Cost Performance Index (CPI)  – 1) A measure of cost efficiency on a project. It is the ratio of earned value (EV) to actual costs (AC). CPI= EV divided by AC.T 2) the ratio of budgeted costs to actual costs (BCWP/ACWP). CPI is often used to predict the magnitude of a possible cost overrun using the following formula: original cost estimate/CPI = projected cost at completion.