I'm attempting to create a calculator that I can use to try different debt repayment scenarios. If I have multiple debts with different rates I'm wondering what algorithm to use to figure out how long it will take to repay and how much interest will be saved.
I have figured out how to calculate these numbers with EXCEL and NPER and CUMIPMT for adding blank amounts to all of them, but I'd like to figure out how to apply it to highest interest rate debt first.
I'm wondering if I just need to calculate the principal and interest payment on a per month basis as if it was a new loan each month to figure this out.
To simply even if I have just one loan, say a mortgage at 3.75% that currently has 300k left on it, my minimum monthly payment is 1435, say I put 60k on it in 4 months, and then in 6 months I start adding another 1200 to the payment for a year... how do I calculate the new payoff date and interest saved over just the minimum payment.