Is there an Excel function to create a hash value?

You don't need to write your own function - others already did that for you.
For example I collected and compared five VBA hash functions on this stackoverflow answer

Personally I use this VBA function

• its called with =BASE64SHA1(A1) in Excel after you copied the macro to a VBA module
• requires .NET since it uses the library "Microsoft MSXML" (with late binding)

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Public Function BASE64SHA1(ByVal sTextToHash As String)

    Dim asc As Object
    Dim enc As Object
    Dim TextToHash() As Byte
    Dim SharedSecretKey() As Byte
    Dim bytes() As Byte
    Const cutoff As Integer = 5

    Set asc = CreateObject("System.Text.UTF8Encoding")
    Set enc = CreateObject("System.Security.Cryptography.HMACSHA1")

    TextToHash = asc.GetBytes_4(sTextToHash)
    SharedSecretKey = asc.GetBytes_4(sTextToHash)
    enc.Key = SharedSecretKey

    bytes = enc.ComputeHash_2((TextToHash))
    BASE64SHA1 = EncodeBase64(bytes)
    BASE64SHA1 = Left(BASE64SHA1, cutoff)

    Set asc = Nothing
    Set enc = Nothing

End Function

Private Function EncodeBase64(ByRef arrData() As Byte) As String

    Dim objXML As Object
    Dim objNode As Object

    Set objXML = CreateObject("MSXML2.DOMDocument")
    Set objNode = objXML.createElement("b64")

    objNode.DataType = "bin.base64"
    objNode.nodeTypedValue = arrData
    EncodeBase64 = objNode.text

    Set objNode = Nothing
    Set objXML = Nothing

End Function

Customizing the hash length

• the hash initially is a 28 characters long unicode string (case sensitive + special chars)
• You customize the hash length with this line: Const cutoff As Integer = 5
• 4 digits hash = 36 collisions in 6895 lines = 0.5 % collision rate
• 5 digits hash = 0 collisions in 6895 lines = 0 % collision rate

There are also hash functions (all three CRC16 functions) which doesn't require .NET and doesn't use external libraries. But the hash is longer and produces more collisions.

You could also just download this example workbook and play around with all 5 hash implementations. As you see there is a good comparison on the first sheet

I don't care very much about collisions, but needed a weak pseudorandomizer of rows based on a variable-length string field. Here's one insane solution that worked well:

=MOD(MOD(MOD(MOD(MOD(IF(LEN(Z2)>=1,CODE(MID(Z2,1,1))+10,31),1009)*IF(LEN(Z2)>=3,CODE(MID(Z2,3,1))+10,41),1009)*IF(LEN(Z2)>=5,CODE(MID(Z2,5,1))+10,59),1009)*IF(LEN(Z2)>=7,CODE(MID(Z2,7,1))+10,26),1009)*IF(LEN(Z2)>=9,CODE(MID(Z2,9,1))+10,53),1009)

Where Z2 is the cell containing the string you want to hash.

"MOD"s are there to prevent overflowing to scientific notation. 1009 is a prime, could use anything X so that X*255 < max_int_size. 10 is arbitrary; use anything. "Else" values are arbitrary (digits of pi here!); use anything. Location of characters (1,3,5,7,9) are arbitrary; use anything.

For a reasonably small list you can create a scrambler (poor man's hash function) using built-in Excel functions.

E.g.

 =CODE(A2)*LEN(A2) + CODE(MID(A2,$A$1,$B$1))*LEN(MID(A2,$A$1,$B$1))

Here A1 and B1 hold a random start letter and string length.

A little fiddling and checking and in most cases you can get a workable unique ID quite quickly.

How it Works: The formula uses the first letter of the string and a fixed letter taken from mid-string and uses LEN() as a 'fanning function' to reduce the chance of collisions.

CAVEAT: this is not a hash, but when you need to get something done quickly, and can inspect the results to see that there are no collisions, it works quite well.

Edit: If your strings should have variable lengths (e.g. full names) but are pulled from a database record with fixed width fields, you will want to do it like this:

 =CODE(TRIM(C8))*LEN(TRIM(C8))
       +CODE(MID(TRIM(C8),$A$1,1))*LEN(MID(TRIM(C8),$A$1,$B$1))

so that the lengths are a meaningful scrambler.