Loan APR Calculator: Estimate Payments and Interest

This loan APR calculator estimates a fixed monthly payment and total interest from the amount borrowed, annual percentage rate (APR), and loan term. Enter the figures provided by a lender to see the estimated monthly payment and the interest paid across the scheduled term.

Formula

The calculator uses the standard fixed-payment loan formula. It converts the annual APR to a monthly rate, calculates the payment that amortizes the balance over the term, rounds that payment to cents, and subtracts the original principal from the sum of scheduled payments to estimate total interest.

• Monthly payment: M = round(P × r / [1 − (1 + r)⁻ⁿ], 2)
• Total interest: I = M × n − P
• Monthly rate: r = APR / 1200

P is the amount borrowed in dollars; APR is the annual percentage rate entered as a percentage; r is the monthly rate as a decimal; n is the number of monthly payments, calculated as the term in years × 12; M is the monthly payment in dollars; and I is estimated total interest in dollars. The calculator assumes a level payment each month. Its total-interest result uses the rounded monthly payment, so rounding can make the final scheduled payment differ slightly in a real amortization schedule.

Worked Example

Using the calculator’s default inputs—$20,000 borrowed, 8% APR, and a five-year term:

1. Convert the APR to a monthly rate: 8 ÷ 1200 = 0.0066667, or about 0.66667% per month.
2. Find the number of monthly payments: 5 × 12 = 60.
3. Apply the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = about $405.53 per month, rounded to cents.
4. Calculate the total of scheduled payments using the rounded payment: $405.53 × 60 = $24,331.80.
5. Subtract the principal: $24,331.80 − $20,000 = $4,331.80 in estimated total interest.

The headline result is the monthly payment, $405.53. The second result is the estimated interest, $4,331.80. The amount paid overall, $24,331.80, is shown here to explain the calculation; it is not a separate calculator output.

Assumptions

• The interest rate is fixed for the full term.
• Payments are equal and made monthly, with the first payment one month after borrowing.
• Interest is compounded monthly using APR divided by 12.
• Fees, taxes, insurance, PMI, escrow, late charges, and optional add-ons are not included. The APR entered is used as the rate in the payment formula; the calculator does not separately itemize any costs included in a lender’s APR.

APR, Interest Rate, and What the Number Means

An interest rate is the rate used to calculate interest on a loan balance. APR is a yearly percentage measure that can reflect the interest rate and certain borrowing costs, depending on the loan and how the lender presents its disclosures. APR is useful when reviewing loan cost information, but it is not a complete description of every possible charge or payment condition.

This calculator treats the entered APR as a fixed nominal annual rate divided into monthly periods. That is a simplifying assumption: it does not determine how a particular lender calculates APR, which costs are included in a disclosed APR, or whether its payment schedule follows this exact convention. For a precise payment comparison, use the loan’s disclosed payment amount and full cost information as well as the estimate.

The monthly payment includes both interest and principal. Early in a typical amortizing schedule, a larger share of each payment goes toward interest because the outstanding balance is larger. As principal is repaid, the interest portion generally falls. Total interest is not the same as the total paid: total paid includes repayment of the original amount borrowed too.

How to Use and Interpret the Estimate

Enter the principal amount, APR as a percentage (for example, enter 8 for 8%, not 0.08), and term in years. The result is a scheduled payment estimate under the assumptions above. If a lender quotes a monthly payment that differs, check whether the quoted figure includes fees, a different term, a variable rate, a different payment date, or other costs that this calculation omits.

Changing the term affects both the payment and the total interest. Holding the amount and rate constant, a longer term spreads repayment over more months and generally lowers the calculated monthly payment, while more payments can increase total interest. A shorter term generally does the reverse. This is a mathematical relationship, not a recommendation about which term to select.

For a useful estimate, confirm that the amount is the actual financed principal rather than a price before down payment or other adjustments. Use the APR and term from the lender’s current written disclosure, and check whether the rate is fixed or variable. A variable rate can change, so a single fixed-rate calculation cannot show future payment changes.

Common Calculator Limits and Mistakes

• Mixing up percent and decimal: The input expects APR as a percent, so enter 8 for 8%. The formula itself converts that value by dividing by 1200.
• Using the wrong loan amount: The calculation begins with the amount financed. Upfront charges, a down payment, or a later balance change can make that different from a listed purchase price.
• Assuming all borrowing costs are included: The calculator does not add fees or other charges separately. A lender’s APR and payment disclosure should be checked for the loan-specific details.
• Ignoring rounding: Payments are rounded to the nearest cent before total interest is calculated. Actual servicing schedules may adjust the final payment to clear a small remaining balance.
• Applying it to irregular repayment: Extra payments, skipped or late payments, interest-only periods, balloon payments, and changing rates are outside this level-payment formula.

Use the estimate to understand the arithmetic, not to determine eligibility or affordability. Approval, contract terms, and actual costs depend on information and conditions the calculator does not model. It does not account for a borrower’s wider finances or replace a lender’s loan documents.

Frequently Asked Questions

Does APR equal the loan’s interest rate?
Not necessarily. The interest rate is used to calculate interest on the balance, while APR may also reflect certain borrowing costs. The exact treatment depends on the loan disclosure; this calculator simply uses the entered APR as its fixed rate input.

Should I enter APR or the interest rate?
Enter the APR if you want to follow the calculator’s stated input. For an exact payment estimate, confirm with the lender which rate and payment terms apply, since the calculator does not model fees separately or reproduce every APR method.

Why is my lender’s payment different?
The lender may use a different principal, rate, term, payment timing, or calculation method. Its payment may also reflect charges or loan features not included here. Check the written disclosure and ask the lender which assumptions explain the difference.

Does the monthly payment include fees and insurance?
No. The output estimates principal and interest only under the formula’s assumptions. Fees, insurance, taxes, and escrow are not added unless they are already reflected in the loan terms in a way the calculator cannot identify.

Can I use this calculator for a variable-rate loan?
It can show a hypothetical payment at one entered rate, but it cannot forecast future payments if the rate changes. The estimate is therefore not a full repayment schedule for a loan with changing interest.

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