Auto-Generate a number for each item in a SharePoint list

I would like to auto-generate a number for each item (row) in my SharePoint list. I can't use the default ID column, because i can't use the ID column in a calculated field. I will need to use this auto-generated number in a CONCATENATE formula for another column in this list. (I want to use this number to concatenate a column that outputs "PRJ###" for each item) Thanks for any help!