Interest Rate Calculator Online
Back out the interest rate on a loan from its amount, payment and term, right in your browser. Estimates only, not financial advice.
The Interest Rate Calculator runs entirely in your browser. The loan amount, payment and term you enter stay on your device and are never uploaded to ArrayKit. Results are estimates only, not financial advice.
Open the Loan Calculator
About Interest Rate Calculator
Interest Rate Calculator works backwards from a loan's amount, fixed monthly payment and term to find the interest rate that produced them. Enter the loan amount, the payment you're making (or quoted), and the number of months, and it solves the standard amortization equation for the monthly rate, then annualizes it to a familiar APR figure. It is handy for checking whether a lender's advertised rate matches the numbers on your statement, comparing a financing offer that only lists a payment amount, or sanity-checking a car dealer's or retailer's installment plan before you sign. When the payment can never pay off the amount in the given term, a clear error explains why instead of showing a misleading number. These are estimates only, not financial advice — confirm the exact rate with your lender. Everything runs locally in your browser; your loan numbers are never uploaded.
Features
- Solves for the monthly and annualized interest rate from amount, payment and term
- Uses the standard amortization present-value equation, solved by bisection
- Handles a near-zero implied rate (payment barely covers the principal)
- Clear error banner when no positive rate can produce the entered payment
- Copy a plain-text summary of the loan amount, payment, term and solved rate
- Works for any fixed-payment installment loan, not just mortgages
- Estimates only — labelled as not financial advice
- Runs entirely in your browser with no loan data sent anywhere
How to use the Interest Rate Calculator
- Enter the loan amount that was financed
- Enter the fixed monthly payment you pay (or were quoted)
- Enter the loan term in months
- Read the solved monthly rate and annualized interest rate (APR)
Example
Input
$10,000, $299.71/mo, 36 mo
Output
Interest rate: 5.00% APR
A $299.71 payment for 36 months on a $10,000 loan implies about 5% APR.
Common errors & troubleshooting
- The Interest Rate Calculator shows an error instead of a rate. — The entered payment times the term must exceed the loan amount — if $200/mo for 36 months only totals $7,200 on a $10,000 loan, no positive rate can produce that payment. Raise the payment, lengthen the term, or lower the amount.
- The solved rate is much higher than expected. — Double-check the term is in months, not years — entering '3' instead of '36' inflates the implied rate because the same payment is being asked to retire the loan much faster.
- The result doesn't match the lender's advertised rate. — This solves for the pure interest rate implied by the payment stream; it does not account for upfront fees or points the way an APR disclosure does — use the APR Calculator if fees are involved.
- The rate comes back as (almost) 0%. — That's expected when the payment times the term lands right at the loan amount — it means the payments barely cover the principal with essentially no interest.
Frequently asked questions
- What does the Interest Rate Calculator solve for?
- Given a loan amount, a fixed monthly payment and a term in months, it solves the amortization equation for the monthly interest rate that produced that payment, then reports both the monthly rate and the annualized rate (APR).
- What formula does the Interest Rate Calculator use?
- It solves P = M × (1 − (1 + r)⁻ⁿ) / r for the monthly rate r, where P is the principal, M is the fixed monthly payment and n is the number of payments, using bisection since the equation has no closed-form solution for r.
- Why does the Interest Rate Calculator sometimes show an error?
- If the monthly payment multiplied by the term is less than the loan amount, no positive interest rate can produce that payment — the loan would need a negative rate to be paid off, which the calculator flags instead of returning a misleading result.
- Can I use the Interest Rate Calculator for a car loan or personal loan?
- Yes. It works for any fixed-payment installment loan — auto loans, personal loans, retailer financing, or a mortgage — as long as you know the amount financed, the fixed monthly payment and the term in months.
- Is the Interest Rate Calculator's result financial advice?
- No. The Interest Rate Calculator produces estimates only, not financial advice. It does not account for fees, points or a variable rate — confirm the exact figure on your loan documents or with your lender.
- Does the Interest Rate Calculator send my loan numbers anywhere?
- No. The Interest Rate Calculator runs entirely in your browser. The amount, payment and term you enter stay on your device and are never uploaded to ArrayKit.
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