Without the cap, both plans would have been set at 7.1% because March's Retail Price Index (RPI) figure rose to 4.1%. Why Plan 2 and Plan 3 Rates Are Capped at 6%The Department for Education sets annual student loan interest using the March RPI figure before the academic year begins. Interest rates do not directly change monthly repayments. Other Student Loan Rates From SeptemberPlan 5 undergraduate loans will carry a 4.1% interest rate from 1 September, matching RPI. The Student Loans Company says interest rates can change during the academic year when relevant caps apply.