I'm trying to find a formula to calculate the cumulative interest paid on a loan after x amount of time. I can do this with data science software but it has to amortize every loan for every customer and is very slow and not realistic to do so was hoping there was a shortcut formula.
I have a customer's loan with an APR of 149%, they pay biweekly so instead of monthly I'd need to calculate for biweekly interest/payments (26 / year payments), I already know they're monthly payment is 48.56, and because I've already amortized this I know after n periods (in this case, I only want to know how much cumulative interest has been paid after 16 periods) that the answer of cumulative interest paid should be 277. I just don't know how to do this mathematically without amortizing every biweekly payment....can anyone help? This particular example their loan is to be paid off in <1 year, but others it's more than a year, some pay monthly, some semi monthly, some biweekly. I can make the adjustments for those I just need to know if there's a shortcut to amortizing the entire loans.