Interest Rate Calculator – Find the Rate on a Loan or Investment

Lenders and sellers love to talk about the monthly payment, but the number that tells you whether a deal is good is the interest rate. This calculator works backwards from what you know. Give it the loan amount, the payment and the number of payments, and it finds the rate. Switch to growth mode to find the yearly rate at which an investment grew from one value to another.

How to use the interest rate calculator

For a loan or financing offer:

Leave Find the rate of set to A loan.
Enter the loan amount: the amount you actually borrow, after any down payment.
Enter the monthly payment and the number of monthly payments.

For an investment or a price change:

Choose Growth.
Enter the starting value, the ending value and the number of years between them.

The results show the annual interest rate, the rate per period, the effective annual rate after compounding, and the total interest paid or growth earned.

How the interest rate is found

For a loan, the payment, the amount and the rate are linked by the standard amortization formula:

payment = amount × r ÷ (1 − (1 + r)^−n)

Here r is the monthly rate and n is the number of payments. You can’t rearrange this to get r on its own, so the calculator searches for the rate that makes the equation balance. It narrows the range until the answer is exact to many decimal places. The annual rate is r × 12, and the effective annual rate is (1 + r)¹² − 1.

For growth, there is a direct formula, the compound annual growth rate:

rate = (ending value ÷ starting value)^(1 ÷ years) − 1

Worked example

A dealer offers $20,000 of financing at $400 a month for 60 months but won’t say the rate.

The payments total 400 × 60 = $24,000, so the loan costs $4,000 in interest.
The rate that fits is 0.618% a month, which is an annual rate of 7.42%.
With monthly compounding, the effective annual rate is 7.678%.

For growth: an investment that rose from $10,000 to $14,000 in 5 years grew at (1.4)^(1/5) − 1 = 6.961% a year.

When to use this calculator

Checking a dealer or store financing offer that only quotes a payment.
Comparing “buy now, pay later” and rent-to-own deals, where the rate is often very high but never stated.
Finding the return on an investment you held for several years.
Checking a loan agreement, to confirm the rate matches the payment schedule.

If the loan also has upfront fees, the APR calculator shows how they raise the true cost. To solve for any other part of a loan, such as the number of payments or the future value, use the finance calculator.

Common mistakes

Using the price instead of the amount borrowed. Subtract your down payment and trade-in, or the rate comes out too low.
Mixing up months and years. The number of payments for a 5-year monthly loan is 60, not 5.
Comparing a monthly rate with an annual one. 0.6% a month sounds small, but it is over 7% a year.
Ignoring add-ons in the payment. Insurance, warranties and service plans folded into a payment make the implied rate look higher than the loan’s real rate.

Frequently asked questions

Can I work out the interest rate from just the monthly payment?

You need three things, the amount borrowed, the payment and the number of payments. With those, there is exactly one interest rate that makes the payments repay the loan, and the calculator finds it.

Why is there no simple formula for the interest rate?

The loan payment formula can't be rearranged to isolate the rate, so it has to be found by trial and error. The calculator does this with a numerical search, the same way Excel's RATE function and financial calculators do.

What is the difference between the annual rate and the effective annual rate?

The annual rate is the monthly rate × 12. The effective annual rate includes compounding, (1 + monthly rate)^12 − 1, so it is a little higher. Loans are usually quoted with the first, and savings accounts with the second (as APY).

The rate I calculated is higher than my lender quoted. Why?

The quoted rate may leave out fees that were added to the loan, or the payment may include add-ons such as insurance or a service contract. Use the amount you actually received, and the payment without add-ons, to find the true rate.

Last updated October 31, 2026 · By the ZTool team