How to Calculate Earned Value Schedule Performance Index
Schedule Performance Index (SPI) is the earned value management ratio that converts schedule progress into an interpretable, numeric signal. It lets project leaders quantify whether work is ahead or behind plan without resorting to subjective status updates. This walkthrough details the formula behind the earned value SPI calculator and shows how to interpret schedule variance alongside broader financial analytics like the net present value tool and the internal rate of return walkthrough. Together these metrics keep portfolio decisions grounded in both timing and financial impact.
SPI is deceptively simple, but accuracy hinges on aligning earned value with your schedule baseline and ensuring planned value reflects the approved work scope. This guide defines the variables and units, provides a step-by-step computation workflow, and explains how to validate the metric before using it for governance decisions.
Definition and interpretation
Schedule Performance Index is the ratio of earned value to planned value. It measures schedule efficiency, not cost efficiency. An SPI above 1.00 indicates the project has earned more value than planned by a given date, meaning it is ahead of schedule. An SPI below 1.00 means work is behind plan. Because the metric is dimensionless, it is easy to compare across projects and reporting periods.
SPI does not replace critical path analysis. A project can have an SPI above 1.00 while still missing critical milestones if the earned value is concentrated in non-critical tasks. Use SPI as a high-level signal, then drill down into schedule logic for tactical decisions.
Variables and units
The calculation uses two primary values, both expressed in the same currency units:
- EV – Earned value. The budgeted value of work actually completed by the measurement date.
- PV – Planned value. The budgeted value of work scheduled to be completed by the same date.
EV and PV must share the same baseline scope, currency, and time cut-off. If EV is calculated from a different baseline or at a different time than PV, the resulting SPI will misrepresent schedule performance. Confirm that both numbers come from the approved project baseline and the same reporting period.
Formula and schedule variance
SPI is computed using the ratio of earned value to planned value. You can also compute schedule variance to express the absolute gap:
Schedule Performance Index: SPI = EV ÷ PV
Schedule variance: SV = EV − PV
SPI is dimensionless. SV is expressed in the same currency as the baseline budget, indicating the value of work ahead of or behind schedule. Report both values together to provide a complete view: SPI indicates proportional efficiency, while SV shows the absolute magnitude of the schedule gap.
Step-by-step calculation workflow
1. Confirm the approved baseline
Validate that the schedule baseline and budget baseline are approved and unchanged. If the project has undergone a rebaseline, use the most recent approved baseline for both EV and PV. Document any changes in scope or sequencing that affect planned value.
2. Calculate earned value consistently
Determine EV using the earned value method defined in your project controls plan, such as percent complete, milestones, or physical progress. Apply the same method across all work packages and ensure that progress measurements are verified before inclusion.
3. Extract planned value for the same cut-off
Pull PV from the baseline schedule for the same reporting date used for EV. Avoid mixing weekly and monthly data; the time cut-off must match exactly for the ratio to be meaningful.
4. Compute SPI and schedule variance
Apply the formulas above or use the embedded calculator. Compare SPI against your program governance thresholds. If SPI is below target, quantify the magnitude of the schedule variance to help the team understand the scale of the recovery plan required.
5. Interpret in context of critical path
Compare SPI with the critical path status. If SPI is below target but critical milestones are intact, the schedule risk may be manageable. If SPI is near 1.00 but critical milestones are slipping, prioritize critical path recovery over aggregate ratio improvements.
Validation and reporting guidance
Validate SPI by reconciling the earned value ledger with schedule progress reports. Look for anomalies such as EV exceeding total budget for a work package or PV not matching baseline loading. If schedule variance is unusually large, confirm that progress measurement methods have not shifted between reporting periods.
When reporting, pair SPI with cost performance index (CPI) and a narrative summary of critical path status. Stakeholders should understand whether the schedule variance is driven by resource constraints, procurement delays, or scope changes. This context turns SPI into a decision-ready insight rather than a stand-alone ratio.
Limits and assumptions
SPI assumes that earned value equates to schedule progress, but earned value can be front-loaded or back-loaded depending on work package design. Large procurement milestones or lump-sum deliverables can distort SPI if they are not aligned with actual schedule risk. Review the earned value measurement method before relying on SPI for day-to-day decisions.
The ratio also does not capture float consumption. A project may appear on schedule even while buffer is depleted. Combine SPI with float metrics, milestone trend analysis, and risk registers to gain a full picture of schedule health.
Embed: Earned value schedule performance index calculator
Provide earned value, planned value, and an optional target SPI to calculate schedule performance and schedule variance in one step.