AmortSheet

Estimates only · not lender advice

Mortgage amortization calculator

Turn principal, rate, and term into a full monthly amortization schedule. Add extra monthly or one-time principal and see the new payoff date and interest saved — in the browser, with a CSV you can keep.

The ledger

Build the schedule

U.S. fixed-rate P&I. Taxes and insurance are not included.

Loan inputs

One-time extra payments

Payment #1 is the first due date. A tax-refund extra in year one is often payment 6–12.

Estimates only. AmortSheet models principal and interest on a U.S. fixed-rate mortgage. It does not include taxes, insurance, PMI, HOA dues, or lender fees, and it isnot lender advice, a loan offer, or a commitment to lend. Confirm figures with your servicer. Full disclaimer.

Monthly P&I

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Does not change when you prepay unless your servicer recasts.

Payoff date

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Baseline without extras: —

Total interest

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Without extras: —

Total paid

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If you keep the extras

Interest saved —· Time saved —

A true biweekly plan (26 half-payments a year) is roughly the same as adding— extra each month.

Why that works

Amortization schedule

Year by year, then every month

Interest Principal

How to read this amortization schedule

Front-loaded interest

Early payments are mostly interest because they are calculated on a large remaining balance. Extra principal in year one buys more than the same dollars in year twenty.

Payment stays put

Extra principal does not automatically lower the required monthly P&I. It shortens the loan instead — unless you ask the servicer for a recast.

P&I only

Your lender draft may be larger because of escrow. This calculator is the loan itself: principal, interest, extras, and the remaining balance.

How amortization works·Extra payments guide·Anatomy of a schedule