Cat On A Spreadsheet

Cat On A Spreadsheet

Setting Up Event-Based Calculation Logic with VBA

Excel’s default calculation mode is input-driven—when a cell changes, Excel recalculates all dependent formulas. This works well for smaller models, but large workbooks can become painfully slow when hundreds of formulas recalculate unnecessarily. Event-based calculation logic lets you take control of when calculations occur. Instead of letting Excel decide, you can trigger calculations only when specific cells, sheets, or conditions change.

 

Why Use Event-Based Calculations?

 

With event-based logic, you can:

  • Prevent unnecessary recalculations in large workbooks.

  • Run complex formulas only when needed, reducing wait times.

  • Make models more predictable by avoiding volatility.

  • Integrate with VBA macros for advanced processing.

 

The Core Idea

 

The goal is to replace constantly updating formulas with VBA code that recalculates values only when specific triggers (events) occur.

 

Events you can use in VBA include:

  • Worksheet_Change — when a cell’s value changes.

  • Worksheet_Calculate — after a recalculation finishes.

  • Workbook_Open — when the file opens.

  • Workbook_SheetChange — when a change happens on any sheet.

 

Example 1: Calculating Only When a Cell Changes

 

Suppose you have a calculation that depends on B2 but it’s slow to run. Instead of using a formula, we can use VBA to trigger it only when B2 changes.

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("B2")) Is Nothing Then
     Application.EnableEvents = False
     Range("C2").Value = Range("B2").Value * 1.25
     Application.EnableEvents = True
End If
End Sub

 

How it works:

  • Intersect checks if the changed cell overlaps B2.

  • If yes, it temporarily disables events (to avoid loops), does the calculation, then re-enables them.

 

Example 2: Conditional Calculations

 

Let’s say we only want to run a calculation when a cell is changed and a specific checkbox is ticked.

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("B2")) Is Nothing And Range("E1").Value = TRUE Then
     Application.EnableEvents = False
     Range("C2").Value = Range("B2").Value * Range("D2").Value
     Application.EnableEvents = True
End If
End Sub

 

Now, the calculation only runs if:

  • B2 changes

  • The value in E1 is TRUE

 

Example 3: Workbook-Level Event Calculation

 

You can also place event code at the workbook level to run calculations based on changes anywhere.

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
If Not Intersect(Target, Sh.Range("A1")) Is Nothing Then
     Sh.Range("B1").Value = Sh.Range("A1").Value ^ 2
End If
End Sub

 

This example updates B1 on whichever sheet’s A1 changes.

 

Example 4: Delayed Calculations for Efficiency

 

Sometimes you don’t want to run the calculation immediately—especially if the user might change multiple related cells. You can delay execution using Application.OnTime.

Dim NextRun As Double
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("B2:B10")) Is Nothing Then
     On Error Resume Next
     Application.OnTime EarliestTime:=NextRun, Procedure:="RunMyCalculation", Schedule:=False
     NextRun = Now + TimeValue("00:00:02")
     Application.OnTime EarliestTime:=NextRun, Procedure:="RunMyCalculation", Schedule:=True
End If
End Sub
Sub RunMyCalculation()
Range("C2").Value = WorksheetFunction.Sum(Range("B2:B10"))
End Sub

 

This waits 2 seconds after the last edit before running the calculation—great for data entry scenarios.

 

Best Practices for Event-Based Calculation Logic

 

  • Always disable events (Application.EnableEvents = False) before making changes inside event handlers, then re-enable them—otherwise you risk infinite loops.

  • Keep event code lightweight—if it’s too heavy, the workbook will lag during normal editing.

  • Combine with named ranges for easier maintenance.

  • Document the logic—future maintainers may wonder why certain formulas are missing from the sheet.

  • Test for edge cases—e.g., pasting multiple cells, undo operations, or clearing values.

 

Final Thoughts

 

Event-based calculation logic is one of the most effective ways to improve Excel performance in large, complex workbooks. By replacing volatile or constantly recalculating formulas with event-driven VBA, you can make spreadsheets faster, more responsive, and easier for your team to use.

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("B2")) Is Nothing Then
     Range("C2").Value = Date
End If
End Sub

 

This automatically adds a date in C2 whenever B2 is changed—no volatile =TODAY() formula required.

 

Bonus: Avoid Volatile UDFs

 

User Defined Functions (UDFs) can also be volatile if they:

  • Call NOW(), TODAY(), or other volatile functions internally

  • Use Application.Volatile=True explicitly

  • Refer to other volatile cells

 

Avoid writing UDFs that mimic volatile behavior unless absolutely necessary.

 

Final Thoughts

 

Rewriting volatile formulas doesn't mean eliminating all dynamic behavior—it means taking control of when and how your spreadsheet recalculates. By replacing volatile functions with static values, structured formulas, or VBA, you'll make your workbooks:

  • Faster

  • More stable

  • Easier to debug and maintain

 

11 August 2025

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Cat On A Spreadsheet

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