BMI Calculator (Metric and Imperial)

BMI icon

Use this BMI calculator tool to work out your body mass index and get an indication of whether your weight may be affecting your health. This metric and imperial BMI tool is intended for men and women of 18 years and over. If you need to convert units, we have a weight converter and height converter.


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Please note:

*The results given by this bmi calculator should be used only as a guide and should not replace medical advice. Please bear in mind that, when interpreting the results of this body mass index calculator, other factors such as muscle structure and ethnic origin should be considered. For example, a person with a lot of muscle might have a higher BMI, when their body is actually perfectly healthy, due to the fact that muscle weighs more than fat.

Always speak to a Doctor or health professional for advice and guidance before making any dramatic changes to your lifestyle.

How to calculate BMI

Should you wish to calculate your BMI manually, I've included some simple instructions and formulae below. It's important that if you use these, you ensure that your units are in the same unit system (metric or imperial) and the correct corresponding unit.

To work out your BMI (kg and m)

  1. Divide your weight (kg) by your height (m).
    Example: 65kg ÷ 1.83m = 35.519
  2. Divide your answer from part 1 by your height again.
    Example: 35.519 ÷ 1.83 = 19.41

To work out your BMI (pounds and inches)

  1. Divide your weight (lb) by your height (in).
    Example: 154lb ÷ 71in = 2.169
  2. Divide your answer from part 1 by your height again.
    Example: 2.169 ÷ 71 = 0.03055
  3. Multiply your answer from part 2 by 703.
    Example: 0.03055 × 703 = 21.48

BMI formula

The official formula for BMI was devised in the 1830s by Belgian mathematician Adolphe Quetelet.

Metric BMI Formula

BMI = weight (kg) / height2 (m2)

Imperial BMI Formula

BMI = ( weight (lb) / height2 (in2) ) x 703

You can learn more about how to calculate your BMI with these formulae in our in-depth article, how to calculate your BMI. An interactive BMI chart is also available to use.

What Is BMI?

The body mass index measurement is the calculation of your body weight in relation to your height and is commonly used as an indicator of whether you might be in a risk category for health problems caused by your weight. Using a BMI calculator such as the one on this page, you enter your weight and height measurements. Those figures are entered into the BMI formula (more on that below) to produce your current BMI reading.

The BMI classifications established by the World Health Organization (WHO) are as follows:

Body Mass Index (kg/m2) Classification
less than 18.5: underweight
18.5 - 24.9: normal weight
25 - 29.9: overweight
30 - 34.9: class I obese
35 - 39.9: class II obese
40 upwards: class III obese

It is important to remember that BMI measurements do not make a distinction between whether you are male or female, whether you have a large or small body frame, whether you have high muscle mass and the areas in which fat is distributed around your body. So, someone who has a very muscular physique (a body builder, for example) may have a high BMI without having excess fat. For this reason, BMI is not an indicator of obesity on its own.

Some alternative methods to BMI, including the waist-hip ratio, are discussed in our article, 5 alternatives to BMI. There's also a good section about BMI on the CDC website.

BMI calculations formula

BMR calculator

As well as BMI, this website also contains a BMR calculator that allows you to calculate your basal metabolic rate (the number of calories you might burn in a day if you are inactive).

If you have any questions about this calculator then please contact me.