About the Interest Rate calculator
Know your loan amount, monthly payment and number of payments, but not the rate? This interest rate calculator solves for it. It is useful for checking a dealer’s offer, a buy-now-pay-later plan or any loan quoted only as a monthly payment.
You get the APR, the effective annual rate after monthly compounding, total interest and total paid. Since there is no simple formula for the rate, the calculator searches for it numerically and the working shows the monthly rate it found and how it becomes the APR.
How to use it
- Enter the Loan amount you receive.
- Enter the Monthly payment you will make.
- Enter the Number of months in the loan.
- Read the APR, the effective annual rate and the total interest.
The formula
The calculator finds the monthly rate r that makes the standard payment formula match your payment:
M = P × r ÷ (1 − (1 + r)^−n)
- M — monthly payment
- P — loan amount
- n — number of months
- r — monthly interest rate (the unknown)
There is no algebraic way to isolate r, so it uses a bisection search: try a rate, compare the resulting payment with yours, and narrow the range until they agree. Then:
APR = r × 12Effective annual rate = (1 + r)^12 − 1Total interest = M × n − P
Worked example
A $25,000 loan with 60 monthly payments of $495 means you pay $29,700 in total, so $4,700 is interest. The calculator finds a monthly rate of about 0.583%, which is an APR of about 7.00%. Because interest compounds monthly, the effective annual rate is a little higher, about 7.23%.
You can check the answer by putting the APR into the loan calculator: a $25,000 loan at 7% over 5 years gives a payment very close to $495.
If the payments add up to less than the loan amount, no positive rate can work, and the calculator asks you to check the numbers. If they add up exactly to the loan amount, the rate is 0%.
APR, effective rate and hidden costs
The APR is the yearly rate quoted on most US loans; it is the monthly rate times 12. The effective annual rate shows the true yearly cost once monthly compounding is counted, so it is always a bit higher than the APR when the rate is above zero.
This calculator is a good way to spot expensive financing. If a seller advertises a low price with “easy monthly payments,” enter the price, the payment and the term to see the rate you are really paying. Fees taken out of the loan matter too: if you receive less than the loan amount but repay the full payment, enter the amount you actually receive to see the higher true rate.
Lender disclosures can differ slightly because of payment timing and rounding, but the result should be close.
Frequently asked questions
How do I find the interest rate on a loan?
Enter the loan amount, monthly payment and number of months. The calculator searches for the rate that produces exactly that payment.
Why is there no formula for the interest rate?
The payment formula cannot be rearranged to isolate the rate, so it has to be solved numerically by trial and refinement.
What is the difference between APR and effective annual rate?
APR is the monthly rate times 12. The effective annual rate compounds the monthly rate over 12 months, so it is slightly higher.
What if my payments total less than the loan amount?
That would mean a negative interest rate, so the calculator asks you to check your inputs.
Last updated October 24, 2026. Results are estimates for education and planning — see our disclaimer.