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The detailed guide below is currently available in English.
Working backwards from the payment
Lenders quote you a payment; this calculator recovers the rate inside it. The loan payment formula links four values — principal P, term n, payment M, and monthly rate r:
P = M × (1 − (1 + r)−n) / r
Given the other three, r cannot be isolated algebraically, so the calculator solves it numerically with bisection: it narrows a bracket around the true rate until the annuity equation balances to within a tiny tolerance.
What the solved rate tells you
- Compare dealer or lender offers on payment alone and you can mistake an expensive loan for a cheap one — compare the solved APR instead.
- Total interest in the summary shows the full cost of the financing embedded in that payment.
- If the result seems implausibly high, check whether fees or add-on products are buried in the payment.
Sık sorulan sorular
Why can’t the rate just be interest divided by the loan?
Because each payment includes principal, the balance you are borrowing against shrinks every month. The interest charge applies to declining balances, which is what makes the equation nonlinear and worth solving numerically.
Is the result the same as APR?
It is the nominal annual rate (monthly rate × 12). A regulatory APR would also spread lender fees across the term, so if fees are rolled into your payment, enter the amount borrowed net of nothing — the result reflects the payment as entered.
What if the payment is too low?
Below straight-line principal (amount ÷ months), no interest rate — even 0% — repays the loan in time, and the calculator explains that instead of showing a nonsense rate.