About the Interest Rate Calculator
Sometimes you know what you borrowed and what you pay each month, but not the interest rate. This interest rate calculator works it out. Enter the amount borrowed, the regular payment and the number of payments, and it finds the yearly rate by trial and error, with the effective yearly rate and the total interest you will pay. A second mode finds the yearly rate of return of a saving or investment from its starting value, ending value and number of years. It is useful for checking a dealer's finance offer, a personal loan, a buy-now-pay-later plan or the growth of an investment.
Updated October 2026
How to use the Interest Rate Calculator
- 1Choose loan or savings and investment.
- 2For a loan, enter the amount, the payment each period and the number of payments.
- 3Pick how often payments are made.
- 4For an investment, enter the start value, end value and years.
- 5Read the yearly rate and the totals.
Checking a finance offer
Dealers and shops often quote a monthly payment and a total price rather than a rate. Enter the amount you borrow after any deposit, the monthly payment and the number of months, and the calculator tells you the real yearly rate. Compare that with a bank loan to see whether the offer is competitive. If the rate looks very high, ask what fees and insurance are included in the payment.
Good habits
- Use the amount actually financed, not the sticker price.
- Include every payment, even a final larger one, in a separate calculation if needed.
- Compare the effective rate between offers, not the headline rate.
- Look at the total repaid, not only the monthly figure.
- Remember that a longer term lowers the payment but raises the total interest.
Related tools
The Loan Calculator finds payments from a known rate, the APR Calculator includes fees, the Effective Interest Rate Calculator converts between nominal and effective rates, the CAGR Calculator finds growth rates and the Amortization Calculator shows each payment. This is a calculation aid and not financial advice.
Features
- Loan rate from payment, amount and term
- Yearly growth rate from start and end value
- Monthly, fortnightly, weekly, quarterly or yearly payments
- Effective yearly rate (APY)
- Total repaid and total interest
- Clear messages if the numbers do not add up
- Runs in your browser
Frequently asked questions
How do I find the interest rate on a loan?
There is no simple formula, because the rate sits inside the payment equation. The calculator tries rates until the payment matches the one you entered. A 20,000 loan paid at 386.66 a month for 60 months works out to 6 percent a year.
Is this the same as APR?
For a loan with equal payments and no fees, the nominal rate it finds equals the APR. If the lender also charges fees, the true cost is higher. In that case use the APR Calculator, which includes upfront charges.
What if the total of my payments is less than the loan?
That would mean a negative interest rate, which a normal loan does not have, so the tool shows a message. Check that you have the right amount, payment and number of payments, and that the payment is not already net of a deposit.
How do I find the annual return of an investment?
Divide the ending value by the starting value, raise the result to the power of 1 over the number of years, and subtract 1. An investment that grows from 10,000 to 15,000 in 5 years returns about 8.45 percent a year, compounded. Choose the savings option in the tool.
What is the difference between nominal and effective rate?
The nominal rate is the quoted yearly rate. The effective rate includes the effect of compounding, so for monthly payments it is a bit higher. The tool shows both so you can compare with offers that quote either one.
Why is the rate I find different from what the lender told me?
Lenders may include fees, insurance or a balloon payment, may quote a flat rate calculated on the original balance, or may charge interest in a different way. A flat rate looks lower than the true rate, which is why the effective rate from the payments is higher.