Schedule Performance Index (SPI) Formula Explained

<div class="ge-article-wrapper"><nav class="ge-toc" aria-label="Table of contents"><p class="ge-toc-label">In this article</p><ul class="ge-toc-list"><li><a href="#the-formula">The Formula</a></li><li><a href="#the-spi-interpretation-scale">The SPI Interpretation Scale</a></li><li><a href="#worked-example-35m-highway-spi-decline-over-4-months">Worked Example: £35M Highway, SPI Decline Over 4 Months</a></li><li><a href="#the-convergence-problem-why-spi-lies-at-the-end">The Convergence Problem: Why SPI Lies at the End</a></li><li><a href="#when-spi-is-most-useful">When SPI Is Most Useful</a></li><li><a href="#common-mistakes">Common Mistakes</a></li><li><a href="#frequently-asked-questions">Frequently Asked Questions</a></li></ul></nav><article class="ge-article-body"><p>The <a href="/en/earned-value/definitions/schedule-performance-index">Schedule Performance Index (SPI)</a> measures how efficiently a construction project is delivering progress against its baseline programme. SPI = <a href="/en/earned-value/definitions/earned-value">EV</a> / <a href="/en/earned-value/definitions/planned-value">PV</a>. Above 1.0 means ahead. Below 1.0 means behind. It's one of the two core ratios in <a href="/en/earned-value">earned value management</a>, alongside <a href="/en/earned-value/definitions/cost-performance-index">CPI</a>, and it has a critical flaw that makes it unreliable in the final third of any project.</p><p>This page expands on the <a href="/en/earned-value/definitions/schedule-performance-index">SPI definition</a> with deeper worked examples and the convergence problem that every commercial team needs to understand. For the companion time-based metric that fixes this flaw, see <a href="/en/earned-value/definitions/schedule-performance-index-time">SPI(t)</a>.</p><h2 id="the-formula">The Formula</h2><div class="ge-formula-box ge-anim"><span class="ge-formula-label">Formula</span><code>SPI = EV / PV</code></div><p>Where:</p><ul><li><strong><a href="/en/earned-value/definitions/earned-value">EV (Earned Value)</a></strong> = budgeted cost of work actually completed</li><li><strong><a href="/en/earned-value/definitions/planned-value">PV (Planned Value)</a></strong> = budgeted cost of work scheduled to be completed by now</li></ul><p>Both are measured in pounds. SPI is a dimensionless ratio. An SPI of 0.85 means you've delivered 85% of the progress the plan expected by this date.</p><h2 id="the-spi-interpretation-scale">The SPI Interpretation Scale</h2><pre class="ge-ascii-diagram ge-anim">SPI VALUE INTERPRETATION 0.0 0.80 0.95 1.0 1.05 1.20 ├──────────────────┼──────────────────┼──────┼──────┼──────────────────┤ │ CRITICAL │ BEHIND │ OK │ OK │ AHEAD │ │ Recovery │ Investigate │ │ │ Verify │ │ unlikely │ root cause │ │ │ baseline │ │ │ Acceleration? │ │ │ │ ├──────────────────┼──────────────────┼──────┼──────┼──────────────────┤ 🔴🔴🔴🔴🔴🔴🔴🔴🔴🟠🟠🟠🟠🟠🟠🟠🟠🟠🟢🟢🟢🟢🟢🟢🟢🟢🟢🟢🟢🟢🟢🟢🟢🟢 &lt; 0.80: Formal recovery plan. Calculate acceleration cost vs LD exposure. 0.80-0.94: Root cause analysis. Which packages are dragging? Critical path impact? 0.95-1.05: Healthy range. Normal monitoring. &gt; 1.05: Good – but verify the baseline isn't sandbagged. &gt; 1.15: Suspicious. Either great performance or an unrealistic baseline.</pre><h2 id="worked-example-35m-highway-spi-decline-over-4-months">Worked Example: £35M Highway, SPI Decline Over 4 Months</h2><span class="ge-worked-label">Worked Example</span><div class="ge-callout ge-anim"><p><strong>Scenario:</strong> A £35M NEC4 Option C dual carriageway widening programme, 20 months duration. The SPI trend over months 5 to 8 tells a story.</p><div class="ge-table-wrap ge-anim"><table class="ge-table"><thead><tr><th>Month</th><th>PV (Cumulative)</th><th>EV (Cumulative)</th><th>SPI</th><th>Trend</th></tr></thead><tbody><tr><td>M5</td><td>£7.7M</td><td>£7.85M</td><td><strong>1.02</strong></td><td>On track</td></tr><tr><td>M6</td><td>£10.1M</td><td>£9.70M</td><td><strong>0.96</strong></td><td>Slipping</td></tr><tr><td>M7</td><td>£12.8M</td><td>£11.39M</td><td><strong>0.89</strong></td><td>Behind</td></tr><tr><td>M8</td><td>£15.4M</td><td>£13.09M</td><td><strong>0.85</strong></td><td>Seriously behind</td></tr></tbody></table></div><p><strong>Month 5:</strong> SPI of 1.02. The project is slightly ahead. No concerns.</p><p><strong>Month 6:</strong> SPI drops to 0.96. A small slip, the team chalks it up to two weeks of poor weather in October. "We'll make it up next month."</p><p><strong>Month 7:</strong> SPI falls to 0.89. Now it's a pattern. The weather excuse doesn't hold because November was dry. Digging into the data reveals the earthworks package is 3 weeks behind the critical path, a subcontractor mobilised late and hasn't caught up.</p><p><strong>Month 8:</strong> SPI at 0.85. The earthworks delay has cascaded into the drainage package, which can't start until formation levels are achieved. Two packages are now behind, and the critical path has shifted.</p><p><strong>The commercial impact:</strong> At SPI 0.85, the project is delivering progress at 85% of the planned rate. The remaining 12 months of programmed work will take approximately 14.1 months at this rate. That's 2.1 months of delay unless something changes. With LDs at £60,000 per week, the exposure is around £545,000.</p><p>This is the value of tracking SPI monthly. At month 6, the slip was dismissed. By month 8, it's a programme crisis. The trend was visible from month 6, the team just didn't act on it.</p></div><h2 id="the-convergence-problem-why-spi-lies-at-the-end">The Convergence Problem: Why SPI Lies at the End</h2><p>Here's SPI's fundamental flaw. It always converges to 1.0 at project completion.</p><p>Why? At completion, all the planned work is done. EV = <a href="/en/earned-value/definitions/budget-at-completion">BAC</a>. And at the planned completion date (or beyond it), PV also equals BAC. So EV / PV = BAC / BAC = 1.0. Even if the project finishes six months late.</p><pre class="ge-ascii-diagram ge-anim">SPI CONVERGENCE OVER PROJECT LIFE SPI | 1.1 | | . 1.0 |───.────────────────────.───────── Target | . 0.9 |..' | .' ← SPI converges to 1.0 0.8 | .' (even though project | .' finishes 4 months late) 0.7 | .' | .' 0.6 |.' +──────────────────────────────────────── Time M1 M5 M10 M15 M20 M24 M28 ↑ ↑ Planned Actual Finish Finish</pre><p>This means SPI is most reliable in the <strong>first 60-70% of a project</strong>. After that, it becomes increasingly misleading. I've seen projects with SPI of 0.97 at month 16 of 20 that were actually running three months behind. The SPI looked almost healthy, but the project finished in month 23.</p><p>The fix? Use <a href="/en/earned-value/definitions/schedule-performance-index-time">SPI(t)</a>, the time-based SPI from earned schedule methodology. It doesn't converge and gives you an honest picture right through to completion.</p><h2 id="when-spi-is-most-useful">When SPI Is Most Useful</h2><p>SPI's sweet spot is months 3 through to about 65% completion. In that window, it's genuinely powerful:</p><p><strong>Trend detection.</strong> Two consecutive months of declining SPI is a signal. Three months is a pattern. By the time it hits the programme report, you've already got the data to diagnose the problem.</p><p><strong>Package comparison.</strong> Calculate SPI per work package. On a £35M job with five major packages, the aggregate SPI might be 0.92. But one package might be 0.75 while the others are all above 0.95. The aggregate hides the outlier. Package-level SPI finds it.</p><p><strong>Cross-project benchmarking.</strong> SPI normalises schedule performance across projects of different sizes and durations. A £5M project with SPI 0.88 and a £50M project with SPI 0.88 have the same schedule efficiency problem, just at different scales.</p><h2 id="common-mistakes">Common Mistakes</h2><p><strong>1. Treating SPI as a time metric.</strong> SPI measures progress in cost terms, not time. An SPI of 0.85 doesn't mean you're 15% behind in time. It means you've delivered 15% less value than planned. Converting SPI to time requires additional calculation, or better yet, use <a href="/en/earned-value/definitions/schedule-variance-time">SV(t)</a> which gives you the answer directly in months.</p><p><strong>2. Relying on SPI after 70% completion.</strong> The convergence effect is real and well-documented. After about two-thirds through the project, supplement SPI with SPI(t) or direct programme analysis.</p><p><strong>3. Ignoring what's slipping.</strong> SPI is an aggregate number. It can't tell you whether the slippage is on the critical path or on float activities. An SPI of 0.90 driven by non-critical market works is very different from an SPI of 0.90 driven by structural works on the critical path. Always check the programme alongside SPI.</p><p><strong>4. Celebrating high SPI without checking the baseline.</strong> An SPI of 1.15 is either excellent performance or a sandbagged programme. On NEC4, if the Accepted Programme was deliberately conservative, SPI will be artificially high. Check whether the baseline is realistic before drawing conclusions.</p><div class="ge-product-note ge-anim"><p><strong>How Gather helps.</strong> Gather's AI reads your site diaries daily and maps progress against your cost-loaded programme, giving you accurate earned value data without manual spreadsheet updates. <a href="https://gatherinsights.com/contact">Book a demo</a> to see it working on a live NEC4 project.</p></div><h2 id="frequently-asked-questions">Frequently Asked Questions</h2><h3>What happens to SPI when a compensation event extends the programme?</h3><p>If the CE changes the Accepted Programme and the baseline is updated, PV at future dates changes. This can improve or worsen SPI depending on whether the CE added scope (increasing PV) or just extended time (spreading PV over a longer period). Always re-baseline when the programme is formally revised.</p><h3>Can two projects have the same SPI but different schedule health?</h3><p>Absolutely. A £10M project with SPI 0.90 at month 3 of 24 has plenty of time to recover. The same SPI at month 20 of 24 is a crisis, there's almost no time left and the convergence effect is masking the true delay. Context matters as much as the number.</p><h3>Should I report SPI or SV to senior management?</h3><p>Both, but for different audiences. SPI is better for trend analysis and cross-project comparison. <a href="/en/earned-value/definitions/schedule-variance">SV</a> is better when you need to express the gap in pound terms, "we're £1.3M behind on progress" lands differently to "our SPI is 0.89." Use whichever gets the right response from your audience.</p><h3>Is SPI = 1.0 always good?</h3><p>Not necessarily. SPI = 1.0 means aggregate progress matches the plan. But if the plan itself is wrong, an unrealistic programme, an unachievable sequence, then SPI = 1.0 against a bad plan doesn't mean the project is healthy. SPI is only as good as the baseline it's measured against.</p></article></div>