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Mortgage Amortization Calculator

See the payoff month and interest saved when extra principal arrives at different times. The first-payment calendar is explicit.

Mortgage Amortization Calculator estimate

Loan amountNote interest rate (%)Loan term (years)Balance checkpoint (years)Extra monthly principalFirst payment yearFirst payment monthSchedule view

Advanced assumptions

Start monthly extras at payment numberExtra annual principalAnnual extra-payment monthOne-time principal payment 1One-time payment 1 yearOne-time payment 1 monthOne-time principal payment 2One-time payment 2 yearOne-time payment 2 month

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Your estimate

Enter values, then calculate.

How the estimate works

Monthly interest is the remaining balance times the note rate divided by 12. The scheduled payment and applicable extra principal reduce the balance. Each month, yearly totals and saved interest reconcile to the same unrounded schedule.

Method source

CFPB: how paying down a mortgage works. Background guidance; the assumptions and specific model are described above.

Important limitation

The entered rate is the note interest rate, not fee-inclusive APR. This is a fixed-rate monthly model without taxes, insurance, escrow, fees or penalties. Confirm how your servicer applies extra principal. Two one-time payments are supported; annual extras recur in the selected calendar month. Results are estimates only and are not financial, lending, legal, tax, or investment advice.

Assumptions and review

Formula review: September 25, 2026. This tool uses only the editable values shown above and does not pull live rates, prices, balances, or account data.

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See this calculator in a worked example

Follow the assumptions and results, then try the example with your own numbers.

What a $5,000 mortgage lump sum changes a year into repayment — Follow a dated $300,000 mortgage example with one $5,000 principal payment, a capped final payment and a downloadable monthly schedule.

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Frequently asked questions

When is a lump-sum payment applied?

After interest for the selected payment month, together with scheduled and any recurring extra principal. A payment after the modeled payoff is not used.

Why do the annual rows not always contain twelve payments?

Rows group payments by calendar year. Starting in September gives four payments in the first year; the final year may also be partial.

Does this include escrow or daily interest?

No. It models monthly principal and interest only. Actual servicing dates, rounding, fees and prepayment terms can differ.