Schedule Performance Index (SPI) is the ratio of earned value to planned value at a given point in time to measure the schedule efficiency reflecting how quickly or slowly project work is progressing. An SPI lower than one indicates the project is behind schedule. An SPI higher than one indicates the project is ahead of schedule.

Formula: SPI = EV / PV
Where: SPI = Schedule Performance Index; EV = Earned Value; PV = Planned Value.