How extra payments help
Each month: interest = balance × monthly rate. The rest of the payment cuts principal. Extra money goes to principal. The loan ends when the balance hits zero. If the payment does not cover interest, the balance grows and this page will say so.
Confirm with your lender that extra amounts apply to principal and that there is no prepayment penalty.
Common questions
Should I include escrow?
No. Use principal and interest only.
Biweekly payments?
Treat them as a higher monthly extra, or run two weeks as half the monthly bill twice.