Amortization Calculator With Payment Input

An amortization calculator with payment input lets you enter a loan balance, annual interest rate, loan term, and monthly payment to estimate how the balance changes over time. This calculator instead calculates the fixed monthly payment from the loan amount, APR, and term, then shows total interest; use the payment and schedule method below when you already know your payment.

Formula

The calculator finds the level monthly payment that repays a fixed-rate loan in the selected number of months. It then estimates total interest as the total of those monthly payments minus the original principal.

• P is the loan amount in dollars.
• APR is the annual percentage rate entered as a percent.
• r is the monthly rate: APR ÷ 1,200.
• n is the number of monthly payments: years × 12.
• PMT is the monthly payment in dollars.
• I is total interest in dollars, calculated as PMT × n − P.

The payment formula is PMT = P × r ÷ [1 − (1 + r)⁻ⁿ]. The calculator rounds the monthly payment to cents, then uses that displayed payment to estimate total interest. Actual final payments can differ because lenders may adjust the last installment or calculate interest using dates and daily balances.

Worked Example

With the calculator’s default inputs, the loan amount is $20,000, APR is 8%, and term is 5 years.

1. Convert the annual rate to a monthly rate: 8 ÷ 1,200 = 0.0066667.
2. Convert the term to payments: 5 × 12 = 60 months.
3. Apply the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = $405.53 per month, rounded to cents.
4. Calculate total interest using that rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

The result is an estimated monthly payment of $405.53 and total interest of $4,331.80.

Assumptions

• The interest rate stays fixed for the full term.
• Payments are equal, made monthly, and on schedule.
• Interest is modelled monthly at APR ÷ 12.
• Fees, taxes, insurance, PMI, escrow, late payments, and extra principal payments are not included.

Using a Known Payment to Build an Amortization Schedule

If you already know the required payment and want to see how each installment is divided, use the standard monthly schedule calculation. For each month, multiply the opening balance by the monthly interest rate to estimate that month’s interest. Subtract the interest from the payment to find principal repaid, then subtract that principal from the opening balance.

• Monthly interest = opening balance × APR ÷ 1,200.
• Principal paid = payment − monthly interest.
• Closing balance = opening balance − principal paid.

For example, at an 8% APR, the monthly rate is 0.0066667. If the opening balance is $20,000 and the payment is $405.53, first-month interest is about $133.33. About $272.20 goes toward principal, leaving a balance near $19,727.80. The next month’s interest is calculated on that lower balance, so the interest portion generally declines while the principal portion grows, provided the payment exceeds interest due.

To make a complete schedule, repeat those steps once per payment period. Calculations that round interest and balances each month can differ by a few cents from a lender’s schedule, which may use unrounded values internally and adjust the final payment.

When to Use This Calculator

Use the calculator to estimate the scheduled payment and total interest for a level-payment, fixed-rate loan when you know the principal, APR, and repayment term. It can help you understand how those three inputs relate mathematically or check an estimate against figures supplied in a loan disclosure. Enter the actual quoted APR and term for a more relevant estimate; the default values are only an example.

A shorter term generally means fewer payment periods and less time for interest to accrue, but the calculated monthly payment is higher. A longer term spreads repayment over more months and can increase total interest under the same rate and principal. These are mathematical effects, not a determination of whether any loan suits a particular person.

How to Read the Results and Common Errors

The monthly payment is the modelled amount due each month for principal and interest. It is not necessarily the entire amount a borrower pays: a mortgage or other loan may have separate charges, and those are outside this calculation. Total interest is the estimated interest over the full stated term using the rounded monthly payment. It is not the total cost of borrowing if fees or other charges apply.

• Enter APR as a percentage, such as 8 for 8%, not 0.08. The formula converts it to a monthly decimal rate.
• Enter the loan amount, not the desired cash received after fees or other deductions.
• Check that the term is expressed in years in this calculator. It converts years to months automatically.
• Do not treat a payment rounded to cents as an exact lender payoff figure. Payment dates and rounding policies can change the final balance.
• Do not enter a promotional or introductory rate as though it applied for the full term if the rate can change.

The standard formula assumes a positive interest rate, equal monthly payments, and no changes to the balance except scheduled payments. It does not model variable rates, skipped payments, payment holidays, irregular payment dates, daily simple interest, balloon payments, or extra payments. If the loan has any of these features, a basic fixed-payment estimate may not match its actual schedule.

Frequently Asked Questions

Can I enter my monthly payment into this calculator?
This calculator takes loan amount, APR, and term as inputs, then calculates the monthly payment. For a loan with a known payment, use the month-by-month amortization steps above or a calculator specifically designed to accept a payment input.

How do I calculate an amortization schedule from a payment?
For each month, calculate interest as the opening balance times APR divided by 1,200. Subtract that interest from the payment to get principal repaid, then subtract principal from the opening balance to get the next balance.

Why does the interest portion decrease over time?
With a fixed rate, interest for a period is based on the outstanding balance. As scheduled payments reduce that balance, the interest calculated for later periods usually falls.

Does the monthly payment include taxes and insurance?
No. The payment shown is principal and interest under the stated assumptions. Taxes, insurance, escrow, PMI, and fees are not inputs and are not included.

Why might my lender’s numbers differ?
A lender may use different payment dates, unrounded calculations, fees, or an interest method based on actual days. The loan’s disclosure and payment schedule show the figures applicable to that loan.

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