Fitness Education

BMI Formula Explained: How to Calculate It Manually

Understanding the math behind the Body Mass Index (BMI). Whether you use kilograms or pounds, here is exactly how to calculate it.

Skip the math?

Use our free instant calculator to get your BMI score and see a visual prediction of your future body.

What is the BMI Formula?

The Body Mass Index (BMI) is a simple screening tool widely used to categorize weight—underweight, healthy weight, overweight, or obesity. It's strictly a mathematical calculation based on your height and weight.

🌍 Metric System (kg/m²)

Most of the world uses this simple formula.

BMI = weight (kg) / [height (m)]²

Example:

Weight: 70 kg
Height: 1.75 m
Calculation: 70 ÷ (1.75 × 1.75) = 22.86

🇺🇸 Imperial System (lbs/in²)

Used in the US. Requires a conversion factor of 703.

BMI = 703 × weight (lbs) / [height (in)]²

Example:

Weight: 160 lbs
Height: 5'9" (69 inches)
Calculation: 703 × 160 ÷ (69 × 69) = 23.6

Why does the formula matter?

While the formula is simple, it doesn't account for muscle mass, bone density, or fat distribution. A bodybuilder might have the same BMI as someone who is overweight, even though their body compositions are completely different.

Don't just look at the number.

Your BMI score is just a starting point. Want to see what you would actually look like at your ideal weight? Our AI simulates your transformation instantly.

How to improve your BMI

If your BMI falls outside the "Normal" range (18.5 – 24.9), small changes can make a big difference. The most effective way to reach a healthy BMI is a sustainable combination of nutrition and movement.

Whether you are considering tracking your progress or exploring options like GLP-1 medications, knowing your starting point is key.