# BMI Calculator

Calculate your **body mass index (BMI)** with this handy, free **body weight calculator tool**. By calculating your body mass index, you can get an indication of whether your weight may be affecting your health. BMI is commonly used by Doctors and health professionals worldwide and is calculated using your weight and height. You have two calculators to choose from - metric and imperial.
Learn about BMI

## BMI Classifications (International)

#### BMI of less than 18.5kg/m^{2}

A BMI of less than 18.5 indicates that you are underweight, so you may need to put on some weight. You are recommended to ask your Doctor or a dietician for advice.

#### BMI of 18.5 - 25kg/m^{2}

A BMI of 18.5 - 25 indicates that you are at a healthy weight for your height. By maintaining a healthy weight, you lower your risk of developing serious health problems.

#### BMI of 25 - 30kg/m^{2}

A BMI of 25 - 30 indicates that you are slightly overweight. You may be advised to lose some weight for health reasons. You are recommended to talk to your Doctor or a dietician for advice.

#### BMI of over 30kg/m^{2}

A BMI of over 30 indicates that you are heavily overweight. Your health may be at risk if you do not lose weight. You are recommended to talk to your Doctor or a dietician for advice.

#### 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.

## What Is Nick Trefethen's BMI Calculation Method?

Nick Trefethen is a Professor of Numerical Analysis at Oxford University's Mathematical Institute. In early 2013 he wrote to the Economist advocating an alternative formula for BMI. He suggests that the existing formula is flawed, causing confusion and misinformation. He commented:

"If all three dimensions of a human being scaled equally as they grew, then a formula of the form weight/height

^{3}would be appropriate. They don't! However, weight/height^{2}is not realistic either.A better approximation to a complex reality, which is the reform I wish could be adopted, would be weight/height

^{2.5}. Certainly if you plot typical weights of people against their heights, the result comes out closer to height^{2.5}than height^{2}."

You can read more about this in Medical News Today's article, Is The Body Mass Index Formula Flawed? and this BBC article.

**This BMI calculator includes an option to show the results from both methods**. Simply check the box for 'Include Trefethen formula calculation'.

## What Is The Formula For BMI?

The official formula for BMI was was devised in the 1830s by Belgian mathematician Lambert Adolphe Jacques Quetelet (1796-1874).

#### The official formula for BMI:

**Metric BMI** = ( weight (kg) / ( height (m)^{2} ) )

**Imperial BMI** = ( weight (lb) / ( height (in)^{2}) ) x 703

#### Nick Trefethen's unofficial BMI formula:

**Metric BMI** = (1.3 x weight (kg) ) / (height (m)^{2.5})

**Imperial BMI** = (5734 x weight (lb) ) / (height (in)^{2.5})

## How Much Should I Weigh?

When determining your **ideal body weight**, several factors should be considered. These include age, muscle-fat ratio, height, sex, and bone density. Some people say your Body Mass Index (BMI) is the best way to calculate whether your body weight is ideal. Others say BMI is faulty as it does not take into account muscle mass and suggest using a measurement of waist-hip ratio instead.

You can read the rest of this article at Medical News Today: How Much Should I Weigh?