Mortgage Payoff Extra Payment Calculator

The Simulation

The calculator runs two side-by-side month-by-month simulations starting from your current balance:

1. Standard: applies your regular monthly P&I payment each month until the balance reaches zero
2. With Extra Payment: applies your regular payment plus the extra amount to principal each month

In each simulation period:

Interest = Balance × (Annual Rate ÷ 12)
Principal Paid = Monthly Amount − Interest
New Balance = Previous Balance − Principal Paid

Because the accelerated simulation reduces the balance faster, each subsequent month's interest charge is slightly smaller, which means even more of the next payment goes to principal. This compounding effect is what makes the savings grow non-linearly — the savings from month 5 are larger than the savings from month 1, even though the extra payment is the same amount.

Why Early Payments Matter More

Mortgage amortization is front-loaded with interest. On a 30-year $300,000 loan at 6.5%, roughly 82% of your first payment is interest. As the balance falls, that ratio improves — but in the early years, extra principal payments eliminate the most expensive interest dollars in the schedule.

An extra $200/month applied in year 1 saves more total interest than the same $200/month applied starting in year 20, because the earlier payment cascades through fewer interest charges on a higher remaining balance.

Reading the Chart

The chart shows remaining balance by year for both schedules. The two lines start together and diverge as extra payments compound over time. The point where the "With Extra Payment" line reaches zero is your new payoff date — you can see at a glance how many years early you exit the loan.

1. When You Get a Raise or Bonus

A raise is a natural trigger point: before lifestyle inflation absorbs the additional income, run this calculator to see what routing $100–$500 of the increase toward your mortgage does over the remaining term. The concrete numbers — years saved, exact interest avoided — make the tradeoff real.

2. Refinancing Decision Support

If you're considering a refinance to lower your rate, compare the savings from a lower rate against the closing costs (typically 2–5% of the loan). Also run this calculator: sometimes the same total interest savings is achievable by simply adding a modest extra payment to your current loan — with no closing costs, no appraisal, and no resetting your amortization clock.

3. Optimizing a Windfall

If you receive a lump sum — tax refund, inheritance, business distribution — you have two strategies:

• Lump sum principal payment (see Mortgage Recast): reduces balance immediately, optionally lowers monthly payment
• Divide across months: split the windfall into equal extra monthly payments over the next 12–24 months

This calculator handles the second scenario. Use both tools to see which approach delivers better long-term interest savings for your specific balance and rate.

4. Comparing Extra Payments vs. Biweekly

The Biweekly Mortgage Payoff calculator shows savings from switching payment frequency. This calculator shows savings from increasing payment amount. The strategies target the same goal — paying more principal per year — but through different mechanisms:

• Biweekly adds ~8.3% more per year automatically (one extra monthly equivalent)
• Fixed extra payment adds exactly as much as you specify, giving full control

Run both to find your preferred approach or combine them.

5. Deciding How Much Extra to Pay

There's no universal answer. Use this calculator to experiment: try $50, $100, $200, $500. Notice how savings accelerate — going from $0 extra to $100 extra often saves more interest than going from $200 to $300, because the first dollars attack the highest-balance, highest-interest period of the loan. Find the amount that meaningfully shortens your term without straining your monthly budget.