How mortgage amortization works

A fixed payment is not a fixed split. Interest is charged on whatever principal is still outstanding; the rest of the payment retires that principal.

Finance Calculator Pro team·Updated October 14, 2026·4 min read

The payment is solved so the balance hits zero

For a fixed-rate, fully amortizing loan the lender chooses a payment that, if made every month, brings the balance to (about) zero on the last due date. If r is the monthly rate and n is the number of months:

M = P × r / (1 − (1 + r)^−n)
when r = 0:  M = P ÷ n

The formula is an identity, not a negotiation. Changing the term or the rate changes M. Changing extra principal does not change M; it changes how many times you need to pay it. The mortgage calculator uses this identity for principal and interest, then adds taxes, insurance, and optional PMI as separate lines.

Each month has the same recipe

Interest for the month is remaining balance × r. Required principal is M minus that interest, unless the balance is smaller than that remainder, in which case you pay the balance and stop. Extra principal, if you send it, comes after the required split. The new balance is old balance minus required principal minus extra.

Early months, the interest piece is large because the balance is large. Later months the interest piece shrinks and more of the same M becomes principal. That crossover is visible on the area chart as a growing principal band and a shrinking interest band. It is not a change in the contract rate.

A worked path, not a survey

Add $200 extra each month and you are no longer on that 360-month path. The payoff calculator rebuilds the schedule with the extra applied every month and reports the interest you no longer pay and the months you no longer sit on the note.

What sits outside amortization

Property tax and homeowners insurance are usually escrowed. They ride along with the payment you send the servicer, but they do not reduce principal. PMI, when it applies, is also not principal. The CFPB’s page on removing PMI states that you can generally request cancellation at 80% LTV and that PMI must automatically terminate at 78% of original value. Those consumer figures sit under the Homeowners Protection Act, 12 U.S.C. § 4902 (12 U.S.C. 4901 et seq.). Our mortgage calculator drops a flat PMI dollar amount at 80% of original principal — a teaching cutoff, not a servicing promise. For the definition of PMI itself, see the CFPB PMI explainer.

Adjustable-rate mortgages recompute interest when the index plus margin changes. Interest-only periods skip the principal split for a time. Balloons leave a lump sum. None of those are in the simple schedule. A refinance replaces the entire note: new P, new r, new n, plus closing costs.

Why extra principal “saves interest”

Because next month’s interest is charged on a smaller number. You do not get a rebate of interest already paid. You avoid interest that would have been charged on the dollars you retired early. Whether those extra dollars should have gone to a reserve, a higher-rate debt, or an investment is a separate question this site will not answer for you.

Confirm with the servicer that an overage is coded as principal and not as early escrow. A recast — a formal request to lower the required payment after a lump sum — is a different product feature from simple prepayment.

Taxes and the schedule

Deductible home mortgage interest, when it is deductible, follows IRS Publication 936 and Tax Topic 505. Amortization determines the interest you were charged, not whether you can take it, how much is qualified, or what your tax rate does to the after-tax cost. The calculators do not estimate a refund.

Limits

Schedules assume the rate and extras you typed never change. They ignore late fees, forbearance, partial claims, and round-off on a real statement. They are educational estimates, not a replacement for the periodic statement or a Loan Estimate. Explore offers with the CFPB compare and close tools when you have real disclosures.