An ancient macro. I haven't tested that it does what it claims.
''By Leo Heuser, published in David Hager's XL newsletter 6/15/99
Sub DeleteUnusedCustomNumberFormats()
Dim Buffer As Object
Dim Sh As Object
Dim SaveFormat As Variant
Dim fFormat As Variant
Dim nFormat() As Variant
Dim xFormat As Long
Dim Counter As Long
Dim Counter1 As Long
Dim Counter2 As Long
Dim StartRow As Long
Dim EndRow As Long
Dim Dummy As Variant
Dim pPresent As Boolean
Dim NumberOfFormats As Long
Dim Answer
Dim c As Object
Dim DataStart As Long
Dim DataEnd As Long
Dim AnswerText As String
NumberOfFormats = 1000
ReDim nFormat(0 To NumberOfFormats)
AnswerText = "Do you want to delete unused custom formats from the
workbook?"
AnswerText = AnswerText & Chr(10) & "To get a list of used and unused
formats only, choose No."
Answer = MsgBox(AnswerText, 259)
If Answer = vbCancel Then GoTo Finito
On Error GoTo Finito
Worksheets.Add.Move after:=Worksheets(Worksheets.Count)
Worksheets(Worksheets.Count).Name = "CustomFormats"
Worksheets("CustomFormats").Activate
Set Buffer = Range("A2")
Buffer.Select
nFormat(0) = Buffer.NumberFormatLocal
Counter = 1
Do
SaveFormat = Buffer.NumberFormatLocal
Dummy = Buffer.NumberFormatLocal
DoEvents
SendKeys "{tab 3}{down}{enter}"
Application.Dialogs(xlDialogFormatNumber).Show Dummy
nFormat(Counter) = Buffer.NumberFormatLocal
Counter = Counter + 1
Loop Until nFormat(Counter - 1) = SaveFormat
ReDim Preserve nFormat(0 To Counter - 2)
Range("A1").Value = "Custom formats"
Range("B1").Value = "Formats used in workbook"
Range("C1").Value = "Formats not used"
Range("A1:C1").Font.Bold = True
StartRow = 3
EndRow = 16384
For Counter = 0 To UBound(nFormat)
Cells(StartRow, 1).Offset(Counter, 0).NumberFormatLocal =
nFormat(Counter)
Cells(StartRow, 1).Offset(Counter, 0).Value = nFormat(Counter)
Next Counter
Counter = 0
For Each Sh In ActiveWorkbook.Worksheets
If Sh.Name = "CustomFormats" Then Exit For
For Each c In Sh.UsedRange.Cells
fFormat = c.NumberFormatLocal
If Application.WorksheetFunction.CountIf(Range(Cells(StartRow,
2), Cells(EndRow, 2)), fFormat) = 0 Then
Cells(StartRow, 2).Offset(Counter, 0).NumberFormatLocal =
fFormat
Cells(StartRow, 2).Offset(Counter, 0).Value = fFormat
Counter = Counter + 1
End If
Next c
Next Sh
xFormat = Range(Cells(StartRow, 2), Cells(EndRow, 2)).Find("").Row - 2
Counter2 = 0
For Counter = 0 To UBound(nFormat)
pPresent = False
For Counter1 = 1 To xFormat
If nFormat(Counter) = Cells(StartRow, 2).Offset(Counter1,
0).NumberFormatLocal Then
pPresent = True
End If
Next Counter1
If pPresent = False Then
Cells(StartRow, 3).Offset(Counter2, 0).NumberFormatLocal =
nFormat(Counter)
Cells(StartRow, 3).Offset(Counter2, 0).Value = nFormat(Counter)
Counter2 = Counter2 + 1
End If
Next Counter
With ActiveSheet.Columns("A:C")
.AutoFit
.HorizontalAlignment = xlLeft
End With
If Answer = vbYes Then
DataStart = Range(Cells(1, 3), Cells(EndRow, 3)).Find("").Row + 1
DataEnd = Cells(DataStart, 3).Resize(EndRow, 1).Find("").Row - 1
On Error Resume Next
For Each c In Range(Cells(DataStart, 3), Cells(DataEnd, 3)).Cells
ActiveWorkbook.DeleteNumberFormat (c.NumberFormat)
Next c
End If
Finito:
Set c = Nothing
Set Sh = Nothing
Set Buffer = Nothing
End Sub
