Access VBA Len function returning 2 as length of single digit number

Not sure... If field "ID" is a numeric field, try

strID = Trim(Cstr(strID)) 'trim off invisible spaces

Why are you opening a recordset twice? I've never seen that syntax before.

I would recommend against storing your ID as a string. Instead set the format property of the control in the form/report to "0000" in order to achieve what you are looking for, with no at all, and while still maintaining a numeric ID.

What data type is the field ID and how is it populated?

What do you see in arrASCII if you run this code?

Dim arrASCII() As Byte

    arrASCII = StrConv(strID, vbFromUnicode)

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The array should contain the ASCII codes for all the characters in ID.

Have to admit that I have never seen this syntax before either:

strID = CurrentDb.OpenRecordset(F).OpenRecordset.Fields("ID")

Normally you would have something like:

Dim rs as DAO.REcordset
set rs = currentdb.Openrecordset(F)

strID = rs!ID

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But if your ID field is a string, is there a chance that you could have some unicode characters in there which are not displaying?

Try:

For intLoop = 1 to Len(strID)
    debug.print mid(strID, intLoop, 1), ASC(mid(strID, intLoop, 1))
Next

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If ID is numeric, you are converting it to a string and that could be the problem.  However, there is a very simple method to turn a number into a string with leading zeros.  use the  Format() function.  You need only one line of code.

oInvSuffix = Format(strID, "000")

Use as many zeros as you need for the desired length of the result.  You want to end up with three characters so the string contains 3 zeros.  If you wanted a 6 character result, then you would use 6 zeros.   So 1 would be 001, 42 would be 042 and 876 would be 876.

@aikimark
Nice catch.
I never would have suspected that. The help function does state it quite clearly though. Now this issue actually gets more confusing due to the naming of variables used in the initial post:

strID = CurrentDb.OpenRecordset(F).OpenRecordset.Fields("ID")
    strID = Trim(strID) 'trim off invisible spaces

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But based on a bit of testing, its clear that strID is NOT declared as a string, because if it was, then the code would work as originally intended. It must be declared as an integer (Which btw, I wouldn't recommend. Use long for any ID, to avoid integer overflow)

For the original issue, use either the format function if you need to do more string manipulation in code, or use the format property of the control in the form/report.

Thank you  Anders. Pity I had accepted answers because that is very interesting