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Interest Rate Calculator
Determine the true cost of borrowing or the real return on your savings.
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Determine the true cost of borrowing or the real return on your savings.
Enter details to reverse-calculate the rate.
Reverse-engineered from your payments
Total Interest
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Total Payment
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Identifying the true cost of a loan can be difficult when only the monthly payment is clear. Our Interest Rate Calculator uses advanced numerical methods to reverse-engineer any fixed-payment loan, revealing the actual Annual Percentage Rate (APR) you are paying.
P = M × [ (1 - (1+r)^-n) / r ]Our tool solves for 'r' (the periodic interest rate) using the Newton-Raphson iterative method. Because 'r' cannot be isolated through basic algebra, this algorithm performs multiple calculations to find the exact point where the loan balance reaches zero.
Because the interest rate appears in both the numerator and as an exponent in loan formulas, it cannot be solved for 'r' using standard arithmetic. It requires complex calculus or computer-based iterative solvers like the one built into this tool.
The nominal rate is the basic periodic interest rate multiplied by the number of periods. The APR provides a more comprehensive look at the cost of the loan, often including fees and the effect of compounding.
The calculator will return an error. In a standard loan, the total of all monthly payments must exceed the original loan amount to account for the interest charged by the lender.
Yes, this tool works perfectly for any fixed-rate, fixed-payment mortgage. It will give you the underlying interest rate based on your monthly principal and interest payment.
"Federal tax estimates are based on 2024 brackets. Consult a tax professional for official filing."