Amortization Calculator With Extra Payments

An amortization calculator with extra payments estimates how additional principal payments change a loan’s balance, total interest, and payoff time. Enter the loan amount, APR, term, and regular extra amount per month to see an estimated payoff schedule and the effect of paying more than the scheduled installment.

Formula

This standard amortization calculator computes a scheduled monthly payment and the total interest over the original term. It does not include an extra-payment input, so it cannot calculate the impact of extra payments directly; the formulas below show the baseline that an extra-payment calculator would compare against. For an extra-payment projection, the balance is updated month by month using the actual extra amount credited to principal.

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

P is the starting 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 (term in years × 12), M is the scheduled monthly payment in dollars, and I is total interest in dollars. The payment is rounded to cents in the displayed output; the total-interest output uses that displayed payment and the full scheduled number of payments.

Worked Example

Using the calculator’s default values, P = $20,000, APR = 8%, and term = 5 years:

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

The calculator’s outputs for these defaults are therefore a monthly payment of $405.53 and total interest of $4,331.80. An extra-payment calculator would need the extra amount and the timing or frequency of those payments to produce a separate payoff estimate.

Assumptions

• The interest rate remains fixed throughout the calculation.
• Payments are monthly and are applied at the end of each monthly period.
• Interest is modelled monthly at APR divided by 12.
• Fees, taxes, insurance, PMI, escrow, and lender-specific payment allocation are not included.

How Extra Payments Change Amortization

With a fixed-rate installment loan, each payment covers interest accrued for the period and reduces principal with whatever remains. As the balance falls, the interest portion of later scheduled payments generally falls too. An extra amount applied directly to principal reduces the balance sooner, which can reduce later interest and shorten the time needed to repay the debt.

A common month-by-month calculation is: interest = beginning balance × monthly rate; principal reduction = scheduled payment − interest + extra principal; ending balance = beginning balance − principal reduction. In the final month, the payment must be limited to the amount actually owed rather than allowing the balance to go below zero. Real servicing calculations may use daily interest or other contract-specific timing, so an estimate can differ from a lender’s payoff figure.

For example, if a borrower makes a one-time $1,000 extra principal payment, the balance drops by that amount when the lender applies it to principal. It does not mean the scheduled payment automatically changes. Often the regular installment remains the same and the loan pays off earlier, but payment handling depends on the loan terms and servicer. A borrower should check how to designate an extra payment and whether any payment must first cover an amount due.

How to Use an Extra-Payment Estimate

1. Use the current principal balance, not necessarily the original amount borrowed.
2. Enter the contractual APR and remaining term in the units the calculator requests.
3. Specify whether extra payments are monthly, occasional, or a one-time amount; do not treat these patterns as interchangeable.
4. Confirm that the extra payment is applied to principal and note when it is applied.
5. Compare the baseline schedule with the revised balance, estimated interest, and payoff date.

A schedule is more informative than a single interest-saved number: it shows when the extra money is assumed to arrive and how each period’s interest and principal change. If the calculator only accepts a recurring monthly extra amount, it cannot accurately represent a lump sum or irregular payments without separate calculations.

Reading Results and Common Mistakes

“Interest saved” is usually the baseline projected interest minus interest under the revised payment schedule. It is an estimate tied to the entered balance, rate, payment dates, and extra-payment pattern—not a guaranteed saving. “Time saved” compares the projected payoff date with the original schedule. Small input differences can change both figures, especially over long terms.

• Confusing APR and monthly rate: APR is entered as a yearly percentage; the formula divides it by 1,200 to get a monthly decimal.
• Using the wrong balance: An existing loan’s remaining principal is different from its original loan amount.
• Assuming extra money reduces the next installment: A principal-only extra payment commonly affects payoff time, not the required payment, but confirm the loan’s process.
• Ignoring rounding: A cent-rounded payment and a lender’s exact daily accrual can create small differences over time.
• Adding extra money to an already paid-ahead account: Payment dates or allocation instructions can affect when principal is reduced; verify the account’s treatment.

Also check whether the loan has a prepayment charge or other relevant contract conditions before relying on an estimate. The calculator does not determine contract rights or fees. For an exact payoff amount on a particular date, request the figure from the servicer.

What This Calculator Does Not Show

The calculator above gives a standard fixed-rate, fully amortizing baseline. It does not model variable rates, skipped or late payments, a changing payment, daily interest, fees, or extra payments. Those features require additional inputs and may require the contract’s precise interest and payment rules. The balance formula also assumes a consistent rate and payment interval; it is not a substitute for a lender’s amortization table or payoff statement.

When evaluating extra-payment scenarios, distinguish interest savings from cash-flow changes. Sending more principal requires more money earlier; an interest estimate alone does not represent the effect on a household budget or other financial priorities. This page calculates loan arithmetic and does not make a recommendation about paying extra.

Frequently Asked Questions

Does an extra mortgage payment go to principal?
It depends on how the servicer processes it and on any instructions or contract terms. Check that the additional amount is credited to principal rather than held, applied to a future installment, or used to cover another amount due.

How do extra payments reduce loan interest?
When credited to principal, an extra payment lowers the balance on which future interest is calculated. The amount and timing of any reduction depend on the rate, payment schedule, and servicing method.

Does paying extra lower my required monthly payment?
Not necessarily. Many amortizing schedules keep the required installment unchanged and reach payoff sooner. A loan may have different rules, so confirm with the servicer rather than assuming a recast or payment reduction occurs.

Can I make a one-time extra payment in this calculator?
No. The calculator block on this page computes the scheduled payment and baseline total interest only. A one-time payment requires a schedule that accepts the payment amount and date and recalculates the remaining balance.

Why does my lender’s payoff amount differ from the calculator?
A payoff quote may include interest accrued through a specific date and applicable charges, while this estimate assumes monthly periods and excludes fees. Differences in balance, rate, payment timing, rounding, or payment allocation can also affect the result.

This result is an estimate; 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