TEAC: Time Estimate at Completion 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-diagram">The Diagram</a></li><li><a href="#why-teac-beats-the-planners-forecast">Why TEAC Beats the Planner's Forecast</a></li><li><a href="#worked-example-18m-rail-signalling-upgrade">Worked Example: £18M Rail Signalling Upgrade</a></li><li><a href="#teac-and-spi-convergence">TEAC and SPI Convergence</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>Time Estimate at Completion (TEAC) forecasts when your project will actually finish based on current schedule efficiency. Not when the planner hopes it'll finish. Not what the programme shows. What the data says. If your planned duration is 24 months and your <a href="/en/earned-value/definitions/schedule-performance-index">SPI</a>(t) is 0.85, TEAC says you're looking at 28.2 months. That's 4.2 months of delay the programme probably isn't showing yet.</p><p>TEAC is part of the <a href="/en/earned-value/definitions">earned value definitions glossary</a>. For the full schedule performance framework including SPI and SV, see the <a href="/en/earned-value/cpi-spi">CPI and SPI formula page</a>.</p><h2 id="the-formula">The Formula</h2><div class="ge-formula-box ge-anim"><span class="ge-formula-label">Formula</span><code>TEAC = PD / SPI(t)</code></div><p>Where:</p><ul><li><strong>PD</strong> = Planned Duration (total baseline duration in months or weeks)</li><li><strong>SPI(t)</strong> = Schedule Performance Index measured in time units (from earned schedule analysis)</li></ul><p>There's also an alternative form using Earned Schedule:</p><p><strong>TEAC = AT + (PD - ES) / SPI(t)</strong></p><p>Where:</p><ul><li><strong>AT</strong> = Actual Time elapsed</li><li><strong>ES</strong> = Earned Schedule (the point in the baseline programme where the current EV would have been achieved)</li></ul><p>Both formulas produce the same result. The second is more intuitive because it separates what's happened (AT) from the forecast for what's left.</p><h2 id="the-diagram">The Diagram</h2><pre class="ge-ascii-diagram ge-anim">TEAC Forecast vs Planned Duration =================================== Planned Timeline (PD = 24 months): |████████████████████████| 0 6 12 18 24 Start Planned Finish Actual Progress at Month 18 (AT = 18, ES = 15.3): |█████████████████████░░░░░░░░░░░| 0 6 12 15.3 18 24 28.2 Start ES AT Planned TEAC ↑ ↑ Finish Finish Where you Where you SHOULD be ARE (in time) SPI(t) = ES / AT = 15.3 / 18 = 0.85 TEAC = 24 / 0.85 = 28.2 months (+4.2 months late) ├── Remaining planned: 24 - 15.3 = 8.7 months of work ──┤ ├── At SPI(t) 0.85, will take: 8.7 / 0.85 = 10.2 months ─┤ ├── Forecast finish: 18 + 10.2 = 28.2 months ─────────────┤</pre><p>The gap between ES and AT is the schedule slip expressed in time. TEAC extends that slip across the remaining work.</p><h2 id="why-teac-beats-the-planners-forecast">Why TEAC Beats the Planner's Forecast</h2><p>Every project has a planner who updates the programme and a commercial team running the numbers. They rarely agree on the completion date. Here's why.</p><p>The planner's forecast is subjective. They interview subcontractors, check lookaheads, assess logic links, and produce a revised completion date. It's informed judgment. Sometimes it's excellent. Often it's optimistic because the planner is under pressure to show recovery.</p><p>TEAC is arithmetic. It takes the rate at which you've been earning schedule credit and projects it forward. No opinions. No recovery assumptions. Just the trend.</p><p>I'm not saying TEAC is always right and the planner is always wrong. But when TEAC says 28.2 months and the planner says 25.5 months, someone needs to explain the 2.7-month gap. That conversation is worth having.</p><h2 id="worked-example-18m-rail-signalling-upgrade">Worked Example: £18M Rail Signalling Upgrade</h2><span class="ge-worked-label">Worked Example</span><div class="ge-callout ge-anim"><p><strong>Scenario:</strong> An £18M NEC4 Option C signalling upgrade on the East Coast Main Line. Planned duration is 24 months, starting January 2025. At the end of month 18 (June 2026), the project manager requests a schedule health check.</p><p><strong>Progress data:</strong></p><ul><li>PD = 24 months</li><li>AT = 18 months</li><li><a href="/en/earned-value/definitions/earned-value">EV</a> = £11.475M (63.75% complete)</li><li><a href="/en/earned-value/definitions/planned-value">PV</a> at month 18 = £13.5M (75% was planned)</li></ul><p><strong>Earned Schedule calculation:</strong> The baseline programme shows that £11.475M of PV was achieved at approximately month 15.3. Therefore ES = 15.3 months.</p><p><strong>SPI(t)</strong> = ES / AT = 15.3 / 18 = <strong>0.85</strong></p><p><strong>TEAC</strong> = PD / SPI(t) = 24 / 0.85 = <strong>28.2 months</strong></p><p>The data says the project will finish in late April 2027, not July 2026 as planned. That's 4.2 months of delay.</p><p>The planner's revised programme shows completion in mid-September 2026 (25.5 months), assuming signalling testing accelerates in the final phase. The commercial manager presents both figures to the project director:</p><ul><li>Planner's forecast: 25.5 months (1.5 months late)</li><li>TEAC: 28.2 months (4.2 months late)</li><li>Gap: 2.7 months of assumed recovery that hasn't been evidenced</li></ul><p>The project director asks: "What specifically will cause SPI(t) to improve from 0.85 to the 0.94 the planner's programme implies?" Good question. That's exactly the conversation TEAC is designed to trigger.</p></div><h2 id="teac-and-spi-convergence">TEAC and SPI Convergence</h2><p>One thing to watch. Traditional SPI (cost-based) suffers from a convergence problem where it trends towards 1.0 as the project nears completion, even if the project is late. See the <a href="/en/earned-value/definitions/schedule-performance-index">SPI page</a> for the full explanation.</p><p>TEAC based on SPI(t) avoids this problem because it uses time-based earned schedule rather than cost-based SPI. That's the whole point of earned schedule analysis. SPI(t) doesn't converge. It stays honest right through to the end.</p><p>This is why the earned schedule community developed these time-based metrics. The original EVM schedule indicators were broken in the final third of a project. TEAC using SPI(t) works throughout.</p><h2 id="common-mistakes">Common Mistakes</h2><ol><li><strong>Using cost-based SPI instead of SPI(t).</strong> If you calculate TEAC = PD / SPI (cost-based), you'll get increasingly unreliable results past 60% completion. Always use SPI(t) from earned schedule analysis. The formulas look similar but the inputs are fundamentally different.</li><li><strong>Ignoring the trend.</strong> A single month's TEAC is a snapshot. Plot TEAC over time. If it's been steadily increasing from 25.5 to 26.8 to 28.2 over three months, the trajectory matters more than any single number. See the <a href="/en/earned-value/definitions/trend-analysis">trend analysis page</a> for how to read these patterns.</li><li><strong>Treating TEAC as gospel.</strong> TEAC assumes current schedule efficiency continues. If something genuinely changes (a major blocker is removed, resources double, a critical subcontractor mobilises), SPI(t) will improve and TEAC will come down. TEAC is a forecast, not a fate.</li><li><strong>Not comparing TEAC against contractual milestones.</strong> A TEAC of 28.2 months is interesting. A TEAC of 28.2 months against a contractual completion date of 24 months with liquidated damages of £35,000 per week is a £519,000 commercial exposure. Always translate TEAC into money.</li></ol><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>How is TEAC different from just looking at the programme?</h3><p>The programme is the planner's informed estimate of remaining duration. TEAC is a mathematical projection based on historical schedule efficiency. They answer the same question from different angles. When they agree, you can be confident. When they diverge, TEAC usually turns out to be closer to reality because it's based on what's actually happened rather than what's planned to happen.</p><h3>Can TEAC improve over the course of a project?</h3><p>Absolutely. If SPI(t) improves (the team accelerates, a blocker is removed, resources increase), TEAC will decrease. That's recovery in action. But watch the trend. Genuine recovery shows a sustained SPI(t) improvement over three or more periods, not a one-month blip.</p><h3>What SPI(t) value should I worry about?</h3><p>Below 0.90 for two consecutive months is a trigger for action. At SPI(t) of 0.85 on a 24-month programme, you're already looking at 4+ months of delay. The earlier you catch the trend, the cheaper the recovery options are. By the time SPI(t) drops to 0.75, your options are acceleration (expensive) or accept the delay (and the LD exposure).</p><h3>Does TEAC account for acceleration?</h3><p>No. TEAC projects current performance forward. If you're planning acceleration, model it separately: estimate the improved SPI(t) after acceleration and recalculate TEAC with the new value. Then compare the cost of acceleration against the cost of delay. That's the business case.</p></article></div>