Extra Mortgage Payment Calculator
Paying a little extra toward your mortgage each month is one of the simplest ways to save money, and this calculator shows exactly how much. Enter your loan amount, interest rate, term and an extra monthly amount, and it compares two paths: the standard schedule at the regular payment, and an accelerated schedule where you add the extra to every payment. The result is the new payoff date, the number of months you cut from the loan and the total interest you avoid. The saving works because interest is charged on the outstanding balance: every extra dollar applied to principal shrinks the balance, which lowers the interest charged in all the months that follow, so the benefit compounds over the life of the loan. The calculator runs a full month-by-month simulation of both schedules rather than an approximation, so the figures reflect real amortization to the cent. For the extra to count toward principal, your servicer must apply it that way; the US Consumer Financial Protection Bureau has guidance on instructing your servicer. Figures here cover principal and interest only and exclude taxes and insurance. Every result is computed deterministically, with a worked example below that reconciles exactly to the calculator so you can trust the comparison.
Paying extra principal shortens a loan fast. Adding $200 a month to a $300,000 loan at 6.5% pays it off 83 months early and saves $103,448.79 in interest.
How the comparison works
M = P * r * (1 + r)^n / ((1 + r)^n - 1) (standard payment)
r = annual rate / 100 / 12, n = years * 12
Each month: interest = balance * r, principal = payment - interest
Baseline payment = M, accelerated payment = M + extra
Run each until balance reaches zero, then compare months and interest
First the standard monthly payment M is found from the loan amount, rate and term. Two schedules are then simulated month by month: one paying M, one paying M plus the extra. The months and total interest from each run are compared to give the months and interest saved.
Worked example
A 300,000 loan at 6.5% over 30 years has a standard payment of 1,896.20 (r = 0.065 / 12, n = 360). Adding 200 a month makes the accelerated payment 2,096.20.
- Baseline: pay 1,896.20 a month, the loan clears in 360 months with 382,633.47 of interest
- Accelerated: pay 2,096.20 a month, the loan clears in 277 months with 279,184.67 of interest
- Months saved = 360 - 277 = 83 months (about 6 years and 11 months)
- Interest saved = 382,633.47 - 279,184.67 = 103,448.79
Paying an extra 200 a month clears the loan 83 months early and saves 103,448.79 in interest. These are the calculator's default inputs, so the results above match the widget exactly.
Effect of different extra payments (300,000 at 6.5% over 30 years)
Larger extra payments shorten the loan and save more interest. These rows come from the same month-by-month simulation the calculator uses.
| Extra per month | New payoff (months) | Months saved | Interest saved |
|---|---|---|---|
| 100 | 312 | 48 | 60,994.79 |
| 200 | 277 | 83 | 103,448.79 |
| 300 | 250 | 110 | 135,115.17 |
| 500 | 210 | 150 | 179,759.08 |
Figures in US dollars and whole months. Method: US Consumer Financial Protection Bureau, Owning a Home.
Extra mortgage payment calculator: frequently asked questions
How does paying extra each month save interest?
Interest is charged on the outstanding balance, so any extra amount applied to principal reduces the balance more than scheduled. A smaller balance means less interest in every month that follows, and the loan reaches zero sooner. The saving compounds over the life of the loan: a modest extra payment can shorten a 30-year mortgage by several years and cut total interest by tens of thousands of dollars.
How is the saving calculated here?
The calculator first finds the standard monthly payment from your loan amount, rate and term. It then runs two month-by-month simulations: one at the standard payment and one at the standard payment plus your extra amount. Each loop charges interest on the balance, applies the rest to principal, and continues until the balance is cleared. The difference in months and in total interest between the two runs is your saving.
Does the extra payment have to go to principal?
Yes, to get this result the extra must be applied to the loan principal, not held as a prepayment of future interest or future scheduled payments. Most US lenders apply extra payments to principal, but some do not by default, so it is worth confirming with your servicer. The US Consumer Financial Protection Bureau has guidance on telling your servicer how to apply extra payments.
Are there downsides to paying a mortgage off early?
Possibly. Money used to prepay a mortgage is no longer available for emergencies, higher-return investments or higher-interest debt. A few loans also carry prepayment penalties. Paying extra is most attractive when you have an emergency fund, no higher-interest debt, and the mortgage rate is higher than what you could safely earn elsewhere. This tool shows the saving so you can weigh it against other uses of the money.
Can I change the extra amount?
Yes. The extra monthly payment is an editable input. Enter any amount to see its effect on the payoff date and total interest. Even a small consistent extra payment helps, and larger amounts shorten the loan more sharply. You can also adjust the loan amount, rate and term to model a different mortgage entirely.
Official sources
- Mortgage basics, applying extra payments to principal and the costs of owning a home: US Consumer Financial Protection Bureau, Owning a Home. As at 24 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 24 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.