Express Interest: Calculate Loan Interest Costs

Use this calculator to estimate the monthly payment and total interest for a fixed-rate loan with regular monthly payments. Enter the loan amount, annual percentage rate (APR), and term to see the payment and the interest paid over the full term.

Formula

The calculator uses the standard fixed-payment amortization formula. It first finds the monthly payment, then subtracts the original principal from the sum of all scheduled payments to estimate total interest.

Monthly payment: M = P × r ÷ [1 − (1 + r)⁻ⁿ]

Total interest: I = M × n − P

• P is the starting loan principal, in dollars.
• APR is the annual percentage rate entered as a percentage, such as 8 for 8%.
• r is the monthly rate as a decimal: APR ÷ 100 ÷ 12, or APR ÷ 1200.
• n is the number of monthly payments: loan term in years × 12.
• M is the monthly payment, in dollars.
• I is total interest over the scheduled term, in dollars.

The calculator displays the payment to the nearest cent. Its total-interest expression uses the displayed, rounded monthly payment multiplied by the number of months, then subtracts principal. This makes the result reflect the rounded payment shown; an actual lender’s final payment may be adjusted to settle a small remaining balance.

Worked Example

Using the default inputs: a $20,000 principal, 8% APR, and a five-year term.

1. Convert the APR to a monthly decimal rate: 8 ÷ 1200 = 0.0066667.
2. Find the number of monthly payments: 5 × 12 = 60.
3. Substitute into the payment formula: M = 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
4. Calculate and round the monthly payment to cents: M = $405.53.
5. Calculate total interest using the displayed payment: $405.53 × 60 − $20,000 = $4,331.80.

For these inputs, the estimated monthly payment is $405.53, and estimated total interest is $4,331.80.

Assumptions

• The interest rate stays fixed for the entire term.
• Payments are equal, made monthly, and occur at the end of each monthly period.
• Interest compounds monthly using APR divided by 12.
• The estimate excludes fees, taxes, insurance, PMI, and escrow; these are not inputs in this calculator.

Step-by-Step

1. Find the figures to enter. Check the loan documents or request the proposed principal, APR, and repayment term. Use the APR as a yearly percentage, not as a decimal: enter 8 for 8%, not 0.08. Use the amount actually borrowed as the principal; if fees are added to the balance, the applicable starting balance may differ from the amount received.
2. Enter the principal. Type the loan’s starting balance in dollars. Do not enter the amount of each payment or the total amount you expect to repay. The input accepts values from $1 to $100,000,000.
3. Enter the annual APR. Use the rate expressed as a percentage per year. The accepted range is 0.01% to 100%. For a variable-rate loan, a single APR cannot represent future changes, so treat the result as a scenario based on the rate entered.
4. Enter the term in years. Convert a term stated in months by dividing by 12. For example, 48 months is 4 years. This calculator accepts terms from 0.0833 to 50 years and calculates the payment count as years multiplied by 12.
5. Read the monthly payment. This is the estimated amount due each month under the formula’s assumptions. It includes principal and interest, not expenses such as insurance or escrow.
6. Read total interest. This estimates the interest across all scheduled payments, not the total amount repaid. Add the principal to total interest to see the modeled total of payments.
7. Check against the lender’s figures. Compare the entered inputs and assumptions with the loan disclosure or payment schedule. Differences may result from fees, payment dates, daily interest calculations, a final payment adjustment, or other loan-specific terms.

When to Use This Estimate

Use the calculation to understand how principal, APR, and repayment length affect a standard fixed-rate loan’s scheduled payment and interest. It can also help you check arithmetic in a quote or compare hypothetical combinations of inputs. To make a meaningful comparison, keep the assumptions consistent and enter the actual quoted figures rather than relying on a sample rate.

A longer term generally spreads repayment over more months, while the APR determines the interest rate used in each monthly calculation. The payment and total-interest outputs describe different things: the payment is a recurring amount; total interest is the modeled borrowing cost over the full schedule. Neither figure includes costs that are not entered.

Common Mistakes

• Entering APR as a decimal: Enter 8 for 8%, not 0.08. The calculator converts percentage APR to a monthly rate itself.
• Confusing years and months: The term field is in years. Entering 60 for a 60-month term would mean 60 years, not five.
• Using the amount received instead of the balance: If an agreement finances fees or other amounts, its principal may be larger than the cash paid out. Use the relevant starting balance from the loan terms.
• Treating the payment as the full monthly bill: The formula covers principal and interest only. Other required charges can change the amount due.
• Assuming a changing rate stays constant: A result based on one APR is a snapshot. It cannot forecast later payments on a variable-rate loan.
• Expecting exact lender totals: Rounding and payment timing can create small differences, and loan-specific charges are outside this calculation.

Tips

• Confirm whether the quoted rate is an APR and whether it is fixed before using the estimate.
• Keep a written note of the principal, APR, and term you entered so you can reproduce a result.
• For a term given in months, divide by 12 before entering it; use sufficient precision for fractional years when needed.
• Use the total-interest result as an estimate for the entire stated term, not as a yearly amount.
• For a detailed payment-by-payment balance or payoff figure, use the lender’s amortization schedule and account for any extra payments separately.

Troubleshooting

The result is rejected: Check that the principal is at least $1, APR is between 0.01% and 100%, and the term is within the permitted range. Remove currency symbols and commas if the input field expects a number only.

The payment seems unexpectedly high or low: Recheck the APR format and term units. Then verify that the principal matches the balance used for the lender’s quote. A shorter term or a higher APR increases the formula’s payment, all else equal.

Your lender’s number differs: Ask which principal, rate, term, payment dates, and charges were used. The lender may include fees or additional monthly costs, apply a different day-count method, or adjust the final payment. This calculator cannot reproduce terms that are not represented by its inputs.

The loan has a variable rate or irregular payments: Treat this result only as a fixed-rate, equal-payment scenario. For a reliable schedule, use the lender’s current repayment details and request an updated projection when the rate or payment changes.

Frequently Asked Questions

What does it mean to express interest in a loan?
It means stating the loan’s borrowing cost as a rate, commonly an APR. This calculator uses the APR you enter to estimate monthly payments and total interest for a fixed-rate repayment schedule.

Is APR the same as the monthly interest rate?
No. APR is entered as a yearly percentage. The formula converts it to a monthly decimal rate by dividing by 1200. Loan disclosures may define APR to include certain costs, but this calculator does not add fees as separate inputs.

Does total interest include the amount borrowed?
No. Total interest is the estimated cost beyond principal. Add the principal to the displayed total interest to calculate the model’s total scheduled payments.

Can I use this for a variable-rate loan?
You can model one fixed-rate scenario by entering an APR, but the result does not predict rate changes. Future payments and total interest may differ if the rate changes.

Why is the estimate different from a lender’s payment?
The lender may include fees or other charges, use different payment timing or interest calculations, or adjust the last payment. Verify the assumptions and figures against the lender’s disclosure.

This result is an estimate, 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