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Amortization Calculator

Build the full payment schedule for any amortizing loan, then compare it against the same loan with extra principal to see exactly what the extra money buys.

Written by , Editor

Reviewed by Ugo Candido, MBA

Last reviewed

An amortization schedule is the only honest description of a loan. The headline rate tells you little; what matters is how each payment divides between interest and principal, and how that division shifts over the term. On a 30-year loan at 6.25%, the first payment is 85% interest. The last one is almost entirely principal. The schedule shows exactly where the crossover happens for your loan.

This calculator produces that schedule payment by payment, and alongside it a second schedule with extra principal applied — monthly, once a year, or as a single lump sum, in any combination. The two run side by side so the interest saved and the months removed are measured rather than estimated. The contractual payment never changes because you pay extra: the loan simply ends sooner.

It works for any amortizing loan — a mortgage, a car loan, a student loan, a business term loan. It deliberately stops at the amortization itself: no property tax, no insurance, no escrow. Those belong to a housing-cost estimate, and mixing them in would obscure the arithmetic this page exists to show.

Loan amortization

Every payment split into interest and principal, with the contractual schedule and the extra-payment schedule side by side.

Loan
The quoted nominal annual rate, divided by twelve for each monthly payment — the lending convention. That is not the same as an effective annual rate.
For terms that are not a whole number of years.
First payment date
This is the date of the first payment, not the origination date.
Later payments keep this day, clamped to the length of each month.
Extra principal (optional)
Paid alongside every scheduled payment.
Which payment of each year carries the annual amount.
Scheduled monthly payment
$2,155.01

Principal and interest only, for 360 scheduled payments. Adding extra principal does not change this amount — it shortens the loan instead.

Total interest$425,80454.9% of everything you pay
Total principal$350,000
Total of payments$775,804
Number of payments360Ending December 1, 2055
Extra principalNone enteredAdd extra principal to see the interest and time it saves
Where your money goes over the full term
  • Principal: $350,000 (45.1%)
  • Interest: $425,804 (54.9%)
Contractual schedule against the extra-payment schedule
MetricStandard planWith extra paymentsDifference
Monthly required payment$2,155.01$2,155.01—
Payoff dateDecember 1, 2055December 1, 2055—
Number of payments360360—
Total interest$425,804$425,804—
Total paid$775,804$775,804—
Extra principal paid$0$0—

No extra principal is entered, so both columns describe the same loan.

Scroll the table sideways on a narrow screen to see every column.

What different amounts of extra principal would do to this loan
StrategyExtra cash a yearPayoffSooner byTotal interestInterest saved
No extra payments$0December 1, 20550 months$425,804$0
+100 a month$1,200July 1, 20523 years 5 months$367,231$58,573
+200 a month$2,400November 1, 20496 years 1 month$324,334$101,470
+500 a month$6,000August 1, 204411 years 4 months$242,996$182,807

Preset strategies computed from the loan above. They are arithmetic, not a recommendation about what to do with spare cash.

Scroll the table sideways on a narrow screen to see every column.

Amortization by year
YearStarting balanceScheduled paymentsExtra paymentsPrincipal repaidInterest paidEnding balance
1$350,000$25,860$0$4,101$21,759$345,899
2$345,899$25,860$0$4,365$21,495$341,534
3$341,534$25,860$0$4,646$21,214$336,888
4$336,888$25,860$0$4,945$20,915$331,943
5$331,943$25,860$0$5,263$20,597$326,680
6$326,680$25,860$0$5,601$20,259$321,079
7$321,079$25,860$0$5,962$19,899$315,118
8$315,118$25,860$0$6,345$19,515$308,773
9$308,773$25,860$0$6,753$19,107$302,019
10$302,019$25,860$0$7,187$18,673$294,832
11$294,832$25,860$0$7,650$18,210$287,182
12$287,182$25,860$0$8,142$17,718$279,040
13$279,040$25,860$0$8,666$17,195$270,375
14$270,375$25,860$0$9,223$16,637$261,152
15$261,152$25,860$0$9,816$16,044$251,336
16$251,336$25,860$0$10,448$15,413$240,888
17$240,888$25,860$0$11,120$14,741$229,769
18$229,769$25,860$0$11,835$14,025$217,934
19$217,934$25,860$0$12,596$13,264$205,338
20$205,338$25,860$0$13,406$12,454$191,932
21$191,932$25,860$0$14,269$11,592$177,663
22$177,663$25,860$0$15,186$10,674$162,477
23$162,477$25,860$0$16,163$9,697$146,314
24$146,314$25,860$0$17,203$8,657$129,111
25$129,111$25,860$0$18,309$7,551$110,802
26$110,802$25,860$0$19,487$6,373$91,315
27$91,315$25,860$0$20,740$5,120$70,574
28$70,574$25,860$0$22,074$3,786$48,500
29$48,500$25,860$0$23,494$2,366$25,006
30$25,006$25,860$0$25,006$855$0

Yearly rows are aggregates of the monthly schedule, which is the calculation itself.

Scroll the table sideways on a narrow screen to see every column.

How to read this result

The scheduled payment covers principal and interest only. On a mortgage a lender's monthly bill will be higher because it also collects tax and insurance; those are pass-through amounts, not a cost of borrowing, so they are not modelled here.

Total interest is the number worth sitting with. On the default loan it is $425,804 against $350,000 borrowed — 54.9% of everything paid. That is not a hidden fee; it is the arithmetic consequence of borrowing for thirty years, and the composition bar shows it as a proportion rather than a number to skim past.

When extra principal is entered, read the comparison table rather than the headline. Every dollar of principal removed today also removes all the interest that dollar would have accrued for the rest of the term, which is why a modest monthly addition removes years rather than months. The strategy table extends the same logic across preset amounts so you can see the shape of the trade-off without re-running the calculator.

The yearly view answers 'what will I still owe in eight years?' at a glance. The per-payment view is the calculation itself: every row shows the split, the extra principal applied that month, and the balance carried forward. The final row is truncated to whatever is actually owed, so the balance lands on zero rather than overshooting.

  • Scheduled monthly payment, total principal, total interest and total of payments.
  • Number of payments and the payoff date, taken from the last row of the schedule.
  • Interest saved in dollars and as a percentage of the interest you would have paid.
  • Time saved in months and in years and months.
  • Total extra principal paid — the cash the acceleration actually cost you.
  • Strategy comparison across preset extra-payment amounts.
  • Schedule by year or by payment, with dates that respect month lengths.

Formula and methodology

The scheduled payment is the level amount that repays the balance exactly over the term at the stated rate. It follows from the present value of an annuity, solved for the payment.

The schedule is then built one month at a time. Interest is charged on the opening balance, the remainder of the scheduled payment reduces principal, and any extra principal is applied afterwards — capped so the balance can never go negative. The loan terminates the moment the balance reaches zero, which is earlier than the contractual term whenever extra principal is applied.

Scheduled monthly payment

M = P × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ]
  • P — the loan amount; r — the monthly rate (annual rate ÷ 12)
  • n — the number of scheduled monthly payments
  • At r = 0 this reduces to M = P / n, which the code branches to explicitly

Each month of the schedule

Interest_t = Balance_(t−1) × r Principal_t = M − Interest_t Balance_t = max(0, Balance_(t−1) − Principal_t − Extra_t)
  • Extra_t is capped at the balance remaining after the scheduled principal, so it can never overshoot
  • The final payment is truncated: scheduled principal is capped at the opening balance
  • M never changes when Extra_t is added — the term shortens instead

Extra principal timing

Extra_t = monthly + (annual if t mod 12 = chosen payment) + (one-time if t = chosen payment number)
  • The monthly amount is paid with every scheduled payment
  • The annual amount lands on the payment number you select within each year (the 12th by default)
  • The one-time amount lands on a single payment number you choose

Payment dates

date(t) = first payment date + (t − 1) months, day clamped to the month length
  • The start date you enter is the FIRST PAYMENT date, not the origination date
  • A schedule anchored on the 31st runs 31 Jan, 28 Feb (29 in a leap year), 31 Mar — the anchor day is preserved rather than degraded
  • Date arithmetic is integer-based, so there is no timezone or daylight-saving exposure

Precision: balances and interest are carried at full floating-point precision through the entire schedule and rounded to cents only when a row is displayed, so a 360-row schedule accumulates no rounding drift. This is a mathematical model of a loan rather than a servicer's ledger — a lender that rounds every payment to the cent may differ by a few cents on the final payment. The schedule is checked by automated tests to repay exactly the amount borrowed and to end at a zero balance.

Worked example

Take $350,000 over 30 years at 6.25%, first payment 1 January 2026. The monthly rate is 0.0052083 and there are 360 payments, giving a scheduled payment of $2,155.01. Over the full term that is $775,804 paid against $350,000 borrowed — $425,804 of interest, or 54.9% of everything paid, with the loan clearing on 1 December 2055.

Look at the first payment: $1,822.92 of it is interest and only $332.09 reduces the balance. That ratio is why early extra principal is so effective, and why the same $200 applied in year one does far more than in year twenty.

Now add $200 a month of extra principal. The loan clears in 287 payments instead of 360 — 73 months sooner, which is 6 years and 1 month — and total interest falls to $324,334. The $200 monthly addition saves $101,470 of interest, or 23.8% of the interest otherwise due, against $57,200 of extra principal actually paid. Roughly $1.77 of interest disappears for every extra dollar paid in.

The table below runs that comparison across a range of monthly amounts on the same loan.

Effect of adding monthly principal to a $350,000 30-year loan at 6.25%
Monthly extraPaymentsPayoff time savedTotal interestInterest saved
$0360—$425,804—
$1003193 years 5 months$367,231$58,573
$2002876 years 1 month$324,334$101,470
$3002628 years 2 months$291,232$134,572
$50022411 years 4 months$242,996$182,807

Principal and interest only, first payment January 2026. Every figure is generated by the calculation module on this page and pinned by an automated test.

Assumptions and limitations

What this calculator does not do

  • It does not model variable rates, interest-only periods, balloon payments, payment holidays or recasts.
  • It is not a housing-cost estimate: property tax, insurance, association fees and mortgage insurance are deliberately out of scope.
  • It does not decide whether extra principal beats investing the same money — that comparison needs a return assumption, which the future value calculator supplies.
  • It does not model the tax treatment of interest, which varies by borrower and jurisdiction.
  • It assumes monthly payments; bi-weekly payment plans are not modelled.

Educational tool. This is an educational amortization model, not a loan offer or a quote. Actual lender figures differ because of fees, accrual conventions and rounding rules specific to the loan agreement.

Related calculators

  • college tuition planner — Households often carry a mortgage and an education bill at once; the tuition planner projects that second obligation on its own timeline.
  • savings goal planner — Deciding between extra principal and building a cash reserve is easier once the savings goal planner has priced the deposit each target needs.
  • life insurance needs calculator — An outstanding mortgage balance is one of the largest single inputs to the life insurance needs calculator, which treats payoff as an obligation to cover.
  • future value calculator — A loan is the mirror image of an investment: the same compounding that builds a balance in the future value calculator is what the lender charges you here.

Further reading

Sources and references

  • What is amortization? — Consumer Financial Protection Bureau
    The regulator's own definition of amortization and of the interest-then-principal split that every row of the schedule above performs, including why early payments are mostly interest.
  • Regulation Z, Appendix J — Annual Percentage Rate Computations for Closed-End Credit Transactions (12 CFR part 1026) — Consumer Financial Protection Bureau, via the Electronic Code of Federal Regulations
    The actuarial method behind a disclosed APR, and the source of the convention this page uses: a nominal annual rate divided into unit-period rates. It also marks the gap this page states in its assumptions — the APR a lender must disclose folds in finance charges that the note rate modelled here does not.
  • Publication 936: Home Mortgage Interest Deduction — Internal Revenue Service
    The federal treatment of mortgage interest, which this calculator deliberately does not model: the schedule reports interest paid gross, and whether any of it is deductible depends on rules set out here.

The schedule relies on no external data — only the loan terms you enter. These references cover the conventions the page states and the treatments it deliberately leaves out.

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Income multiple, DIME and a full needs analysis side by side, with the coverage gap left after the resources you already hold.

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About the author

This page was written by , editor of Quantus Tools & Intelligence, who is the named author of the calculator documentation and the written analysis published on this site.

It was reviewed before publication by Ugo Candido, MBA, and last verified on . The editorial policy sets out what that review checks and how a correction is made.