between metric and imperial measurements:

Conversion	Formula
Pounds to Kilograms	kg = lbs ÷ 2.205
Kilograms to Pounds	lbs = kg × 2.205
Inches to Meters	m = inches ÷ 39.37
Meters to Inches	inches = m × 39.37

Practice Problems

Try calculating BMI for these examples:

Practice Examples

Example 1: Weight: 65 kg, Height: 1.60 m

Example 2: Weight: 180 lbs, Height: 6'0"

Example 3: Weight: 55 kg, Height: 1.65 m

Answers: 1) BMI = 25.39, 2) BMI = 24.41, 3) BMI = 20.20

When Manual Calculation is Useful

Understanding how BMI calculators work
Verifying calculator results
Educational purposes and teaching
When digital calculators aren't available
Building confidence in the BMI concept

Skip the Math - Use Our Calculator

While knowing how to calculate BMI manually is valuable, our free BMI calculator does all the math for you instantly and accurately.

Use BMI Calculator