The cost performance index (CPI) is a measure of the conformance of the actual work completed (measured by its earned value) to the actual cost incurred: CPI = EV / AC. What is the Cost Performance Index (CPI)? Cost performance index (CPI) also known as earned vs. burned, measures the financial effectiveness and efficiency of a project. It represents the amount of completed work for every monetary unit spent. CPI is expressed as a ratio calculated by diving the budgeted cost of work completed (BCWP) or earned value, by the actual cost of the work performed (ACWP) or burned costs. As an example, if a project has an earned value of $30,000 but burned costs were $40,000. CPI = EV / AC = 30,000 / 40,000 = 0.75