Schedule Performance Index (SPI)

The Schedule Performance Index (SPI) compresses schedule health into a single ratio: the Earned Value of work completed divided by the Planned Value of work that should have been completed by now (SPI = EV / PV).

An SPI of exactly 1.0 means the project is precisely on schedule in earned-value terms. Above 1.0 means more value has been earned than planned, so the work is ahead; below 1.0 means it is behind. For example, if EV is 90,000 and PV is 100,000, the SPI is 0.90 - the team has delivered 90% of the value it should have by this point.

SPI is a budget-weighted measure of progress, not a calendar date, so it is most reliable mid-project; near completion it naturally trends toward 1.0 even on late projects because the remaining Planned Value shrinks. Read it alongside the critical path and the Cost Performance Index for a full picture.