Interest Rate Calculator

Sometimes you know everything about a loan except the one number that matters most for comparing it: the actual interest rate. A dealer or private seller quotes a monthly payment and a term, or an old loan statement lists a balance and payment but no rate — either way, you're missing the figure you'd need to shop it against other offers.

This calculator works backward from what you do know — the loan amount, the payment, and the term — to the rate that reconciles them, which is exactly the situation anyone verifying a financing offer or reconstructing an old loan's terms runs into.

Nothing about the loan you're checking — amount, payment, or term — is sent off your device; the whole search runs locally in your browser.

There's no algebraic formula that solves directly for the interest rate given a payment, loan amount and term — the standard payment formula can't be rearranged that way. Instead, this calculator searches for it numerically: it repeatedly tests a candidate rate, computes the payment that rate would produce, and narrows the range up or down until the computed payment converges on the one you entered.

That search (a binary search, sometimes called bisection) typically needs only a few dozen iterations to close in on a rate accurate to many decimal places, which is why it returns instantly despite having no closed-form shortcut.