I have a 30 year fixed-rate mortgage. Yearly interest rate is r. If my initial principal is P and monthly payment is p, but this month i decide to pay p+Delta, how do i calculate how much of the (p+Delta) goes to the principal, and how much goes to the interest?
The bank, upon request, should give you an amortization table. The remaining principal dropping a bit each month which means the next month, the payment will have a slightly lower amount going to interest. The table will show you each month's principal, and each month's interest.
The addition payments you're trying to make will go 100% to principal.
I recall an example, $200K 6% 30year mortgage, the Payment was $1200. Very early on in the mortgage, this was $1000 interest, $200 principal. If you paid $200 extra that month, you'd skip ahead a full month on the amortization table. By looking ahead at the next month's principal, you might keep this up for a number of years, effectively paying 2 months off at a time with this slightly higher payment.
Years ago, I wrote an amortization spreadsheet, which would help illustrate my response, and let you tinker with the idea of prepayments. It was part of a series to counter a mortgage acceleration scam, at that time sold as "Money Merge Account" and reincarnated more recently as "Wealth Unlimited". In a few hours, I wrote a spreadsheet that reproduced what either of these $3500 products claimed to do.