Home buying · Worked example
What an extra $100 a month changes on a mortgage
In this hypothetical $300,000, 30-year mortgage at a fixed 6.5% interest rate, adding $100 to principal every month pays the loan off 48 months earlier and reduces modeled interest by $60,994.79. That is an illustration, not a quote or a recommendation for your budget.
The example, with all the assumptions visible
The starting balance is $300,000. There are 360 scheduled monthly payments, no changing interest rate, no missed payments and no prepayment charge. The regular principal-and-interest payment is $1,896.20. The second scenario adds $100 from the first payment onward, making the planned payment $1,996.20 until the smaller final payment. Taxes, insurance and HOA charges are excluded because they do not repay principal.
Why a small extra amount changes later payments
The model computes each month's interest on the remaining balance, then applies the rest of that month's payment to principal. An extra principal payment lowers the balance on which the next month's interest is calculated. The monthly interest saving starts small and builds across the remaining schedule. The final payment is capped at the remaining balance plus that month's interest, so the model does not pretend the final full payment is required.
Do not confuse an extra principal payment with paying a future monthly installment early. Ask your servicer how to designate extra principal and confirm how it appears on the statement. Review your loan documents for any relevant charges or restrictions. The CFPB's explanation of principal, interest and amortization is a useful starting point.
The same $100 at different hypothetical rates
| Fixed rate | Regular P&I | Months earlier | Interest saved |
|---|---|---|---|
| 3% | $1,264.81 | 40 | $19,437.31 |
| 6.5% | $1,896.20 | 48 | $60,994.79 |
| 8% | $2,201.29 | 54 | $88,300.57 |
These rates are comparison assumptions, not current market offers. Changing the balance, rate or remaining term changes the answer. An older mortgage with only ten years left is not the same calculation as a new 30-year loan, even when the balance matches.
What to test before making a decision
Run the calculator with your actual remaining balance and remaining term. Compare $0, $50 and $100 extra and save the first result as Scenario A. Look at both the cash commitment and the interest difference. The tool does not evaluate your emergency savings, other debts, investment alternatives or taxes; those decisions need more context than an amortization table can supply.
The example deliberately excludes lender fees and refinancing. If you are comparing new mortgages rather than accelerating an existing one, use the mortgage-offer comparison to separate interest, upfront costs and principal repaid. Keep the question and the tool aligned.