What Does SPI Mean? Schedule Performance Index Values From 0.5 to 1.5

How to read your result

The schedule performance index (SPI) compares the work you have finished with the work you planned to have finished by now. It is a ratio, so it has no unit, and it reads the same for a 10,000 project as for a 10 million one. This page explains what each SPI value means, shows the delay each implies for a 20-week plan, and lists the traps in reading it. To calculate your own, use the schedule variance and SPI calculator.

SPI is earned value (EV) divided by planned value (PV). Earned value is the planned value of the work that is actually done. If you planned 50,000 of work by this week and finished work that was planned at 40,000, SPI is 0.8.

What SPI says about the schedule

SPI = EV ÷ PV    forecast duration = planned duration ÷ SPI

An SPI of 0.5 doubles the duration, 0.8 lengthens it by a quarter, 1.25 shortens it by a fifth. Because the forecast divides by SPI, the effect is not symmetrical: a project at 0.8 is 25% late, while one at 1.25 is only 20% early.

Worked examples

Each example uses numbers you can load into the calculator above; the results match the tool exactly.

SPI bands from 0.5 to 1.5

This table uses a 20-week plan. For another plan length multiply the forecast by length ÷ 20. The band names are the ones the calculator shows.

What each band means

• Far behind (below 0.5): Less than half of the planned work is done. The plan needs a reset: re-estimate, cut scope or add capacity.
• Seriously behind (0.5 up to 0.7): At this pace the project takes well over 40% longer than planned. Expect a missed date unless something changes.
• Behind (0.7 up to 0.9): Noticeably behind. Find the cause now, while a 10% to 30% slip can still be recovered.
• Slightly behind (0.9 up to 1.0): A small slip. Often within normal noise, but watch the trend over the next periods.
• On schedule (exactly 1.0): Earned value equals planned value: progress matches the plan.
• Ahead (above 1.0 up to 1.2): More work is done than planned. Check that quality and the plan itself (was it padded?) explain it.
• Well ahead (1.2 and above): Far ahead of plan. Verify the baseline: early finishes this large are more often a planning error than a speed-up.

Why one SPI value is not the whole story

SPI is based on value, not on the critical path. A project can show a good SPI because easy, high-value work was finished early, while the task that sets the end date is late. Always compare SPI with the critical path. Also note that SPI drifts to 1.0 at the end of any project, since EV equals PV once everything is finished; the figure is most meaningful in the middle of the project. SPI around 0.9 on a long project is normal noise; the same value two weeks before the end is a real delay.

How to read your own SPI

1. Calculate SPI as EV ÷ PV at a fixed status date.
2. Find the band in the table above.
3. Convert to a forecast: planned duration ÷ SPI. The difference from the plan is the expected delay.
4. Check the trend. One reading is a snapshot; three in a row is a direction.
5. Look at the critical path before deciding what to do.

Assumptions, limits and common mistakes

Assumptions and limits

• The forecast assumes today's schedule efficiency continues for the rest of the project.
• SPI tends to 1.0 as a project completes, so late-project values say little about delay.
• The table is for a 20-week plan with no holidays or non-linear effort.

Common mistakes

• Reading 0.9 as "10% late". The forecast is 20 ÷ 0.9 = 22.2 weeks, which is 11% late, not 10%.
• Treating SPI as a date. SPI is a ratio; convert it with the planned duration to get a duration or date.
• Ignoring SPI above 1. Early work may mean the plan was padded or quality is being cut.
• Using SPI without a status date. PV is only defined at a specific date.

Frequently asked questions

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