See how extra monthly payments, biweekly schedules, or refinancing affect your mortgage payoff date and total interest. Multi-currency, free, no sign-up.
Calculator
Estimated monthly payment
$2,398.20/ mo
Based on 20 yr remaining at 6% APR.
Total interest
$240,825.63
Total paid
$575,568.55
Payoff date
May 2046
Amortization schedule
Year
Interest
Principal
End balance
11
$19,841.45
$8,936.98
$325,805.94
12
$19,290.24
$9,488.17
$316,317.76
13
$18,705.06
$10,073.38
$306,244.38
14
$18,083.74
$10,694.68
$295,549.69
15
$17,424.13
$11,354.31
$284,195.38
16
$16,723.82
$12,054.64
$272,140.76
17
$15,980.30
$12,798.13
$259,342.63
18
$15,190.93
$13,587.48
$245,755.14
19
$14,352.90
$14,425.55
$231,329.61
20
$13,463.16
$15,315.25
$216,014.34
21
$12,518.54
$16,259.87
$199,754.47
22
$11,515.67
$17,262.77
$182,491.71
23
$10,450.95
$18,327.46
$164,164.23
24
$9,320.53
$19,457.88
$144,706.35
25
$8,120.43
$20,658.00
$124,048.35
26
$6,846.28
$21,932.15
$102,116.21
27
$5,493.55
$23,284.87
$78,831.34
28
$4,057.41
$24,721.04
$54,110.31
29
$2,532.66
$26,245.76
$27,864.55
30
$913.88
$27,864.55
$0.00
How it's calculated
The level monthly payment for a fixed-rate mortgage uses the standard amortization formula: M = P · r · (1+r)^n / ((1+r)^n − 1), where P is the principal, r is the monthly interest rate (annual ÷ 12) and n is the number of months. At 0% the payment is simply principal ÷ n.
Each month's interest is charged on the remaining balance, so early payments are mostly interest and late payments are mostly principal. Extra payments go entirely to principal, which cuts every following month's interest charge — this is why a small extra payment today can save thousands over the life of the loan.
Biweekly schedules effectively make 13 monthly payments per year instead of 12; the one extra payment per year typically retires a 30-year mortgage in about 25–26 years.
Frequently asked questions
Does paying biweekly really save money?
Yes, but only because you make one extra full payment each year — biweekly amounts to 26 half-payments = 13 monthly equivalents. The same effect can be achieved by adding one-twelfth of your monthly payment to each month.
Should I pay off my mortgage or invest the extra cash?
It depends on your loan rate vs. expected investment return, your tax situation, and how much liquidity you need. A mortgage at 4% is functionally a guaranteed 4% return when paid down. Many advisors suggest clearing high-interest debt and building an emergency fund before extra mortgage payments.
Will my lender charge a prepayment penalty?
Most current US mortgages (and all FHA, VA, and federally-insured loans) prohibit prepayment penalties. Some older or private loans may include them — check the loan documents or ask your lender.
What if I don't know my exact remaining term?
Switch the calculator to "Don't know term". Enter your unpaid principal balance, monthly payment, and interest rate — the calculator derives the remaining months from your payment.
Results are estimates. Verify with a professional for important decisions.
About this calculator
This calculator shows how extra payments shorten your mortgage and reduce the interest you pay over the life of the loan. Enter your loan details, choose a payoff strategy — extra monthly, extra yearly, a single lump sum, or biweekly — and see your new payoff date alongside the months and interest you save.
How to read your results
The two headline figures are months saved and interest saved compared with making only your scheduled payment going forward. A shorter bar or lower interest total in the summary panel means your extra payments are working. The amortisation schedule table below tracks each month's interest charge, principal applied, and remaining balance — look for the row marked as your first extra payment to see how quickly the balance starts falling faster. Note that these figures reflect future savings from today; past payments are not recalculated.
Worked example
A 300,000 loan at 6% annual interest over 30 years (360 months, starting fresh) with an extra 200 added to principal every month from payment one.
The base monthly payment is 1,798.65. With the extra 200, the loan pays off in 279 months instead of 360 — saving 81 months (nearly 7 years) and cutting total interest from 347,515 down to 256,341, a saving of roughly 91,173.
Frequently asked questions
Does it matter whether I pay extra at the start or later in the loan?
Earlier extra payments save significantly more interest because the outstanding balance is higher and more of each regular payment goes to interest. Starting even a modest extra amount in the first few years produces outsized savings compared with the same amount added in the final decade.
How does the biweekly option work?
Switching to biweekly payments means you make 26 half-payments per year instead of 12 full payments. That equals one full extra monthly payment each year, which is applied entirely to principal. The calculator models this by adding one-twelfth of the base payment as extra principal each month.
What is a one-time lump-sum payment?
A lump-sum payment is a single extra amount applied to principal in a specific month — for example, a tax refund or bonus. Enter the month number and amount, and the calculator shows how that single reduction in balance shortens the remaining term.
Does the calculator include taxes, insurance, or PMI?
No. Only principal and interest are modelled. Property taxes, homeowner's insurance, and any private mortgage insurance (PMI) are paid separately and do not affect the amortisation schedule this calculator produces.
Can I use this if I don't know my original loan term?
Yes. Switch to the unknown-term mode and enter your current unpaid balance and your actual monthly payment. The calculator derives the remaining term from those two numbers and proceeds as normal.
How it's calculated
The calculation uses standard mortgage amortisation. The scheduled monthly payment is derived from the level-payment formula: P × r(1+r)^n / ((1+r)^n − 1), where P is the original principal, r the monthly interest rate (annual rate divided by 12), and n the total number of payments. From there, the calculator runs a month-by-month simulation: each period it applies the monthly rate to the current balance to find the interest charge, subtracts that from the payment (base plus any extra) to find the principal reduction, and carries the lower balance into the next period. Extra payments are added to the principal portion only, directly reducing the outstanding balance. The simulation stops when the balance reaches zero, and the total months elapsed becomes the new payoff term. Savings in interest and months are the difference between the no-extra simulation and the with-extra simulation, measured from the current period forward. The finance utilities in src/lib/finance.ts supply the level-payment and balance-after-k-payments helpers used in the preliminary setup step.
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