The BMI Formula

by Simon Hartley

Imperial Formula

BMI =
(weight in pounds * 703)


height in inches²

A guide with examples are shown below

Metric Formula

BMI =
(weight in kilograms)


height in meters²

A guide with examples are shown below

In this article I will show you how to calculate your BMI and how to find your weight category. BMI is a measurement which determines which weight category a person belongs to. Depending on their height and weight, a person can belong to one of the following weight categories:

  • underweight (BMI less than 18.5)
  • normal weight (BMI between 18.5 & 24.9)
  • overweight (BMI between 25.0 & 29.9)
  • obese (BMI 30.0 and above)

If your math is a little rusty, that’s ok. Calculating BMI is straight forward, the formula is easy to compute and I will provide step by step examples.

In fact, the simplicity of this formula was part of the reason for its success (and also it's main criticism).

It was developed by Belgium Statistician Adolphe Quetelet approximately 150 years ago. This was before the era of electronic calculators; any formula used to indicate weight status needed to be easy for physicians to manually calculate.

Only the height and weight values of a person are needed in the equation.

Adolphe Quetelet Belgium Statistician
Adolphe Quetelet: Creator Of The BMI Formula

How To Calculate BMI

Metric Method

Metric Body Mass Index Formula

The metric formula accepts height measurements in meters and weight in kilograms. If you know your height in centimeters only, simply divide the number of centimeters by 100 convert it to meters.

For example, a person who is 183cms tall is 1.83m tall (183cm / 100 = 1.83m).

Using the metric formula is even easier than the imperial method as it's a two step process

  1. Multiply your height by itself.
  2. Divide your weight in kilograms by the value calculated in step 1.

The resulting number is your BMI. Compare this BMI value with the weight status table below.

Example:

Paul weighs 150kgs and is 1.8m tall. He wants to know if he is overweight.

1. First we multiply Paul's height by itself: 1.8 x 1.8 = 3.24 ²

Next we divide Paul's weight by his height in meters ² just calculated: 150 / 3.24 = 46.3

Paul's BMI is 46.3


We compare this value to the weight categories listed on the BMI table and find that he is obese.

 

Imperial (US) Method

Imperial Body Mass Index Formula

The imperial formula accepts height measurements in inches and weight in pounds. It's popular in the US where the imperial system is mostly used. Many people know their height in feet and inches, but not in inches only.

If this applies to you, we need to convert your height into inches so we can use it in the equation. There are 12 inches in a foot, so multiply your number of feet by 12 and add them to the number of extra inches.

For example, if your height is 5 feet 10 inches, multiply 5 by 12 (which gives 60") and add them to the extra 10 inches (which gives 70").

Now we have the right measurements we can use them in the formula.

There are three simple steps for computing BMI with imperial values:

  1. Multiply your weight in pounds by 703.
  2. Multiply your height in inches by itself
  3. Divide the figure from step 1 by the figure in step 3.

The resulting number is your BMI. Compare this BMI value with the weight status table below.

Example:

Jane weighs 150lbs and is 5 feet 4 inches tall. She wants to know if she is overweight.

Jane's height in inches is (5 * 12) + 4 = 66"

1. Using the first part of the formula we multiply her weight by 703. 150 * 703 = 105450

2. Using the second part of the formula we multiply Jane's height by itself. 66 x 66 = 4356

3. Finally we divide the first figure by the second. 105450 / 4356 = 24.21

Jane's BMI is 24.21


We compare this value to the weight categories listed on the BMI table and find that she is of a normal weight.

BMI Weight Status Categories

The weight status categories opposite, are those currently used by the Centers for Disease Control and Prevention (CDC). They are suitable for adults who have stopped growing.

For children and teens who are still growing, the CDC base the weight categories on a BMI percentile. A child's weight status is based on where their BMI value lies compared to children of a similar age.

BMI Weight Categories
BMI Weight Status
Below 18.5 Underweight
18.5 - 24.9 Normal
25 - 29.9 Overweight
30.0 + Obese

Limitations Of The BMI Formula

There's no question that the body mass index calculation has been useful for some physicians.

However, since it's creation many have stated that this method of calculating BMI is not fit for purpose.

BMI does not take into account other factors which may affect a persons height or weight:

  • Body builders and athletes with a high proportion of muscle mass.
  • The elderly (whose height and muscle mass may fall over time), frail people, and those affected by certain illnesses.
  • Lactating or pregnant women.
  • Children and teenagers who have not reached physical maturity and are still growing.
  • A tendency for natural differences in height and weight ratios between races.