How to Calculate BMI: Step-by-Step Guide

Body Mass Index (BMI) is a numerical value derived from your weight and height that provides a standardized measure of body composition.

Whether you’re tracking fitness progress, preparing for a medical appointment, or simply curious about your current measurements, knowing how to calculate BMI yourself gives you immediate access to this baseline metric without relying on online tools.

Understanding the calculation process helps you verify results, troubleshoot potential errors, and gain confidence in the numbers you’re working with. The math itself is straightforward, but getting accurate results requires attention to detail with units, conversions, and proper formula application.

Why Knowing How to Calculate BMI Matters

Learning to calculate BMI manually serves several practical purposes beyond basic curiosity. Healthcare providers often request BMI values during appointments, and being able to calculate yours beforehand saves time and ensures accuracy.

Fitness enthusiasts frequently track BMI alongside other metrics to monitor body composition changes over time.

The calculation skill becomes particularly valuable when you need quick assessments without internet access or when double-checking results from digital calculators.

Insurance applications, employment physicals, and sports participation forms commonly require BMI information, making personal calculation ability a useful life skill.

Additionally, understanding the math behind BMI helps you recognize when results seem unusual or potentially incorrect, allowing you to recalculate or seek clarification when needed.

BMI Formula (Metric System)

The metric BMI formula is: BMI = weight (kg) ÷ height² (m)

This calculation requires your weight in kilograms and your height in meters. The height must be squared (multiplied by itself) before dividing your weight by this number.

Step-by-Step Example:

• Weight: 70 kg
• Height: 1.75 m

1. Square your height: 1.75 × 1.75 = 3.0625 m²
2. Divide weight by height squared: 70 ÷ 3.0625 = 22.86
3. Your BMI is 22.86

Another Example:

• Weight: 85 kg
• Height: 1.80 m

1. Square your height: 1.80 × 1.80 = 3.24 m²
2. Divide weight by height squared: 85 ÷ 3.24 = 26.23
3. Your BMI is 26.23

The metric system typically produces the most straightforward calculations since the formula doesn’t require additional conversion factors.

BMI Formula (Imperial System)

The imperial BMI formula is: BMI = (weight (lbs) × 703) ÷ height² (in)

This calculation uses pounds for weight and inches for height, with the multiplication factor of 703 converting the result to the standard BMI scale.

Step-by-Step Example:

• Weight: 154 lbs
• Height: 69 inches (5’9″)

1. Multiply weight by 703: 154 × 703 = 108,262
2. Square your height: 69 × 69 = 4,761 inches²
3. Divide the first result by height squared: 108,262 ÷ 4,761 = 22.74
4. Your BMI is 22.74

Another Example:

• Weight: 180 lbs
• Height: 72 inches (6’0″)

1. Multiply weight by 703: 180 × 703 = 126,540
2. Square your height: 72 × 72 = 5,184 inches²
3. Divide the first result by height squared: 126,540 ÷ 5,184 = 24.41
4. Your BMI is 24.41

The 703 conversion factor ensures imperial measurements produce the same BMI values as metric calculations for identical body measurements.

Convert Between Metric and Imperial Units

Converting between measurement systems ensures accuracy regardless of which units you’re more familiar with. Here are the essential conversions:

Weight Conversions:

• 1 kg = 2.205 lbs
• 1 lb = 0.453 kg

Height Conversions:

• 1 meter = 39.37 inches
• 1 inch = 0.0254 meters
• 1 foot = 12 inches = 0.3048 meters

Quick Reference Table:

When converting, round to one decimal place for practical purposes. Most BMI calculations don’t require precision beyond this level for general use.

Online BMI Calculators

Digital BMI calculators eliminate manual math and reduce calculation errors, making them ideal for quick assessments. These tools automatically handle unit conversions and provide instant results when you input your measurements.

Try our free BMI calculator for immediate results with both metric and imperial unit options. The calculator includes built-in error checking and provides formatted results suitable for record-keeping.

Common Mistakes When Calculating BMI

Online calculators prove especially useful when calculating BMI for multiple people or when working with unusual height-weight combinations that might complicate manual calculations.

Several calculation errors occur frequently when computing BMI manually. The most common mistake involves incorrectly squaring the height measurement.

Wrong Example:

• Weight: 70 kg, Height: 1.75 m
• Incorrect: 70 ÷ (1.75 × 2) = 70 ÷ 3.5 = 20.00

Correct Example:

• Weight: 70 kg, Height: 1.75 m
• Correct: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86

Other frequent errors include mixing metric and imperial units within the same calculation, forgetting the 703 multiplier in imperial calculations, and using height in centimeters instead of meters for metric calculations.

Unit confusion represents another significant error source. Always verify that your weight and height measurements match your chosen formula before calculating.

When to Double-Check Your BMI Calculation

Unexpected BMI results warrant recalculation to ensure accuracy. If your calculated BMI differs significantly from previous measurements or seems inconsistent with your physical appearance, review your math and measurement inputs.

Common red flags include BMI values below 15 or above 40 for typical adults, which may indicate calculation errors rather than actual measurements.

Similarly, if your BMI changes dramatically between calculations without corresponding weight or height changes, verification becomes necessary.

When results seem questionable, try calculating with both metric and imperial formulas to confirm consistency. Learn more about BMI accuracy limitations to understand when the measurement itself might not reflect your situation accurately.

Quick Reference Table

Key Reminders:

• Always square the height measurement
• Use 703 multiplier for imperial calculations only
• Convert units before calculating if mixing systems
• Round final results to one decimal place

Frequently Asked Questions

How do I calculate BMI manually?

Calculate BMI by dividing your weight by your height squared. For metric units, use BMI = kg ÷ m². For imperial units, use BMI = (lbs × 703) ÷ in². Always square your height first, then divide your weight by this number.

Why does the imperial formula use 703?

The 703 multiplier converts imperial measurements (pounds and inches) to produce the same BMI scale as metric calculations. Without this factor, imperial measurements would yield different numerical results despite identical body composition.

Should I use metric or imperial for BMI?

Use whichever measurement system matches your available data. Both formulas produce identical results when applied correctly. Metric calculations are often simpler since they don’t require the 703 conversion factor.

What’s the difference between BMI calculated in kg/m² and lbs/in²?

BMI is always expressed in kg/m² regardless of your input measurements. The imperial formula’s 703 multiplier converts pounds and inches to produce results on the same kg/m² scale as metric calculations.

How accurate should my height and weight measurements be?

Measure weight to the nearest pound or half-kilogram and height to the nearest inch or centimeter. BMI calculations are generally rounded to one decimal place, so extreme precision in initial measurements isn’t necessary.

Can I calculate BMI if I only know my height in feet and inches?

Yes, convert feet and inches to total inches first. Multiply feet by 12, then add remaining inches. For example, 5’9″ becomes (5 × 12) + 9 = 69 inches total.

What if my calculated BMI seems wrong?

Double-check your math, especially the height-squaring step. Verify you’re using the correct formula for your measurement units. If results still seem unusual, try calculating with converted units or use an online calculator for verification.

Do I need to account for clothing weight when calculating BMI?

For casual calculations, clothing weight typically doesn’t significantly affect BMI results. However, for medical or official purposes, weigh yourself without shoes and in minimal clothing for the most accurate measurements.

Ready to skip the math entirely? Try our free BMI calculator for instant, accurate results. Want to understand what your BMI number actually means? Learn more about what BMI means for your health and fitness goals.