Amortization Schedule With Extra Payments

A standard amortization row has four numbers: the payment, how much of it was interest, how much was principal, and the balance left afterward. Adding extra payments adds one column and changes the meaning of the last one.

Here are the first three months of a $400,000 loan at 6.5% over 30 years, with $300 extra applied from month one. The scheduled payment is $2,528.27.

Notice what does not change. The payment column is identical in all three rows and stays identical for the life of the loan. The interest column falls slightly each month, because interest is charged on a balance that is now dropping faster than the lender's schedule assumed. The principal column rises to match. And the balance in row three is $2,004.42 lower than a no-extra schedule would show — the $900 you added, plus $1,104.42 of principal that the smaller balance freed up on its own.

That second component is the part worth understanding. Your extra payments earn extra payments. The effect compounds, quietly, in the interest column.

The table below was generated with the calculator on this page, using $400,000 at 6.5% over 30 years — a loan whose scheduled cost is $510,178 in interest across exactly 360 payments.

Read the $100 row slowly. One hundred dollars a month, for as long as the loan lasts, returns $63,917. Over the 322 months it is actually paid, that is $32,200 contributed against $63,917 saved — a return of roughly two to one, with no market risk, available to anyone with a mortgage and a spare hundred dollars.

The relationship is not linear, and the table shows why. Going from $0 to $100 buys 38 payments. Going from $400 to $500 buys far fewer, because by then the balance is falling quickly on its own and there is less interest left to cancel. The first dollar of prepayment is always worth more than the last.

The reason sits in the interest column of month one. On that $400,000 loan, the first payment is $2,166.67 interest and $361.60 principal — 86% of the money goes to the lender and never touches the debt. Across the whole first year, the split is $25,868 interest against $4,471 principal, and the balance falls only to $395,529.

An extra $1,000 in month one is therefore doing something very different from an extra $1,000 in month 300. In month one it cancels roughly 29 years of compounding on that thousand dollars. In month 300 it cancels five years of it. Same dollar, wildly different job.

This has a practical consequence that surprises people: a lump sum early beats a larger lump sum later, and a small recurring extra started immediately beats a large one started after the kids finish school. On the same loan, a single one-time $10,000 payment made in month 13 retires the mortgage 24 months early and saves $51,174 — more than five times the amount paid.

A fixed monthly amount. The simplest and the one modelled in the table above. It is easy to automate and easy to stop if money gets tight. If you do only one thing, do this.

One-time lump sums. A bonus, a tax refund, proceeds from a sale. These are worth modelling separately because their value depends heavily on timing — the same $10,000 is worth about $51,000 in month 13 and considerably less in month 130. If a lump sum is coming, applying it sooner is almost always correct.

Round-up payments. Paying $2,600 instead of $2,528.27 adds $71.73 a month without feeling like a decision. It underperforms a deliberate $300, but it is the version people actually sustain for twenty years, and a sustained $72 beats an abandoned $300.

The schedule treats all three identically once the money arrives: extra dollars are principal, applied on the day they post. What differs is only your ability to keep doing it.

An extra payment only behaves the way this page describes if the servicer applies it to principal. Send $2,828.27 on a $2,528.27 mortgage with no instruction attached, and a common outcome is that the servicer books $2,528.27 against October and parks the remaining $300 as a partial prepayment of November. Your balance does not drop. Your interest does not fall. You have simply paid next month early.

Three habits prevent this:

• Use the servicer's dedicated additional principal field if the online portal has one, rather than overpaying the total. Most large servicers now have one.
• If you pay by check or bill-pay, write “apply to principal only” on the payment and keep a copy.
• Check the next statement. The balance should have fallen by your regular principal plus the full extra amount. If it did not, call before the second payment compounds the error.

Also confirm there is no prepayment penalty. Most modern owner-occupied mortgages have none, and Qualified Mortgages are restricted in whether and how they may charge one, but the term is loan-specific and worth reading rather than assuming. The Consumer Financial Protection Bureau explains what a prepayment penalty is and when one may apply (consumerfinance.gov, read September 2026).

The numbers in this page are arithmetic, and arithmetic is not advice. Three things sit outside the table and can reverse its conclusion.

Your rate versus other uses of the money. Prepaying a mortgage is a guaranteed, tax-adjusted return equal to your interest rate. At 6.5% that is a strong risk-free return and hard to argue with. At 3.1% — where a great many American mortgages are still fixed — the same $300 has a much better case for going somewhere else, including an employer match you are not capturing.

Liquidity. A dollar paid into a mortgage is extremely hard to get back out. It is not an emergency fund, and it becomes accessible only through a sale or a new loan, which costs money and requires qualifying again. Fill the emergency fund first.

The payment does not shrink. This is the one that catches people. Prepaying shortens the loan; it does not reduce next month's obligation by a cent. If the goal is lower monthly cash flow rather than a shorter loan, prepayment is the wrong instrument — a recast or a refinance is the right one.

Enter the loan amount, rate and remaining term, then add an extra monthly amount, a one-time amount, or both. The schedule regenerates row by row, and the summary reports the new payoff date, total interest, and the interest saved against the untouched loan.

Two habits make the output more useful. First, model your actual current balance and the months you have left, not the original loan — if you are four years into a 30-year mortgage, you have a 26-year loan, and using the original figures will overstate the savings. Second, run the same extra amount at two or three levels rather than one. Seeing that $200 buys 67 payments and $300 buys 91 makes the trade-off concrete in a way a single answer never does.

An amortization schedule with extra payments is only as good as the loan you feed it, and every figure on this page was produced by that calculator rather than estimated. Exporting the full schedule and finding the row where the balance crosses zero is the most direct way to see what your own extra payment is buying.