Calculate 12%APR on outstanding balance of invoices at law firm

[lvf331]

I am trying to figure out how to calculate the 12% APR on any outstanding balance of an invoice sent to a client and continue with charging interest on the increasing running total each month there wasn't a payment. For example, the first month the invoice is $3559.98. I need to do the 12%APR and add that to the original total. Then I need to add the next month total of $1,692 and do the calculation again. For some reason, I'm having trouble figuring this out. Any help would be greatly appreciated.

 

[offthelip]

Are you using a monthly interest rate of 1% or are you using the true monthly rate calculated from 12% PA of 0.9489%
(1.009489 to the power 12 = 1.12)

 

[offthelip]

this should do it for you:

Monthly interest rate    1.09489        
Month               Jan       Feb        Mar
New Invoices                 300          250
Balance             1000     =C3+B5    =D3+C5
Balance+Interest =B4*$B1  =C4*$B1    =D4*$B1

 

[offthelip]

I just realised that I typed the wrong number in for the interest rate it should have been
1.009489