Full definition
RRSO (Rzeczywista Roczna Stopa Oprocentowania) — the Polish implementation of APR — is a standardized metric that allows consumers to compare the true cost of different loan products on a like-for-like basis. Unlike nominal interest rate, RRSO captures every mandatory cost of the loan: interest, origination fee, preparation fees, insurance (if obligatory), account-maintenance fees linked to the loan. It is calculated using the IRR-based formula set out in the Polish Consumer Credit Act of 12 May 2011. Every Polish lender is legally required to disclose RRSO in every advertisement, preliminary offer and loan agreement — failing to do so is a statutory violation subject to sanctions by UOKiK (the Polish consumer protection authority).
Formula
RRSO is the rate X satisfying: Σ Aₖ/(1+X)^tk = Σ A'ₗ/(1+X)^tl, where Aₖ = loan disbursements, A'ₗ = repayments and fees, tk/tl = time in years.
Concrete numeric example
A PLN 30,000 loan over 4 years with nominal interest 7.5% and a 5% origination fee has an RRSO of roughly 10.2%. Real annual cost = 10.2%, not 7.5% as the headline rate would suggest.