Interest Rate Calculator
Solve for the interest rate given principal, payment, and term of any loan.
Interest Rate Results
How to Use the Interest Rate Calculator
Enter the loan principal, your monthly payment, and the number of months/years to find the implied annual interest rate (APR).
Formula
Uses numerical binary search to solve the standard loan payment equation for rate: M = P × r(1+r)ⁿ / [(1+r)ⁿ−1].
Frequently Asked Questions
Why would I solve for the interest rate?
This is useful when you already know a payment amount and want to understand the effective rate — for example, when comparing dealer financing offers.
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