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Interest Rate Calculator

Know your loan amount, term, and monthly payment but not the rate? This tool works backward from those three numbers to estimate the interest rate you're actually paying.

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Estimated Interest Rate
$0
Total Interest
$0
Total Paid
Remaining Balance

How the Interest Rate Calculator works

Every fixed-payment loan links four numbers together: the loan amount, the interest rate, the term, and the monthly payment. If you know any three of them, the fourth can be found. This calculator solves for the interest rate when you already know the amount, the term, and the payment — which is common when a lender quotes you a payment without spelling out the rate.

Unlike the payment formula, there's no simple algebraic way to isolate the rate directly, so this calculator uses an iterative search: it repeatedly tests candidate rates and narrows in on the one whose resulting payment matches yours.

Payment(r) = (P × r) / (1 − (1 + r)^−n)
solved for r by bisection search, since there is no closed-form inverse

If the payment you enter isn't even enough to repay the principal over that term at any positive rate, there's no valid rate to solve for — the calculator will flag this instead of guessing.

Frequently asked questions

Some loan offers only tell you the amount, term, and monthly payment, without stating the interest rate directly. This calculator lets you back into the rate so you can compare the offer against other loans or your own budget.
It's a very close estimate based on standard amortization math. Your actual contract rate should match closely, though small differences can appear due to rounding, fees, or how your lender compounds interest.
If the total of all your payments wouldn't even cover the loan amount, there's no interest rate — positive or otherwise — that fits those numbers. Try increasing the monthly payment or lengthening the loan term.