10% interest isn't specific enough, you need to define over what period (e.g. annually, monthly etc) and that's totally up to you. Then you can do base_amo

10% interest isn't specific enough, you need to define over what period (e.g. annually, monthly etc) and that's totally up to you. Then you can do base_amount * interest_rate * (time_held/interest_period) So for example if I you pay a 10% annual rate, and I have a balance of $100 for two weeks you can pay me 100*0.1*(2/52) which is about 38 cents and my new balance will be $100.38. How often you decide to make that payment is again, totally up to you (this doesn't need to be the same number as the interest period).