Interest Rate Calculator
Two tools: implied return on a single deposit, or APR on a fixed-rate loan from payment and term.
Investment: rate from growth
Given starting balance, ending balance, years, and how often interest compounds.
Nominal annual rate:
Loan: APR from payment
Loan amount, monthly payment, and number of payments (level payment loan).
Estimated APR (nominal, monthly compounding):
How these tools solve for the rate
Unlike calculating a future value or payment directly, solving for a rate has no simple formula — both tools use a binary search (repeatedly narrowing a range and testing the midpoint) to find the rate that makes the math work out, whether that's the rate that grows your starting balance into the ending balance you entered, or the rate that produces the exact loan payment you specified.
Frequently asked questions
Why do I need "compound frequency" for the investment tool? The same nominal annual rate produces a different result depending on how often it compounds — entering the compounding frequency that matches your actual account (e.g., monthly for most savings accounts) gives an accurate implied rate.
Why did I get an error on the loan APR tool? If your entered payment is too low to ever pay off the loan (it doesn't even cover principal at 0% interest, or the math otherwise has no valid solution), the calculator flags that instead of returning a misleading number.
Is the loan APR here the same as the lender's advertised APR? This estimates the interest rate implied purely by the loan amount, payment, and number of payments — advertised APRs sometimes bundle in fees, which would make the lender's quoted APR slightly higher than this rate-only calculation.