What is an interest rate calculator?

An interest rate calculator works backwards from a known monthly payment, loan amount, and term to find the annual percentage rate (APR) that makes the payment schedule balance out.

How is the APR calculated?

The calculator uses a binary-search algorithm. It repeatedly guesses a monthly rate, computes the resulting payment with the standard amortization formula, and narrows the range until the computed payment matches yours to within a tiny tolerance.

What happens with a 0% interest rate?

When the monthly payment equals the loan amount divided by the number of payments, the implied rate is exactly 0% and the calculator returns that directly without the binary search.

Can I use this to compare loan offers?

Yes. If a lender quotes you a monthly payment rather than an APR, enter the loan amount, that payment, and the term to find the true APR and compare it across offers.

Interest Rate Calculator

Overview

Calculate the annual interest rate (APR) on a loan from the principal, monthly payment, and loan term using a precise binary-search solver.

The Interest Rate Calculator solves for the annual percentage rate (APR) when you know the loan amount, the fixed monthly payment, and the loan term. It uses a robust binary-search algorithm that converges for any realistic rate—including 0%—and returns the APR, monthly rate, total payment, and total interest charged.

How to use

Solve for your loan's APR in three quick steps.

Enter the loan amount: Type in the principal—the amount you borrowed or plan to borrow.
Enter the monthly payment: Input the fixed monthly payment amount you are making (or planning to make).
Enter the loan term: Specify the repayment period in years and click Calculate to see the implied APR.

FAQs

What is an interest rate calculator?: An interest rate calculator works backwards from a known monthly payment, loan amount, and term to find the annual percentage rate (APR) that makes the payment schedule balance out.
How is the APR calculated?: The calculator uses a binary-search algorithm. It repeatedly guesses a monthly rate, computes the resulting payment with the standard amortization formula, and narrows the range until the computed payment matches yours to within a tiny tolerance.
What happens with a 0% interest rate?: When the monthly payment equals the loan amount divided by the number of payments, the implied rate is exactly 0% and the calculator returns that directly without the binary search.
Can I use this to compare loan offers?: Yes. If a lender quotes you a monthly payment rather than an APR, enter the loan amount, that payment, and the term to find the true APR and compare it across offers.