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Loans & mortgages

Interest Rate Calculator

Know the loan amount and the payment but not the rate? This works backward to find the interest rate, for example on a car deal or a “low monthly payment” offer.

9.496%Annual interest rate (APR)

Total paid$25,200.00
Total interest$5,200.00
Effective annual rate9.921%

Found numerically: the rate at which this loan and payment schedule match exactly.

How this calculator works

There is no simple formula to solve for the rate, so the calculator searches numerically for the rate at which the loan and payments match exactly.

The result is the annual percentage rate (monthly rate × 12).

The effective annual rate also shows the effect of monthly compounding: (1 + monthly rate)¹² − 1.

If the payments add up to less than the loan, no positive rate fits, and the calculator says so.

Worked example

$20,000 borrowed, $420 a month for 60 months

  1. Total paid: $25,200, so $5,200 is interest.
  2. The matching rate is about 9.50% APR.
  3. Effective annual rate: about 9.92%.

Questions people ask

Why check the rate on a dealer offer?

Offers are often framed by monthly payment. Working out the rate shows the real cost and lets you compare with a bank pre-approval.

Does this include fees?

If fees were added to the loan amount, they are included. If you paid them upfront, use the APR calculator for the fee-inclusive rate.

What is a good interest rate?

It depends on the loan type, your credit and the market. Compare quotes from a few lenders on the same day.

Can I use this for a lease?

Leases use a money factor and residual value, so they need a different calculation.

Last reviewed October 2, 2026