Why can't the interest rate be calculated directly like a payment can?

The payment formula involves the rate raised to a power in a way that can't be algebraically isolated — so instead of solving directly, the calculator numerically searches for the rate that matches your payment.

What if I don't know my exact monthly payment?

Check a loan statement or amortization schedule for the exact fixed payment amount — a rounded or approximate payment will produce a rate estimate, not an exact answer.

Interest Rate Calculator

Find the interest rate on a loan when you know the loan amount, the monthly payment, and the term.

Loan amount

Monthly payment

Term (months)

Estimated Interest Rate

7.5%

How It Works

Working backward from a payment

Unlike calculating a payment from a known rate, there's no direct formula to solve for the rate — instead, this calculator searches for the rate that produces your exact monthly payment, narrowing in step by step until it converges.

Example Calculation

A $20,000 loan over 60 months with a $400.76 monthly payment works out to an interest rate of about 7.5%.

Common Mistakes

Entering the total amount repaid instead of the loan's original principal.
Mixing up the loan term in months vs. years — this calculator expects months.

Frequently Asked Questions

Why can't the interest rate be calculated directly like a payment can?: The payment formula involves the rate raised to a power in a way that can't be algebraically isolated — so instead of solving directly, the calculator numerically searches for the rate that matches your payment.
What if I don't know my exact monthly payment?: Check a loan statement or amortization schedule for the exact fixed payment amount — a rounded or approximate payment will produce a rate estimate, not an exact answer.