Build a full month-by-month payoff schedule for any fixed-rate loan. See exactly how each payment splits between interest and principal — and how extra payments shorten the loan.
The single most expensive financial decision most Americans make is the one they never think about: choosing a 30-year amortization. The contract looks identical to a 15- or 20-year version — same lender, same house, same address. But strung end to end, the amortization schedule decides whether a $315,000 loan costs you $470,000 or $620,000. This page exists to make that schedule visible.
Interest is calculated each month against whatever principal is still outstanding. At payment one of a 30-year loan, the balance is at its maximum, so the interest charge is at its maximum too. Whatever's left over after that interest is paid is the only money that actually reduces the loan.
The math doesn't care about how long you've owned the home, how reliably you pay, or whether you put 20% down. It only cares about the current balance. That's why amortization curves look like hockey sticks: principal repayment is invisible at first and accelerates dramatically in the last third of the loan.
For a fully-amortizing fixed-rate loan, the monthly payment never moves:
M = P × r(1 + r)n ÷ ((1 + r)n − 1)
M is the dollar payment, P is the starting principal, r is the monthly rate (annual APR divided by 12), and n is the total number of monthly payments. The formula sets M at exactly the level needed so the final payment leaves a zero balance — no more, no less. Every month after that, your interest charge is just balance × r, and the principal portion is M − interest.
Payment one breaks down as $1,871 of interest and $252 of principal. That ratio doesn't cross 50/50 until payment 235 — the back half of year 19. If your career path or family plans imply you'll move or refinance before then, you should know in advance that almost none of the early payments are building actual equity.
Every extra dollar you send is subtracted from the principal balance the day it lands. That means the very next interest charge is calculated against a smaller number. The next charge after that is even smaller. The savings compound in your favor.
| Extra monthly principal | Years to payoff | Lifetime interest | Saved vs. baseline |
|---|---|---|---|
| $0 (baseline) | 30.0 yrs | $449,449 | — |
| $150 | 26.1 yrs | $378,610 | $70,839 |
| $250 | 23.8 yrs | $337,584 | $111,865 |
| $500 | 19.0 yrs | $258,420 | $191,029 |
| $1,000 | 14.2 yrs | $184,121 | $265,328 |
Notice the table is not linear. Doubling extra principal from $250 to $500 doesn't double the savings — it nearly doubles them plus shaves additional years off the term. That's the compounding-in-reverse benefit.
A biweekly mortgage pays half the monthly amount every 14 days. Because the calendar contains 26 biweekly periods (52 ÷ 2), you end up making 13 monthly equivalents instead of 12. The extra payment is what shortens the loan — typically 4 to 7 years on a 30-year term.
Here's the catch: third-party biweekly enrollment services charge $9.95 to $15 monthly to "manage" this for you. You can replicate the same outcome for free by dividing your monthly payment by 12 and sending that amount as extra principal every month, or by making one full extra payment annually. The result on your schedule is identical.
The fix is identical across every U.S. servicer: in the online portal, use the explicit "Additional Principal" or "Principal-Only Payment" field. If you mail a check, write "Apply to principal" on the memo line and include a printed note. Then verify on the next statement that the principal balance dropped by exactly the extra amount.
If a windfall lands — inheritance, bonus, business sale — there are three competing options for accelerating a mortgage. Each fits a different situation.
For loans originated after December 15, 2017, mortgage interest is deductible on the first $750,000 of acquisition debt if you itemize on Schedule A. Older loans use the $1 million cap. The standard deduction for tax year 2026 is $14,600 single and $29,200 married filing jointly, which is why roughly 90% of homeowners no longer itemize — the standard deduction simply beats the mortgage interest plus state and local tax (capped at $10,000).
If you itemize, your servicer sends Form 1098 in late January showing total interest paid and points (if any) for the year. Auto loan, personal loan, and credit-card interest are not deductible for personal use.
Interest is balance × monthly rate. At payment one the balance is highest, so the interest charge is highest. The principal portion grows automatically each month thereafter — there is no action required from you for that to happen.
No, not unless you instruct your servicer to apply it to principal. Without instructions, many servicers post extra funds as a prepayment of the next scheduled installment, which does nothing for your interest savings.
On a 30-year mortgage at typical 2026 rates, one annual extra payment removes roughly 4 to 5 years from the term and saves 20% to 25% of total lifetime interest. The exact figure depends on when in the year you make it and your specific rate.
Yes — federal law requires lenders to provide the Loan Estimate and Closing Disclosure showing the same numbers, but neither breaks down a month-by-month split the way this calculator does. Run the same loan amount, rate, and term here before signing and you'll know exactly what years 1, 5, and 15 will look like.
When the scheduled payment is smaller than the interest accruing, the gap is added to the principal — the balance grows. It's intentional on some income-driven student loan plans and graduated-payment mortgages, and it can be destructive on adjustable-rate loans where minimum payments don't cover interest.
Closing the account often produces a temporary 5- to 25-point dip because the loan disappears from your credit mix and your average account age drops. Scores typically recover within 12 months.
No. Only the interest you actually paid in the calendar year is deductible (if you itemize). Extra principal payments simply reduce future interest you would have paid.
Only the fixed-rate portion. For an ARM, this tool is accurate for the initial fixed period; after that, your rate will adjust and the schedule changes. For balloon loans, the final payment is much larger than the monthly amount and the formula doesn't apply.
Editorial note: This calculator is provided for educational purposes only and does not constitute personalized financial, tax, or legal advice. Loan terms, rates, and fees vary by lender; always confirm prepayment policies with your specific servicer in writing before sending extra principal. Last reviewed by Jordan Halloway, CFP®, on February 28, 2026.