Interest Rate Calculator
Know the payment but not the rate? Enter your loan amount, monthly payment, and term, and we'll solve for the interest rate hidden inside.
A $25,000 loan at $500/mo for 5 years isn't free money — it works out to about 7.42% APR and $5,000 in interest.
7.42%
APR implied by a $25,000 loan at $500/mo over 5 years
$5,000
total interest paid over the life of that loan
20%
interest as a share of the amount borrowed
The rate hiding in your payment
Lenders advertise payments; this tool exposes the interest rate behind them.
Reverse-engineer the rate
Lenders quote payments; this tool reveals the rate behind them. Enter the loan, the monthly payment, and the term, and we solve for the APR — 7.42% in the example — so you know exactly what a deal costs.
Solved, not estimated
Because the rate can't be isolated algebraically, the calculator uses a numerical search (bisection) on the payment equation. It converges on the precise rate that reproduces your payment to the cent.
Spot a bad deal
A tempting low payment can hide a high rate or a punishingly long term. Seeing the implied APR and the total interest side by side helps you compare offers on equal footing.
How the Interest Rate Calculator Works
Formula
Payment(r) = P · r / (1 − (1 + r)^−n)
Solve for r (the monthly rate) so Payment(r) = your payment,
then APR = r × 12.
P = loan amount, n = term in months
Solved numerically by bisection — no closed-form solution exists.Enter the loan amount
The principal you borrowed.
Enter the monthly payment
What you pay (principal + interest).
Enter the term
How many years the loan runs.
Solve for the rate
We search for the APR that fits your payment.
Review the cost
Implied APR, total interest, and a rate-by-payment chart.
Most loan calculators start with a rate and give you a payment. This one runs in reverse: you already know the payment, and you want the rate. Because the interest rate can't be isolated with algebra, the calculator searches for it numerically — converging on the exact APR that reproduces your payment. A $25,000 loan at $500/mo over 5 years, for instance, implies about 7.42% APR and $5,000 of interest.
This is especially handy when a dealer or lender pushes a monthly payment without clearly disclosing the rate, or when you want to sanity-check an offer. Pair it with the loan payment and APR calculators to see the full picture — payment, rate, and the all-in cost of borrowing including fees.