Loan APR Calculator: Estimate Your Monthly Payment

A loan APR calculator estimates the fixed monthly payment and total interest for a fully amortizing loan when you enter the amount borrowed, annual percentage rate (APR), and term in years. It assumes equal monthly payments and returns a useful estimate, not a lender’s final payoff or disclosure figures.

Formula

The calculator converts the annual APR to a monthly rate, then calculates the level payment that pays down principal and interest over the stated number of months. The payment is rounded to the nearest cent before total interest is calculated.

Monthly payment: M = round[P × r / (1 − (1 + r)⁻ⁿ), 2]

Total interest: I = (M × n) − P

• P is the loan amount, in dollars.
• APR is the annual percentage rate entered as a percentage, such as 8 for 8% per year.
• r is the monthly rate as a decimal: APR ÷ 1,200.
• n is the term in months: years × 12.
• M is the monthly payment in dollars, rounded to cents.
• I is the total interest estimated across n payments, in dollars.

The payment equation is the standard fixed-payment amortization formula. Each scheduled payment covers interest accrued for the period and applies any remainder to principal. As the balance falls, the interest portion generally shrinks and the principal portion grows.

Worked Example

Using the calculator’s default inputs—$20,000 borrowed, 8% APR, and 5 years—the term is 5 × 12 = 60 monthly payments. The monthly rate is 8 ÷ 1,200 = 0.0066667.

1. Substitute into the payment formula: M = 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
2. The unrounded result is approximately $405.527; rounded to cents, the monthly payment is $405.53.
3. Total of the rounded payments: $405.53 × 60 = $24,331.80.
4. Estimated total interest: $24,331.80 − $20,000 = $4,331.80.

Because the payment is rounded to cents, the final scheduled payment in an actual account may differ slightly from the calculator’s equal-payment estimate. A lender’s schedule may also use its own payment dates and rounding conventions.

Assumptions

• The rate stays fixed for the full term.
• Payments are equal, made monthly, and treated as occurring at the end of each monthly period.
• Interest is modelled monthly using APR ÷ 12; this is a standard estimate and may not match a lender’s day-by-day calculation.
• The calculation excludes fees, taxes, insurance, PMI, escrow, late charges, and other costs because they are not inputs.
• The loan is fully repaid by the end of the stated term, with no missed payments or extra principal payments.

When to Use an APR Payment Calculator

Use it to see how the amount borrowed, rate, and term affect a fixed monthly payment, or to understand the interest implied by a set of assumptions. It can help you make an apples-to-apples arithmetic comparison when the amounts, repayment periods, and included costs are clearly identified. Enter the APR and term shown in the lender’s offer rather than relying on an assumed figure; rates and fees vary by offer and are not determined by this calculator.

APR is intended to express the yearly borrowing cost as a percentage, and may reflect certain charges in addition to interest, depending on the loan and the lender’s disclosure. This calculator uses APR as though it were the nominal annual rate applied in monthly periods. If fees are financed, paid upfront, or treated differently in the disclosure, the calculator’s payment and total-interest figures will not necessarily reproduce the lender’s figures. For a precise comparison, review the disclosed payment schedule and the costs included in each offer.

How to Read the Results and Common Mistakes

The monthly payment is the estimated recurring amount needed to amortize the entered principal over the entered term under the stated assumptions. Total interest is the sum of those rounded payments minus the starting principal. It is not the total amount paid: adding the principal to the interest gives the estimated total of payments.

• Enter APR as a percentage, not a decimal. For 8%, enter 8, not 0.08. The formula divides the entered percentage by 1,200 to get a monthly decimal rate.
• Keep the term in years. This calculator converts the year input to months. Do not enter 60 for a five-year term; enter 5.
• Distinguish APR from a periodic rate. Do not enter a monthly rate as though it were an annual APR.
• Do not treat total interest as all-in borrowing cost. Excluded fees and other expenses can change the total cash outlay.
• Expect small rounding differences. The displayed payment is rounded to cents, while a lender may calculate interest using actual dates or adjust the last payment.

A longer term usually spreads repayment over more monthly payments, which can lower the payment calculated from the same principal and rate while increasing the number of periods over which interest accrues. A higher entered rate raises the calculated payment for a fixed term and principal. These are mathematical effects, not a recommendation about which loan to take.

What This Estimate Does Not Show

The calculator models a fixed-rate, level-payment loan. It does not handle variable rates, interest-only periods, a balloon payment, deferred interest, irregular payment dates, skipped payments, or additional payments. It also does not calculate a lender’s exact payoff balance on a particular date. For those cases, the payment pattern or interest method needs more information than the three inputs provided.

To estimate an offer, use the amount that will actually be borrowed and the rate and term in the lender’s written disclosure. If a fee is rolled into the balance, it may affect the amount financed; if it is paid separately, it may not appear in the regular payment. Check the documents for the amount financed, payment count, scheduled payment, APR, and total payments rather than assuming every cost is represented by the calculator.

Frequently Asked Questions

Does APR determine my monthly loan payment?
APR is one input, alongside principal and loan term. This calculator treats APR as the annual rate for monthly amortization; lender fees or a different interest calculation can make the disclosed payment differ.

Is APR the same as the interest rate?
Not always. APR is a yearly cost measure that may include certain charges, while the interest rate describes the rate charged on the outstanding balance. What is included in APR depends on the loan disclosure.

Why is my lender’s payment different?
The amount financed, rate, term, fees, payment dates, day-count method, and rounding can differ from the calculator’s assumptions. Compare the inputs and the lender’s payment schedule.

Does total interest include the amount I borrowed?
No. Total interest is the estimated cost above principal. Add the loan amount to total interest to get the calculator’s estimated total of payments.

Can I use this for a variable-rate loan?
Only as a limited estimate for a period when the rate is known and unchanged. The result cannot predict future payments if the rate changes.

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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2