Mortgage Calculator With Extra Payments

See your exact payoff date, total interest saved, and how many years you cut — instantly, with your own numbers.

The Mortgage Paydown Calculator lets you model extra payments on top of your minimum repayment — monthly, quarterly, annually, or as one-off lump sums. Enter your loan details and an extra amount and it shows you exactly how much interest you save and when your mortgage ends.

Why Extra Mortgage Payments Save So Much

Every extra dollar you pay reduces your outstanding principal. A lower principal means less interest is charged the following month, which means more of your regular repayment goes to principal the month after — and so on for the remaining life of the loan. This compounding effect is why even a modest extra payment can save a surprisingly large amount over a 25 or 30-year term.

The effect is strongest early in the loan, when interest charges are at their highest. An extra $200 per month applied from year one saves significantly more than the same $200 applied from year five.

A Worked Example

Here is what the numbers look like on a $500,000 loan at 6.00% over 30 years, with an extra $300 per month from the start:

That $139,881 saving comes purely from the compounding effect of reducing the principal faster each month. The extra $300 totals $85,500 in additional payments — but it eliminates $139,881 of interest and frees you from 75 monthly repayments.

How to Use the Calculator With Extra Payments

Step 1 — Enter your loan details

Open the calculator and fill in the four required fields in Section 1 — Loan Details:

• Initial Loan Amount — the original amount you borrowed, not your current remaining balance
• Loan Term — your original term in years (e.g. 25 or 30)
• Initial Interest Rate — your current annual rate as a percentage
• Loan Start Date — the date of your first repayment

Step 2 — Add your recurring extra payment

Tick the checkbox next to Section 2 — Recurring Extra Payments to expand it, then enter:

• Amount — the extra dollar amount per period, on top of your minimum repayment
• Frequency — monthly, quarterly, or annually
• Start date — when you plan to begin (can be today, a past date, or a future date)
• End date (optional) — if the extra payments will stop at a known point

Step 3 — Add lump sum payments (optional)

If you plan to make one-off payments — a bonus, inheritance, or other windfall — use Section 3 — Ad-Hoc Extra Repayments. Add a row for each lump sum with the date and amount. You can add as many as you like and combine them with recurring extra payments in the same scenario.

Step 4 — Calculate and read your results

Press Calculate My Paydown Plan. The results page shows:

• Total interest saved — the dollar difference between your baseline (minimum repayments only) and your plan with extra payments
• Time saved — how many months and years earlier your mortgage ends
• Balance chart — a visual comparison of your loan balance over time, with and without extra payments
• Month-by-month schedule — the full amortisation table showing each payment split between principal and interest

What Affects Your Saving the Most

• When you start. Starting extra repayments today versus in two years can reduce your total saving by 20–30%. The earlier each extra dollar is applied, the longer it has to reduce the interest charged on every future payment.
• How much you pay extra. The relationship is not linear — doubling your extra payment more than doubles the interest saved, because the faster principal reduction accelerates the compounding effect.
• Your interest rate. A higher rate makes extra payments more valuable. Each extra dollar of principal saves more interest per month on an expensive loan than a cheap one.
• How far into the loan you are. Early-stage loans benefit most in total dollar terms. But even if you are 10 years into a 30-year mortgage, extra payments still save significant amounts because compound interest is still working against you for another 20 years.

Frequently Asked Questions

How much can I save with extra mortgage payments?

It depends on your loan size, interest rate, and extra amount. On a $500,000 loan at 6%, an extra $300/month saves around $140,000 in interest and cuts 6 years and 3 months off the term. Use the calculator to get the exact figure for your loan.

Does the calculator support lump sum payments?

Yes. The Ad-Hoc Extra Repayments section lets you add any number of one-off lump sum payments with specific dates — useful for a bonus, tax refund, or windfall. You can combine lump sums with recurring extra payments in the same scenario.

When is the best time to start making extra payments?

As early as possible. Extra payments are most powerful at the start of the loan when the outstanding balance — and therefore the monthly interest charge — is at its highest. That said, extra payments save interest at any stage of the loan. It is always worth running the numbers.

Do extra payments reduce my monthly payment or shorten the loan term?

The calculator keeps your minimum monthly repayment fixed and applies extra payments to reduce your principal faster, which shortens the loan term. Your mandatory monthly payment does not decrease — the mortgage simply ends earlier, and you pay far less interest overall.

Can I model future interest rate changes alongside extra payments?

Yes. Section 4 — Ad-Hoc Rate Changes — lets you add future rate changes with an effective date and new rate. The calculator applies these in chronological order alongside your extra payments, so you can stress-test your plan against rate rises or model the end of a fixed-rate period.