try changing it to text and the use a function such as
provided they're all definitely that length though, may want to run a script to double check all the lengths?
Do you only have Filemaker Pro ? or Advanced ?
If you have Advanced, you can use a custom function such as : http://www.briandunning.com/cf/391
or you could adapt it slightly to work as an auto enter calc as required.
thanks. yes it was a text field already and your autocalculation worked. :)