I am trying to find a faster way of calculating my monthly interest on a auto loan that I have. The remaining balance of my loan is $8400 with an Apr of 4.94%. My monthly payment is $716.94. But I want to make 4 weekly additional payments of $500 to speed up process of paying off my loan. So I have been using I= prt to calculate daily interest and adding up the weeks to get my total for the month. But since my additional payments are weekly is there another way I could do this calculation faster?