BMI body mass index formula in a notepad.
BMI Calculator

Find out your BMI, and what it means, with our easy calculator.

BMI Calculator


Choose your measurement preference then enter your height and weight to calculate your BMI

Kilograms / Metres

Height m
Weight kg

Stone & Pounds / Feet and Inches

Height ft in
Weight st lb

Pounds / Feet and Inches

Height ft in
Weight lb

What is BMI?

BMI stands for body mass index; a formula which relates body weight to height. It is used by health professionals and scientists to determine the health implications of being a certain weight for your height.

It's also a handy number for individuals to know, or look up from time to time, to see how healthy their current weight is. Research has shown that people who monitor their weight are less likely to put weight on and more likely to lose it.

What Your BMI Calculation Means

Under 18.5 Underweight
18.5 - 25 Healthy Weight
25 - 30 Overweight
30 - 40 Obese
Over 40 Severely obese

The BMI Formula

Body mass index (BMI) = weight(kg) ÷ height(m)2

BMI is a number generated by dividing your weight in kilograms by your height in metres squared. (You can convert from stones and pounds to kilos here.)

The easiest way to work this out is to do the height multiplication first: height in meters x height in metres, then divide your weight in kilograms by the result.

Here's an example for a person who is 1.7m tall and weighs 64kg

1.7 x 1.7 = 2.89 (height x height)

64 ÷ 2.89 = 22.14 (weight ÷ height2)

So this person has a body mass index of 22.1

You can verify this result in the calculator to help the maths sink in! If you prefer seeing this type of in graphical form you can find a body mass index chart here.

Limitations of BMI as a Measure of Health

BMI is a broad measure and has 3 main limitations when it comes to determining healthy weight for an individual.

  1. People with a high ratio of muscle such as athletes and body builders can have a body mass index of more than 25 but still be a healthy weight. This is because BMI does not take into account the proportion of body fat a person has, and it is excess body fat that poses a risk to health, not excess muscle.
  2. For the same reason as in point 1, people can fall within a healthy weight range according to BMI, but still be carrying too much fat. This is especially true if fat has accumulated around the waist.  
  3. The healthy weight range applies to both men and women. Since men generally have a larger build and have more muscle than women, they naturally weigh more. If you are setting a personal weight goal it can be helpful to look at ideal weight ranges tailored for men or women


Want to monitor your weight?

Get a free professionally designed Weight Graph PDF with our fortnightly newsletter. Simply enter your first name and email (never shared).

Start a Free Trial Today

See the ideal weight range personal to you, set a Weight Loss goal, and see how many calories you need each day to get there - try the Weight Loss Resources Tools for free.

Free Trial »

Lose a Stone for Christmas Challenge

If you enjoyed this article, try our newsletter. It's free.

Receive the latest on what works for weight loss straight to your inbox. We won't share your email address. Privacy policy