The annual percentage rate (APR) is the simplest measure of an investment's return: it expresses earnings as a percentage of the amount invested per year. If a CD has an APR of r%, then a principal P held for exactly one year earns P × (r/100) in interest.
For investments shorter or longer than one year, the APR is prorated by days: interest = principal × rate × (days / 365). This is the simple-interest formula used for Egyptian bank CDs, T-bills, and savings accounts. APR does not account for compounding — for compound growth on funds and ETFs, see CAGR.