Interest Rate Calculator
Solve for the interest rate when you know principal, monthly payment and loan duration.
How to use
Enter principal, monthly payment and years; rate is calculated numerically.
Formula
Solves amortization formula for r (Newton-Raphson).
Examples
- $10,000, $200/mo, 5 years → ~7.4% annual
Important notes
Estimate; assumes fixed equal payments.
Frequently asked questions
When is this useful?
When a lender quotes monthly payments but not the APR.