Amortization schedule for a $250,000 loan over 20 years

How Is an Amortization Schedule Built?

Every payment on a fixed-rate loan is the same dollar amount, computed with the standard amortization formula. What changes month to month is the split between interest and principal within that fixed payment.

• M — fixed monthly payment
• P — principal (loan amount)
• r — monthly interest rate (annual rate ÷ 12)
• n — total number of monthly payments

Each period, interest is charged only on the current outstanding balance: interest = balance × monthly rate. Whatever is left of the fixed payment after that interest charge goes toward principal, which is why the schedule above shows the principal portion growing — and the interest portion shrinking — every single month.

Amortizing vs. Non-Amortizing Debt

Not all debt is amortized. Credit cards are revolving debt — the balance can be carried indefinitely month to month with no fixed payoff schedule. Interest-only loans have a period where payments cover interest only, with the entire principal still owed at the end of that period. Balloon loans make smaller regular payments followed by one large "balloon" payment of the remaining principal at maturity. This calculator, like most amortization calculators, models fully amortizing fixed-rate loans — the kind where the balance is guaranteed to reach exactly zero on the final scheduled payment.

Amortization schedules generally only apply cleanly to fixed-rate loans. Adjustable-rate loans, variable-rate lines of credit, and other non-fixed products need to be recalculated every time the rate resets, since the schedule is only valid for the rate it was built with.

What Extra Payments Actually Do

An extra payment applied to principal reduces the balance immediately, which means every future month's interest charge — calculated on that now-smaller balance — is permanently lower. This compounds: the loan doesn't just finish faster, it finishes faster while paying less interest in every remaining month along the way. That is why a relatively small extra payment can eliminate a disproportionately large share of total interest, especially early in the loan when the balance (and therefore the interest charge) is largest.

The comparison table above shows this effect for several extra payment amounts on your exact loan. Note that a basic amortization schedule like this one does not account for fees your lender might apply to extra or early payments — always confirm with your lender that extra payments are applied directly to principal, and check for any prepayment penalty before committing to an accelerated payoff plan.

Why the Interest/Principal Split Changes Over Time

In the first year of a 30-year loan, the vast majority of each payment is interest — often 70–90% depending on the rate — because the balance is still close to the full original principal. By the final years, that ratio flips almost entirely to principal. This front-loaded interest structure is standard for all fully amortizing loans and is the mathematical reason why refinancing or selling early in a long-term loan means you have built relatively little equity compared to how much you've paid.

It is also why shortening the term (say, from 30 years to 15) has such a large effect on total interest even though the monthly payment rises: a 15-year schedule spends far fewer total months paying interest on a large balance, so a much larger share of every payment goes to principal from the very first month.

About These Parameters

Loan Amount

The principal — the starting balance the entire schedule is built around. Every interest charge in every future period is a percentage of whatever balance remains, so a larger starting principal produces proportionally larger interest charges throughout the schedule.

Annual Interest Rate

The yearly rate charged on the outstanding balance, divided by 12 for the monthly schedule. This calculator assumes a fixed rate for the entire term — adjustable-rate loans would need a new schedule built at each rate reset.

Amortization Term

How many years the regular (non-extra) payment schedule is designed to take. Shorter terms mean a higher required monthly payment but dramatically less total interest, since less time is spent paying interest on a large balance.

Extra Monthly Payment

An optional amount applied directly to principal every month, in addition to the required payment. It shortens the schedule and reduces total interest — see the comparison table above for the exact effect on your loan. Leave at zero to see the standard, unaccelerated schedule.

Frequently Asked Questions