Mortgage Amortization Calculator

See where every mortgage payment goes

A fixed-rate mortgage can have the same scheduled principal-and-interest payment for hundreds of months, but the split changes every time. Interest is calculated on the balance still owed. The remainder reduces principal. As principal falls, the next month’s interest becomes smaller and more of the fixed payment reaches the balance.

The payment formula is:

M = P × r(1 + r)ⁿ ÷ ((1 + r)ⁿ − 1)

where P is principal, r is the monthly interest rate, and n is the number of monthly payments.

For each month, the schedule uses:

interest = opening balance × monthly rate

principal = payment − interest + extra principal

The final payment is reduced so the balance ends at zero instead of becoming negative.

Worked example: $300,000 at 6.5%

A $300,000, 30-year mortgage at 6.5% has scheduled principal and interest of about $1,896.20 per month. In month one, interest is $1,625 and only about $271 reaches principal. That is not a hidden fee; it is the result of applying 6.5% annual interest to a large outstanding balance.

Later, the balance is smaller. The payment stays approximately $1,896.20, so less goes to interest and more goes to principal. The annual table aggregates twelve months to make that shift easier to read.

Adding $200 of extra principal each month pays the example loan off years earlier and lowers total interest. The exact result appears immediately in the calculator, alongside the base schedule’s time and interest.

How to read the annual table

Each row contains:

• Payments: all scheduled and extra payments made that year;
• Principal: the portion that reduced the debt;
• Interest: the cost of borrowing during that year; and
• Ending balance: principal still owed after the final payment in the row.

The payment total is not the same as the principal reduction because interest is part of every payment. Equity can also change through home-price movement, but appreciation is not part of mortgage amortization.

Extra payments need correct servicing

Before sending additional money, confirm that the mortgage permits prepayment and ask how to mark the amount as principal-only. Review the next statement. A servicer may handle partial or extra payments differently, and a recast is a separate process that can change the scheduled payment after a large principal reduction.

For an accelerated schedule tied to paychecks, compare the biweekly mortgage calculator. To start from a home price and down payment instead of an existing loan balance, use the monthly mortgage payment calculator.

Model limitations

This schedule assumes a fixed rate, monthly compounding, end-of-month payments, and a constant extra amount. Adjustable rates, interest-only periods, daily simple interest, skipped payments, late charges, escrow, and servicing adjustments require a different model. Treat the output as a reproducible estimate rather than an official payoff quote.