Stop Using Add-Ins for UDFs — Here's How to Add a User Defined Function in Excel Properly

Most Excel trainers tell you to 'install an add-in' to get custom functions. That’s like buying a new car just to replace a flat tire. You already have everything you need — right inside Excel’s Visual Basic Editor. No downloads. No admin rights. No version lock-in.

Quick Answer

You add a user defined function in Excel by opening the VBA editor (Alt+F11), inserting a new module (Alt+I+M), typing a Function…End Function block in VBA, then calling it like =MyFunction(A1) from any worksheet. It’s faster than installing an add-in — and works offline, on Mac or Windows.

All the Methods

Method	Steps	Best For	Limitations
VBA Module (Standard)	Alt+F11 → Insert → Module → Paste Function code → Save as .xlsm	Most users — reliable, portable, no setup	Requires macro-enabled workbook (.xlsm)
Excel LAMBDA (365/2021)	Formulas → Define Name → Enter LAMBDA() formula → Use =Name(A1)	Users with Microsoft 365 who avoid VBA	Not available in Excel 2019 or earlier; can’t call other LAMBDAs recursively without helper names
Add-in (.xlam)	Save VBA as .xlam → File → Options → Add-ins → Browse → Load	Teams needing centralized, shared UDFs across workbooks	Fails silently if macro security blocks it; hard to debug across users
COM Add-in (C#/VB.NET)	Build DLL → Register → Load via COM Add-ins dialog	Enterprise IT teams building secure, versioned functions	Requires dev tools, admin rights, and ongoing maintenance

Method 1 Deep Dive

We’ll build a UDF called DaysUntilPayday that calculates business days from today to the next 15th or last day of month — useful for payroll forecasting. Open your workbook (say, Payroll_Q2_2024.xlsx), then press Alt+F11.

In the Project Explorer (Ctrl+R if hidden), right-click VBAProject (Payroll_Q2_2024.xlsm) → Insert → Module. A blank window appears. Paste this:

Function DaysUntilPayday() As Long
    Dim TodayDate As Date, NextPayday As Date
    TodayDate = Date
    If Day(TodayDate) < 15 Then
        NextPayday = DateSerial(Year(TodayDate), Month(TodayDate), 15)
    Else
        NextPayday = DateSerial(Year(TodayDate), Month(TodayDate) + 1, 0)
    End If
    DaysUntilPayday = WorksheetFunction.NetworkDays(TodayDate, NextPayday)
End Function

Close the editor (Alt+Q). Back in Excel, type =DaysUntilPayday() in cell D2. It returns 7 if today is 2024-04-08. Try it in E5 too — same result, because it’s not volatile. (That’s the surprise: unlike NOW() or RAND(), UDFs don’t auto-recalculate unless their inputs change — but ours has no inputs, so you’ll need to force recalc with F9 after date changes.)

Now test with input: modify the function to accept a start date:

Function DaysUntilPayday(StartDate As Date) As Long
    ' Same logic, but uses StartDate instead of Date
    Dim NextPayday As Date
    If Day(StartDate) < 15 Then
        NextPayday = DateSerial(Year(StartDate), Month(StartDate), 15)
    Else
        NextPayday = DateSerial(Year(StartDate), Month(StartDate) + 1, 0)
    End If
    DaysUntilPayday = WorksheetFunction.NetworkDays(StartDate, NextPayday)
End Function

Now try =DaysUntilPayday(B2) where B2 contains 2024-03-22. It returns 22. Works across sheets — no setup needed.

Method 2 Deep Dive

LAMBDA is cleaner — but only if you’re on Microsoft 365. Let’s replicate the same logic without VBA. Go to Formulas → Define Name (or press Ctrl+F3). In the dialog:

Name: DaysUntilPayday_L
Scope: Workbook
Refers to: =LAMBDA(start_date, LET(next15,DATE(YEAR(start_date),MONTH(start_date),15), lastday,DATE(YEAR(start_date),MONTH(start_date)+1,0), payday,IF(DAY(start_date)<15,next15,lastday), NETWORKDAYS(start_date,payday)))

Click OK. Now use =DaysUntilPayday_L(B2) in C2. It returns 22, same as before. But here’s the catch: LAMBDA doesn’t support loops or error-handling like On Error Resume Next. So if B2 is blank or text, it throws #VALUE! — no graceful fallback.

Try this real-world example: in column A, list sales reps — Sarah Chen, Miguel Torres, Leila Park. In column B, their quarterly quota ($125,000, $98,400, $142,100). In column C, actual sales ($118,200, $103,650, $137,900). Use =DaysUntilPayday_L(TODAY()) in F1 to show days until next payroll — then copy that value into G1:G3 for visibility.

One more tip: LAMBDA names appear in the Formula Bar’s autocomplete — but only *after* you’ve used them once. Type =DaysU and press Tab — nothing shows up until you’ve entered and confirmed the name at least once.

Cheat Sheet

Task	Action	Shortcut / Note
Open VBA Editor	Launch Visual Basic environment	Alt+F11
Insert New Module	Add blank code space for UDFs	Alt+I+M
Save as Macro-Enabled	Preserve VBA code on save	File → Save As → Excel Macro-Enabled Workbook (.xlsm)*
Define LAMBDA Name	Make reusable formula accessible	Ctrl+F3 → New → Name + Refers to
Force Recalculation	Refresh all UDF outputs instantly	F9 (or Shift+F9 for active sheet only)
Debug UDF	See what your function returns step-by-step	In VBA editor: highlight line → F9 to set breakpoint → run function from sheet