If you are up for a UDF (User Defined Function), then give the function
below a try. To install it, press Alt+F11 to get into the Visual Basic
Editor, then click Insert/Module on its menu bar and copy/paste the function
below into the code window that opened up. Then you can use the function
just like a built-in function. Making sure your large number are entered as
text (either format the cell as text before entering the number into it or
place an apostrophe in front of the number when you enter it); for example,
put '268435457800737937 in A1) and then put...
=BigDec2Hex(A1)
in the cell you want the Hex value to be place in. Here is the function...
Function BigDec2Hex(ByVal DecimalIn As Variant) As String
Dim X As Integer
Dim BinaryString As String
Const BinValues = "*0000*0001*0010*0011" & _
"*0100*0101*0110*0111" & _
"*1000*1001*1010*1011" & _
"*1100*1101*1110*1111*"
Const HexValues = "0123456789ABCDEF"
Const MaxNumOfBits As Long = 96
BinaryString = ""
DecimalIn = Int(CDec(DecimalIn))
Do While DecimalIn <> 0
BinaryString = Trim$(Str$(DecimalIn - 2 * _
Int(DecimalIn / 2))) & BinaryString
DecimalIn = Int(DecimalIn / 2)
Loop
BinaryString = String$((4 - Len(BinaryString) _
Mod 4) Mod 4, "0") & BinaryString
For X = 1 To Len(BinaryString) - 3 Step 4
BigDec2Hex = BigDec2Hex & Mid$(HexValues, _
(4 + InStr(BinValues, "*" & _
Mid$(BinaryString, X, 4) & "*")) \ 5, 1)
Next
End Function
Note that for your example number, the function returns 3B9ACA06B551491
which is the correct answer (not the answer you showed in your posting).
