Extra Mortgage Payment Calculator
Every Extra Dollar Attacks the Balance Directly
A standard mortgage payment splits between interest and principal according to a fixed amortization schedule, but any payment above that amount goes entirely to principal — there's no interest owed on money that hasn't accrued yet. That's what makes extra payments disproportionately effective early in a loan: reducing the balance sooner means every future month's interest is calculated against a smaller number, compounding the savings for the rest of the term.
The Formula
The calculator runs this month by month until the balance reaches zero, supporting both a recurring extra monthly payment and a one-time lump-sum payment applied at the start.
Where This Calculation Matters
- Deciding how to use a raise or bonus — comparing a lump-sum principal payment against investing the same amount shows which path builds more value given your mortgage rate versus expected investment returns.
- Setting a realistic extra-payment target — even a modest recurring amount produces a meaningfully shorter payoff and lower total interest, useful for setting a sustainable budget goal.
- Evaluating a windfall — an inheritance or asset sale applied as a one-time extra payment shows exactly how many years and how much interest it eliminates.
- Comparing against refinancing — extra payments shorten the existing loan without new closing costs or a new rate negotiation, an alternative worth comparing against a refi.
Impact of Extra Monthly Payments
On a $300,000 loan at 6.5% over 30 years, adding a fixed extra monthly payment on top of the standard payment:
| Extra/month | Payoff time | Total interest | Interest saved |
|---|---|---|---|
| $0 | 30.0 years | $382,633.47 | — |
| $100 | 26.0 years | $321,638.68 | $60,994.79 |
| $200 | 23.1 years | $279,184.67 | $103,448.79 |
| $300 | 20.8 years | $247,518.30 | $135,115.17 |
Returns diminish as the extra amount grows — the jump from $0 to $100/month saves nearly $61,000 in interest, while the next $100 increment saves a smaller additional amount, because the loan is already being paid off faster.
How to Use This Calculator
- Enter the Loan Amount, Interest Rate, and Loan Term (years).
- Enter an Extra Monthly Payment if you plan to pay a fixed amount above the required payment every month.
- Enter an Extra One-Time Payment if you're applying a lump sum now instead of or in addition to recurring extra payments.
- Select Calculate to see the new payoff timeline, total interest, and the time and money saved versus the standard schedule.
Related Calculations
Compare this strategy against switching payment frequency with the Biweekly Mortgage Calculator, or against buying down the rate instead with the Mortgage Points Calculator.