What extra principal payments change
An extra principal payment is money you send above the scheduled amount, marked as principal. It does not change your contract rate. It reduces the balance sooner, so later months charge interest on a smaller number. That is how extra payments cut total interest and move the payoff date forward.
What they do not change
On a typical fixed-rate loan, extra principal does not lower the required monthly payment unless you refinance or recast (and recast is not offered on every loan). If you stop the extras, the required payment is still the original payment. Extra payments also do not replace tax, insurance, or HOA amounts that may sit in escrow.
How to test it here
On the calculator, enter your amount, APR, and term, then put a number in Extra monthly payment. Compare total interest and the number of rows in the schedule with extra set to $0 versus $100 or $200. The payment shown at the top is still the scheduled payment; the extras are applied after that payment each month in this model.
- A small extra on a long mortgage often saves more interest than the same extra on a short auto loan, because more months of interest are avoided.
- Confirm with your servicer that extras apply to principal and are not treated as early payments of next month’s bill.
- Some loans have prepayment rules. Read the note before you send large extras.
For the split of a normal payment, read how amortization works. This site does not tell you whether to prepay or invest. It only shows the arithmetic.