You're right that there isn't a built-in VBA function for pi, but there are a few ways to get an accurate value for pi in VBA.

One option is to use the constant "4 * Atn(1)" which is a commonly used approximation for pi. This is essentially saying that pi is equal to 4 times the arctangent of 1. Here's an example of how you could use this in a VBA function:

Another option is to use the same pi() worksheet function that you mentioned in your question, but call it using the Application.WorksheetFunction object. Here's an example:

Both of these options should give you an accurate value for pi in your VBA function.

1. Use the constant "4 * Atn(1)" to get an approximation for pi.
2. Call the pi() worksheet function using the Application.WorksheetFunction object.