The BMI formula uses your weight (in kg or pounds) and your height (in meters or inches) to form a simple calculation that provides a measure of your body fat. The formula for BMI was devised in the 1830s by Belgian mathematician Adolphe Quetelet and is universally expressed in kg/m 2 .
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.
BMI = weight (kg) / [height (m) ] 2
BMI = 703 × weight (lbs) / [height (in) ] 2
Let's go through a couple of examples to demonstrate how these formulae work.
These simple steps will help you work out your BMI:
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)
BMI = 75 ÷ 3.24
This gives us a BMI figure of 23.15.
Simple steps to work out your imperial BMI:
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.
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.
Note that the results given by the Interactive BMI Formula should be used only as a guide and should not replace medical advice.
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 | Very severely obese |
For a full chart of BMI values, customised for your own height and weight, see my interactive BMI chart.
