Home Amortization Calculator With Extra Payments

A home amortization calculator with extra payments estimates how additional principal payments change your mortgage balance, total interest, and payoff time. Enter the loan amount, annual interest rate, term, and extra amount paid each month to compare an accelerated schedule with the standard monthly payment.

Formula

This calculator uses the standard fixed-payment amortization formula. It estimates the scheduled monthly principal-and-interest payment and the total interest paid over the stated term, assuming every scheduled payment is made on time and the rate remains fixed.

• Monthly payment: M = P × r / [1 − (1 + r)⁻ⁿ]
• Total interest: I = M × n − P
• Monthly rate: r = APR / 1,200
• Number of payments: n = term in years × 12

P is the original loan principal in dollars; APR is the annual percentage rate entered as a percentage; r is the monthly interest rate as a decimal; n is the number of monthly payments; M is the monthly principal-and-interest payment in dollars; and I is total interest in dollars. The displayed payment is rounded to cents. The total-interest expression uses that displayed, rounded payment, so the final scheduled payment may differ slightly from a real lender’s payoff amount.

The required input is the standard amortization schedule, not an extra-payment schedule. The calculator block does not have an extra-payment field, so it cannot calculate a revised payoff date or interest savings from extra payments. To model those effects, the schedule must also account for the extra amount, when it is paid, how the lender applies it, and the remaining balance after each payment. The explanation below describes how those comparisons work without presenting them as outputs of this calculator.

Worked Example

Using the block’s default inputs: P = $20,000, APR = 8%, and term = 5 years.

1. Convert the annual rate to a monthly decimal: r = 8 / 1,200 = 0.0066667.
2. Find the number of monthly payments: n = 5 × 12 = 60.
3. Apply the payment formula: M = 20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰] = $405.53 when rounded to cents.
4. Calculate total interest using the rounded payment: I = $405.53 × 60 − $20,000 = $4,331.80.

These are estimates under the stated assumptions. A mortgage lender’s payment schedule can differ because of payment dates, rounding, fees, escrow, or other loan terms.

Assumptions

• The interest rate is fixed for the entire entered term.
• Payments are monthly, made at the end of each period, with the first payment one month after the loan begins.
• Interest compounds monthly at APR divided by 12.
• The calculation excludes fees, taxes, homeowners insurance, mortgage insurance, and escrow payments.
• No extra principal payments, missed payments, rate changes, or early payoff fees are modelled.

How Extra Mortgage Payments Change Amortization

Each regular payment covers interest accrued since the previous payment, then applies the remainder to principal. In the early part of a long, level-payment loan, the outstanding balance is larger, so more of each payment goes to interest. As principal falls, the interest portion generally shrinks and the principal portion grows.

An extra payment directed to principal reduces the balance sooner. Since later interest is calculated on a smaller balance, the borrower can pay less interest over the remaining schedule and may finish earlier. The size of the effect depends on the loan balance, rate, remaining term, extra amount, and timing. An extra amount paid early in the loan generally affects more future interest periods than the same amount paid near the end, all else equal.

For a schedule that includes extra payments, calculate each period separately: determine interest on the opening balance, subtract the regular payment’s principal portion and the extra principal amount, then use the new balance for the next period. The last payment should be limited to the amount actually owed; otherwise, a simple repeating-payment model may show an overpayment.

How to Compare an Extra-Payment Schedule

1. Use the loan’s current principal balance, remaining term, and actual rate—not necessarily the original home purchase price or original loan term.
2. Get the required payment and payment frequency from the loan statement or lender disclosure.
3. Choose an extra amount and specify whether it is a one-time payment or a recurring monthly amount.
4. Check whether extra payments are applied directly to principal and whether the lender requires a particular payment instruction.
5. Compare the projected interest and payoff timing with the standard schedule, while accounting for rounding and any final partial payment.

For example, a hypothetical $100 monthly principal-only payment is not simply $100 less interest on the next statement. It first reduces principal; the interest difference then accumulates across later periods. A precise comparison needs the actual balance and schedule. The example here is illustrative, not a forecast of any particular mortgage.

Common Mistakes and Limits

• Using the home price instead of the loan balance: The principal is the amount financed, after any down payment and other loan adjustments. For an existing mortgage, use the current principal balance.
• Confusing APR and monthly rate: Enter APR as a percent, such as 8 for 8%. The formula divides it by 1,200 to get a monthly decimal rate.
• Treating the payment as the full housing cost: This result covers principal and interest only. Property taxes, insurance, association charges, and escrow items are not included.
• Assuming every extra payment lowers the required payment: Extra principal usually changes the balance and projected payoff schedule; it does not necessarily recast the required monthly payment. Confirm the loan’s handling with its servicer.
• Ignoring payment application and timing: A payment credited late, or an amount not applied to principal as intended, can produce a different result.

The formula assumes a fixed rate and equal monthly periods. It does not model adjustable rates, biweekly payment arrangements, skipped payments, fees, escrow, or lender-specific rounding. Use the loan’s disclosure or servicing records for its exact terms, and check with the servicer how additional payments are credited.

Frequently Asked Questions

Does an extra mortgage payment go to principal?
It depends on how the loan servicer applies and labels it. Check the payment instructions or contact the servicer to confirm that an amount above the required payment will reduce principal rather than be held or applied to a later installment.

How much interest can extra mortgage payments save?
There is no single amount. It depends on the balance, rate, remaining term, extra payment amount, and timing. Compare two complete schedules using those inputs; this calculator provides the standard schedule but does not calculate the extra-payment savings.

Will paying extra each month shorten my mortgage term?
Reducing principal can bring the projected payoff forward when the extra amount is applied to principal and regular payments continue. The exact timing depends on the loan terms and payment processing.

Should I use my original loan amount or current balance?
Use the original amount to estimate a new loan from its start. To estimate an existing mortgage from today onward, use its current principal balance and remaining term.

Does the calculator include taxes and insurance?
No. The payment and interest figures are for principal and interest only. Taxes, insurance, mortgage insurance, escrow, and other charges are excluded.

This result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• Amortization Formula With Extra Payments: How It Works
• Amortization Calculator With Down Payment
• Amortization Calculator With Known Payment

in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2