Writing DAX functions

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Power BI Desktop updates for September 2025 Part three of a four-part series of blogs
The big changes in this month's update are two preview features which will change the working lives of DAX authors when released: a new way of creating calendars and the ability to write functions in DAX. A summary of the September 2025 Power BI updates The enhanced DAX time intelligence calendar features in Power BI Writing DAX functions (this blog) Button, list and text slicers

Power BI Desktop updates for September 2025

Part three of a four-part series of blogs

The big changes in this month's update are two preview features which will change the working lives of DAX authors when released: a new way of creating calendars and the ability to write functions in DAX.

A summary of the September 2025 Power BI updates
The enhanced DAX time intelligence calendar features in Power BI
Writing DAX functions (this blog)
Button, list and text slicers

Posted by Andy Brown on 25 September 2025

In this blog

Enabling the preview feature
Creating a function
An example of a function

DAX has many built-in functions, but now you can add your own!

When testing this feature I encountered numerous bugs. You seem to have to enter all of the syntax on a single line, I got strange syntax errors despite my code being correct (and accepted) and I couldn't get creating functions in DAX query editor to work at all. It's good to know that DAX functions will be available soon, but I wouldn't enable them just yet.

Enabling the preview feature

Before you can create functions in DAX you first need to enable them from the File -> Options and settings -> Option dialog box:

The option to make DAX functions work (it requires a restart).

Creating a function

There are several ways to create functions in DAX, apparently, but I couldn't get DAX query view to work. So for now to create a function, switch to TMDL view:

The icon to switch to TMDL view.

Type in your function and apply your changes:

This pointless (and badly named) function doubles any number (see below for a more useful example of a function).

When I tried this in my version of Power BI Desktop I got this message:

Power BI uses an Analysis Services database behind the scenes, and it is this that needs updating - but why didn't this happen automatically as part of the monthly update?

When you click on Yes, you will then see your function as part of your semantic model:

You can expand Functions to see any DAX functions you've written.

An example of a function

Suppose you frequently want to calculate the value of a sale by multiplying the quantity of goods sold by the price. You might create this function in TDML view:

createOrReplace

/// a function to return the sales value as quantity multipled by price

function SalesValue = (Quantity : int64, Price : numeric) => Price * Quantity

So this function takes two arguments:

Argument	Data type	What it contains
Quantity	Integer	The number of items bought
Price	Any number	The price of each

The function then returns the price multiplied by the quantity. I could then use this in (for example) a measure:

The names of the two functions I've created appear in Intellisense.

Here for example is a final measure:

Total sales = SUMX(

Sales,

// take quantity * price

SalesValue(

Sales[Quantity],

Sales[Price]

)

I could then show this measure in a visual:

The measure above is showing total sales for each region.

The above example uses a simple scalar function, but DAX functions can also take tables as arguments. You can see much more details about creating DAX functions at this Microsoft blog, but Wise Owl will avoid using them until they are a bit smoother to use!

Parts of this blog
A summary of the September 2025 Power BI updates The enhanced DAX time intelligence calendar features in Power BI Writing DAX functions (this blog) Button, list and text slicers

Some other pages relevant to the above blogs include:

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