SPI Calculator

The Schedule Performance Index (SPI) shows how efficiently a project is progressing compared with its baseline plan. It divides the value of work completed (Earned Value) by the value of work that should have been completed by the status date (Planned Value). An SPI of 1.0 means exactly on schedule, below 1.0 means behind, above 1.0 means ahead.

How to use the calculator

1. Enter the Planned Value (PV) for the status date, from the baseline cost-loaded schedule.
2. Enter the Earned Value (EV), the budgeted value of the work actually completed.
3. Optionally enter the planned duration to estimate the forecast duration.
4. Read SPI, schedule variance and the forecast.

Formulas

• SPI = EV / PV
• SV = EV - PV
• Forecast duration ≈ Planned duration / SPI (a rough estimate)

Worked example

A 20-month project has PV = 3,000,000 and EV = 2,550,000 at month 8. SPI = 2,550,000 / 3,000,000 = 0.85. Only 85% of the planned work has been done. If this rate continues, the project would take about 20 / 0.85 = 23.5 months, roughly three and a half months late.

Interpreting SPI correctly

• SPI drifts back to 1.0 near the end. When all work is complete, EV equals PV, so a late project still finishes with SPI = 1.0. In the last third of a project, earned schedule SPI(t) is a more reliable measure.
• Check the critical path. SPI aggregates all work. Non-critical activities running ahead can hide critical delays.
• Consistent measurement of percent complete is essential; optimistic progress inflates EV and SPI.

Using SPI with CPI

The product CPI × SPI, sometimes called the critical ratio, gives a combined view. A project with SPI 0.85 and CPI 0.92 is both late and over budget, and recovery by adding resources will increase cost further.