How to calculate bmi

Body mass index is a measure of body fat and is commonly used within the health industry to determine whether your weight is healthy. BMI applies to both adult men and women and is the calculation of body weight in relation to height. This article delves into the BMI formula and demonstrates how you can use it to calculate your own BMI.

The first formula we’ve listed is the metric BMI formula, using kilograms and meters. The second one is the imperial BMI formula, which uses units of pounds and inches. Converters are available for kilos to stone, kilos to pounds and ounces and pounds, should you need them.

Metric BMI Formula

BMI = weight (kg) / [height (m) ] 2

Imperial BMI Formula

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

Let’s go through a couple of examples to demonstrate how these formulae work.

Using the BMI formula (metric units)

These simple steps will help you work out your BMI:

Example using formula

For an adult with height of 180 cm and weight of 75 kg. Our first step needs to be to convert the height into meters (British spelling: metres). As there are 100cm in a meter, we divide our figure by 100. This gives us 1.8m.

Let’s plug those figures into our formula:

BMI = 75 ÷ (1.8 × 1.8)

This gives us a BMI figure of 23.15.

Using the imperial BMI formula

Simple steps to work out your imperial BMI:

Example using formula

For an adult with height of 5ft 11 inches and weight of 155 pounds (lbs). Step one is to convert the height into inches only. There are 12 inches in a foot, so we simply multiply the 5ft by 12 and then add the 11 inches. This gives us a total of 71 inches.

Let’s plug those figures into our formula:

BMI = 703 × (155 ÷ (71 × 71))

We do the multiplication inside the brackets first:

BMI = 703 × (155 ÷ 5041)

BMI = 703 × 0.030747867

This gives us a BMI figure of 21.62.

Of course, should you not wish to calculate BMI manually with the formula, you can use my popular BMI calculator or the interactive BMI formula below.

Interactive BMI formula

Use the interactive BMI formula calculator below to demonstrate the formula and resulting BMI calculation for your chosen weight and height figures. Options for both metric and imperial units are available.

Please note that this calculator requires JavaScript to be enabled in your browser.

Metric BMI Formula

Note that the results given by the Interactive BMI Formula should be used only as a guide and should not replace medical advice.

BMI Categorization

The BMI statistical categories below are based on BMI scores and apply to adults of age 20 years and upwards. The World Health Organisation (WHO) regards a healthy adult BMI to be between 18.5 and 25.

BMI	BMI Category
Less than 15	Very severely underweight
Between 15 and 16	Severely underweight
Between 16 and 18.5	Underweight
Between 18.5 and 25	Normal (healthy weight)
Between 25 and 30	Overweight
Between 30 and 35	Moderately obese
Between 35 and 40	Severely obese
Over 40