Mortgage Repayment Calendar

Think of a mortgage as a tank slowly draining through two outlets simultaneously: one carries interest to the lender, the other reduces the balance you owe. In the early years, almost all of your fixed monthly payment flows out the interest outlet. Only a trickle reduces the balance. That imbalance is why paying off a 30-year mortgage in full means paying close to double the original purchase price.

The calculation behind this tool is called amortization. Each month, interest is calculated on whatever principal remains — so as the balance falls, interest charges fall too, and more of each fixed payment flows toward principal. This creates a slow but accelerating drain as the loan matures. The payoff date is simply the month when the balance hits zero.

Extra payments short-circuit this process by reducing the balance faster than scheduled, which reduces the interest charged the following month, which means more of next month's regular payment also goes to principal. The benefit compounds: one extra payment now saves more than one extra payment two years from now, because today's payment removes a larger slice of future interest charges.

Use this tool when you are deciding whether to redirect discretionary income — a bonus, a raise, or money freed up by eliminating another debt — toward your mortgage. The comparison between total interest with and without extra payments gives you the actual return on that decision in dollar terms, which you can weigh against alternatives like investing or eliminating higher-rate debt.

It is also useful when refinancing. If you are considering a shorter loan term, running the numbers on your current loan with extra payments lets you see whether you can achieve a similar payoff date without refinancing — avoiding closing costs of $3,000 to $6,000 that a refinance typically requires.

This tool is not appropriate for adjustable-rate mortgages where the rate resets in the future. The payoff date projection will be wrong the moment your rate changes, and the interest savings estimate will not account for rate movement in either direction. For ARMs, treat the output as a baseline estimate only, and revisit it after each rate adjustment. It is also not suitable for interest-only loans, since the base formula assumes a fully amortizing payment structure.

Entering the original loan amount instead of the remaining balance is the most common input error. Someone who bought a home for $400,000 ten years ago and has paid it down to $310,000 will see a completely wrong payoff date if they enter $400,000. Always pull the current balance from your most recent statement — it changes every single month.

Assuming the extra payment automatically applies to principal. Some mortgage servicers apply overpayments to the next month's scheduled payment rather than to principal, which delays the benefit by a full month and reduces the interest saving. Call your servicer to confirm how to designate extra payments as principal reduction. Many require a specific instruction in a memo field or a separate payment entry online.

Forgetting that the result is estimated, not guaranteed. This tool assumes your rate is fixed and payments are made on time every month. Adjustable-rate mortgages will have a different rate after the fixed period ends, meaning the payoff date can shift significantly. Even on fixed-rate loans, servicer rounding and exact payment dates cause minor real-world variation compared to the projected schedule.

The monthly payment on a fixed-rate mortgage is derived from the present value of an annuity formula: P = L * [r(1+r)^n] / [(1+r)^n - 1], where L is the remaining loan balance, r is the monthly interest rate (annual rate divided by 12), and n is the number of months remaining. This formula produces the exact payment that reduces the balance to zero after n payments.

Once the base payment is known, the amortization schedule runs month by month: interest for the month equals the current balance multiplied by the monthly rate. The principal portion equals the total payment minus that interest charge. The balance for the next month is the previous balance minus the principal portion. Adding an extra payment simply increases the principal reduction in that month, which reduces the starting balance for all future months.

The payoff date is determined by simulating this loop until the balance reaches zero. The tool counts those months from today's date to produce the calendar month and year. Interest saved is the difference between total interest accumulated in two simulations — one with extra payments and one without. This is exact arithmetic given the inputs, but real-world results can vary due to payment timing, rounding by servicers, and rate adjustments on adjustable mortgages.

The amortization formula assumes continuous, on-time monthly payments — but in practice, the exact day a payment is received affects how many days of interest accrue. Most US mortgages use a simple daily interest model during the grace period, meaning a payment received on the 5th versus the 15th of the month costs you roughly ten days of interest on the outstanding balance. On a $300,000 loan at 6.75%, that is about $170 per incident. Over a decade of borderline-late payments, this alone can add thousands of dollars and several months to the effective payoff date — none of which appears in a standard amortization calculator.