The schedule performance index (SPI) measures the accuracy or efficiency of the work done against the work predicted by the schedule. It is a ratio. When it is equal to 1, the amount of work is equal to what was planned. When it is less than 1, the amount of work completed is less than what was expected by this point in the schedule. When it is greater than one, more work has been accomplished than was planned.
The equation to calculate the SPI:
or in words, the schedule performance index is the earned value (EV) divided by the planned value (PV).
This calculation is one of the calculations used in earned value management. The SPI is used in one of the equations for estimate at completion. Along with other measures, such as the critical path, it is used to estimate whether the project will be completed on, behind, or ahead of schedule.
PMBOK, 5th edition, section 18.104.22.168
Related: schedule variance