Interest Rate Explained: How Loan Interest Works

An interest rate is the price of borrowing money, expressed as a percentage of the amount borrowed, or the return paid for saving or lending money. This calculator estimates the monthly payment and total interest for a fixed-rate, level-payment loan; enter the loan amount, annual rate, and term to see both results.

Formula

For a fixed-rate loan repaid in equal monthly payments, the payment is calculated using the standard amortizing-loan formula:

Monthly payment = P × r / [1 − (1 + r)−n]

• P is the starting loan principal, in dollars.
• r is the monthly interest rate as a decimal: annual percentage rate (APR) ÷ 12 ÷ 100.
• n is the number of monthly payments: term in years × 12.

The calculator rounds the monthly payment to the nearest cent. It estimates total interest as the rounded monthly payment multiplied by the number of payments, minus the original principal: total interest = payment × n − P. Small differences from a lender’s final total can occur because actual payment schedules may adjust the last payment or use different conventions.

Worked Example

Using the calculator’s defaults, the principal is $20,000, APR is 8%, and the term is 5 years.

1. Convert the APR to a monthly decimal rate: r = 8 ÷ 1200 = 0.0066667.
2. Convert the term to payments: n = 5 × 12 = 60.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)−60] ≈ $405.53 per month.
4. Use the rounded payment for total interest: $405.53 × 60 − $20,000 = $4,331.80.

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

Assumptions

• The interest rate is fixed for the full loan term.
• Payments are equal, made monthly, and paid at the end of each monthly period.
• Interest compounds monthly using APR divided by 12.
• The estimate excludes fees, taxes, insurance, PMI, and escrow, and assumes the stated loan amount is the principal being repaid.

What an Interest Rate Means

When you borrow, interest is the amount charged for using someone else’s money. When you deposit or lend money, interest is the return paid to you. A rate is usually quoted as a percentage per year, but the amount of interest that accumulates depends on the balance, the time involved, how often interest is calculated, and whether payments reduce the balance.

For example, 8% is not a promise that every loan will cost exactly 8% of its original principal each year. On an amortizing loan, the outstanding balance declines as payments are made, so the interest portion generally declines too. A loan’s APR can also reflect certain borrowing costs in addition to its stated interest rate; the exact disclosure and calculation depend on the product. Use the lender’s quoted figures when estimating a real loan.

How Loan Interest Builds

With a typical amortizing payment, each payment covers interest accrued since the previous payment and reduces principal. Early in the schedule, the balance is larger, so more of a level payment goes to interest. Later, as the balance shrinks, more goes toward principal. The payment formula is designed to pay off the balance after the stated number of periods if its assumptions hold.

Interest rate and total interest are not interchangeable. The rate is a percentage; total interest is a currency amount accumulated over the loan. In the example, the rate is 8% annually, while the estimated interest over five years is $4,331.80. A longer term generally means more payment periods and can increase total interest, even if the monthly payment is lower. The exact result depends on the rate, principal, term, and payment structure.

Interest Rate, APR, and Compounding

The stated interest rate describes the rate applied to a balance under the loan’s terms. APR is a standardized annualized measure intended to represent borrowing cost more broadly for certain products, but it is not always identical to the note rate. Not every charge is necessarily included in APR, and conventions differ by product. This calculator treats the APR input as the annual rate used directly in the monthly payment formula; it does not separately add fees.

Compounding is the process by which interest is calculated on a balance, potentially including previously accumulated interest. For a loan with regular payments, the schedule also depends on when payments are credited and how the lender calculates accrued interest. The calculator assumes monthly periods and a monthly rate equal to APR divided by 12. It is a useful simplified estimate, not a substitute for the loan’s payment schedule.

Using the Estimate Carefully

Enter the principal you expect to repay, the annual APR as a percentage (for example, enter 8 for 8%), and the term in years. The result assumes the full principal is disbursed and repaid over the selected term. If a lender gives a monthly payment or total that differs, check whether the quote includes fees, an adjustable rate, a different payment date, a balloon payment, or other features not represented here.

• Do not enter 0.08 for an 8% APR; the input is already expressed as a percentage.
• Check whether the quoted rate is fixed or can change over time.
• Compare like with like: principal, term, payment frequency, and costs all affect the figures.
• Rounding the payment to cents can create a small difference, especially if a lender adjusts the final installment.

This formula does not model variable rates, irregular payments, interest-only periods, fees financed into the balance, skipped payments, or extra principal payments. It also does not determine eligibility, approval, or the rate a lender may offer. Rates, fees, and loan terms are inputs to obtain from the lender, not predictions made by this calculator.

Frequently Asked Questions

What is an interest rate in simple terms?
An interest rate is the percentage charged for borrowing money or paid for lending or saving it. The actual amount depends on the balance, time, and calculation method.

How do I calculate monthly interest from an APR?
For this calculator’s monthly-compounding assumption, divide APR by 12 and then by 100. An 8% APR becomes a monthly decimal rate of 0.0066667.

Why does a longer loan term usually mean more total interest?
A longer term typically means the balance remains outstanding for more payment periods. Even if each payment is smaller, interest can accumulate over a longer span.

Is APR the same as the interest rate?
Not necessarily. APR may include certain borrowing costs as well as interest, depending on the product and applicable calculation. Review the lender’s disclosures to see what is included.

Does this calculator include fees or a changing rate?
No. It assumes a fixed rate and excludes fees and other costs. It cannot represent a variable-rate schedule or lender-specific payment rules.

This result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• 20-Year Fixed Interest Rate Calculator and Payment Guide
• Interest Rate Adjustment Calculator and Payment Changes
• Effective Interest Rate: Calculate the True Annual Cost

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