Home Loan Amortization Calculator With Extra Payments

A home loan amortization calculator with extra payments estimates your regular monthly payment, interest, and payoff schedule when you add money to principal. Enter the loan amount, APR, term, and extra payment per month to see how much faster the modeled loan is repaid and how the extra payments change total interest.

Formula

This required calculator computes a standard fixed-payment loan, but its inputs do not include an extra-payment amount. Therefore, it cannot calculate an extra-payment payoff date or the interest savings from extra payments; its outputs are the baseline payment and interest for the entered term. The formulas are:

• Monthly payment: M = P × r / [1 − (1 + r)⁻ⁿ], where r = APR / 1,200.
• Total interest: I = M × n − P.

P is the original loan principal in dollars; APR is the annual percentage rate entered as a percent; r is the monthly rate as a decimal; n is the number of monthly payments (years × 12); 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 calculator’s total-interest expression uses the unrounded formula result, so its estimate can differ slightly from a schedule that rounds every payment to cents.

Worked Example

Using the defaults—$20,000 principal, 8% APR, and a five-year term:

1. Convert the APR to a monthly rate: r = 8 / 1,200 = 0.0066667.
2. Find the number of payments: n = 5 × 12 = 60.
3. Calculate the payment: M = 20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰] ≈ $405.53 per month.
4. Calculate total interest using the formula’s unrounded payment: I = M × 60 − 20,000 ≈ $4,331.83.

The first output is the monthly payment, rounded to cents. These figures describe the baseline loan only; no extra payment is entered or included.

Assumptions

• The interest rate is fixed throughout the modeled term.
• Payments are level monthly payments made at the end of each monthly period.
• Interest compounds monthly using APR divided by 12.
• The calculation excludes extra payments, fees, taxes, insurance, PMI, and escrow.

How Extra Payments Change Amortization

Amortization divides each scheduled payment between interest and principal. Early in a fully amortizing loan, more of each payment generally goes to interest because the outstanding balance is larger. As principal falls, the interest portion declines and more of the regular payment reduces principal.

An additional amount applied to principal lowers the balance sooner. Since later interest is calculated on that smaller balance, paying extra can reduce total interest and shorten the repayment period. The exact effect depends on the payment amount, timing, rate, remaining term, and how the loan servicer applies the money. A payment described as extra is not automatically treated as principal reduction; confirm the payment instructions and transaction record.

For an extra-payment estimate, a calculator needs at least the extra amount and its frequency, in addition to the loan details. It should calculate each month’s interest and principal, apply the extra amount to the balance, and stop when the balance reaches zero. The calculator on this page does not have those inputs, so do not interpret its results as an extra-payment comparison.

How to Compare an Extra-Payment Scenario

To evaluate a specific schedule, compare two amortization schedules using identical loan terms: one with no extra payment and one with the proposed extra amount. Useful outputs include the revised payoff date, total interest in each scenario, and the difference between those interest totals. A schedule should also account for the final payment being smaller than the usual payment if the remaining balance is less than the planned amount.

• Check whether extra money is applied to principal rather than held for a future installment.
• Specify whether the extra amount is monthly, annual, one-time, or irregular.
• Use the lender’s actual balance, rate, payment due date, and payment rules when available.
• Compare like with like: keep the same rate and scheduled payment in both scenarios.

Some loans may have terms or charges that affect additional payments. The calculator does not model these conditions. Consult the loan documents or servicer for the applicable details rather than assuming an extra payment has a particular effect.

Common Errors and Limits

A frequent mistake is treating the monthly payment as the full housing cost. The formula estimates principal and interest only; property-related costs and other charges are not included. Another is entering an interest rate as a decimal: enter 8 for 8%, not 0.08, because the formula divides the percent input by 1,200.

Rounding can also cause small differences. A real payment schedule commonly records payments and interest to cents each period, while this calculator applies the formula directly and calculates interest from the unrounded payment. The result is a mathematical estimate, not a lender’s payoff quote. Variable rates, irregular payment dates, missed or partial payments, fees, and changes to the loan are outside this formula.

Frequently Asked Questions

Does this calculator include extra mortgage payments?
No. The required inputs include principal, APR, and term only. It calculates the baseline monthly payment and total interest, not an extra-payment payoff schedule.

How do extra payments reduce loan interest?
When an additional payment is applied to principal, the balance used to calculate later interest is lower. The amount saved depends on the loan terms, timing, and payment handling.

Should I enter an extra payment as part of my monthly payment?
This calculator has no extra-payment field. A separate scenario needs the extra amount and frequency, and should distinguish it from the scheduled principal-and-interest payment.

Does the monthly payment include taxes and insurance?
No. The formula estimates principal and interest only. Taxes, insurance, PMI, escrow, and fees are not included.

Why does my lender’s figure differ from this estimate?
Payment rounding, actual dates, loan fees, rate details, payment application, and other contract terms can affect lender figures. Use the lender’s disclosure or account information for exact amounts.

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

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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