How to Calculate BMI Manually (Body Mass Index)

Body Mass Index (BMI) is a widely used screening tool that helps evaluate whether a person has a healthy body weight for their height. While there are countless tools available online, including our own highly accurate BMI Calculator, knowing how to do the math manually can be quite empowering.

The Metric Formula for BMI

For those using the metric system, the formula is delightfully straightforward. You simply need your weight in kilograms and your height in meters.

Step-by-step example:

1. Let's say you weigh 75 kilograms.
2. Let's say you are 1.80 meters tall.
3. First, multiply your height by itself: 1.80 × 1.80 = 3.24.
4. Next, divide your weight by that number: 75 / 3.24 = 23.1.

Congratulations, your BMI is 23.1!

The Imperial Formula for BMI

If you live in the United States or otherwise use pounds and inches, the formula changes slightly. You must add a conversion factor (703) to account for the difference in measurement units.

Step-by-step example:

1. Let's say you weigh 160 pounds.
2. Let's say your height is 5 feet 8 inches. First, convert this entirely to inches: (5 × 12) + 8 = 68 inches.
3. Multiply your height in inches by itself: 68 × 68 = 4,624.
4. Divide your weight by this number: 160 / 4,624 = 0.034602...
5. Finally, multiply by 703: 0.034602 × 703 = 24.3.

Understanding Your Results

Now that you have your number, what does it mean? The World Health Organization defines the following categories:

• Underweight: BMI less than 18.5
• Normal weight: BMI 18.5 to 24.9
• Overweight: BMI 25.0 to 29.9
• Obesity: BMI 30.0 or higher

While doing it manually is a great exercise, you can always save time and ensure absolute precision by using the Calcodo BMI Calculator. You can also figure out what your target optimal weight range should be using our Ideal Weight Calculator.