Hidden Interest Rate Calculator
Saw a product priced at $1,000 cash or 10 × $120? This calculator reveals the embedded monthly and annual interest rate. Essential before accepting any installment plan.
Monthly vs annual rate
Retailers often advertise low monthly rates that hide high annual costs. Because interest compounds, 3% per month is roughly 42.6% per year. Use this calculator to see the real rate before signing.
Find the hidden interest rate in any installment plan
This calculator uses Newton's method to solve the Price amortization equation in reverse: given a cash price, a number of installments and the value of each installment, it returns the exact monthly interest rate and the equivalent annual rate (effective APR). Use it any time a retailer or lender offers 'easy installments' without disclosing the rate.
Example: a product listed at $1,000 cash or 10 × $120. The 'extra' is only $200 (20% on paper), but the true installment formula in compound interest math gives you about 3.5% monthly — roughly 51% effective APR.
Nominal vs effective rate (APR)
The nominal rate is the headline number (e.g. 2%/month). The effective rate accounts for compounding — 2%/month compounds to 26.82%/year, not 24%. The CET (Brazil) or full APR (U.S.) adds fees, taxes and insurance on top of the effective rate, and it's the only number that shows the true cost of credit.
This calculator returns the effective rate hidden in the installments you typed in. If the lender's quoted rate is much lower than what this calculator finds, the difference is being charged as fees.
Is 'zero-interest financing' really zero?
Rarely. The cash price usually embeds the financing cost already. To check, ask the retailer for a cash discount: if there's a meaningful discount for paying in full, that discount is the hidden interest. Run this calculator with the discounted cash price as 'asset value' and the regular installment to see the true rate.
FAQ
How do I calculate the interest rate on an installment plan?
Enter the cash price, the number of installments and the value of each installment in this calculator. It returns the monthly rate and the effective annual rate.
Difference between nominal and effective interest rate?
The nominal rate is the advertised number; the effective rate (APR) accounts for compounding over time. 2%/month compounds to 26.82%/year — that's the effective annual rate.
Does this work for credit card installments?
Yes. Enter the cash price, number of months and monthly installment. The calculator returns the rate the card issuer is actually charging — which is often hidden behind 'no-interest' marketing.