Formula for adding interest to a figure for each 30 days

[olimajor123]

Hi there,

I am looking for a formula to help speed up a process.

I have a spreadsheet which details invoice values and I need add 2% interest on for each 30 days that the invoice is overdue.

I have columns titled; invoice number, invoice date, invoice due date (30 days from date), invoice value, interest

Does anyone know of a formula I can use for the interest column so it adds 2% onto the value for each 30 days after the invoice date that the invoice remains unpaid? Ie if the invoice is dated 01/10/2013 it adds 2% on for the 01/12/2013, and then 2% on top of the new value for 01/01/2014.

I can do this manually however I have quite a few to do and a formula would really help!

I hope this makes sense. Any help would be greatly appreciated.

Thanks