# Need formula for APR with long/short odd days.May 23, 2012 1:18 PM   Subscribe

Need formula for APR with long/short odd days.

I need to programmatically calculate APR based on the following inputs:

Principal Amount
Number of payments
Payment each period

Second: most APR formula assume an equal number of payments and an equal number of days between each payment.

In my case, we may have loans where there is an odd number of days for the first payment (either less than or more than the frequency of their payments).

Can someone help me with getting the formula(s) for APR that can help me find the adjusted APR that takes into account the first payment odd days?
posted by joshuamcginnis to Education (2 answers total) 1 user marked this as a favorite

I used to have various spreadsheets that did this; they're not at hand, so this is from memory.

If you're using Excel, I think you need the IRR function, which can calculate yield/APR with periodic payments that vary. So, I'm assuming you are generating values based on a long or short first period with a 30/360 convention. You first value is the principal, say (100) and then the first payment is 15, and then all other payments are 10, until the final payment of 110.

Your APR would be the =irr(a1:a5) range. You can give the function a hint as to what the answer should be; sometimes it has trouble calculating.