**Creating user-defined Excel functions (UDFs) – Temperature Conversion and BMI Calculation functions from scratch.**

Our last two posts focused on using, or working with somebody else’s VBA code. This article will focus on creating UDF functions from scratch. We will start with a simple temperature conversion from Celsius to Fahreheit, and vice versa. We will then calculate metric Body Mass Index (BMI), graduating to an imperial BMI calculation with 3 variables.

Since we’ve already worked with VBA code, we know some basic syntax rules for our future functions:

1. Start out with ` Option Explicit`

statement.

*Note *:**Option Explicit** statement forces explicit declaration of our variables, and gives us an error message if we have a typo instead of performing incorrect calculations.

2. Define our function name and its parameter(s)

` Function FunctionName(Parameter DataType) As DataType`

.

** Note :** VBA makes definining data types optional, but it is a good habit to have.

3. Perform our calculations:

` Your code goes here`

.4. Exit function and return it’s value:

` End Function`

.