IEAC Formula: Independent Estimate at Completion 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-core-formula">The Core Formula</a></li><li><a href="#the-five-ieac-variants">The Five IEAC Variants</a></li><li><a href="#ieac-vs-managers-eac-the-gap-that-matters">IEAC vs Manager's EAC: The Gap That Matters</a></li><li><a href="#why-the-gap-exists">Why the Gap Exists</a></li><li><a href="#worked-example-whos-right">Worked Example: Who's Right?</a></li><li><a href="#when-to-use-ieac">When to Use IEAC</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 Independent Estimate at Completion (IEAC) is a mathematically derived forecast of final project cost calculated purely from earned value data. No management judgement, no optimism, no "we'll recover it next quarter." It exists for one reason: to cross-check the manager's <a href="/en/earned-value/definitions/estimate-at-completion">EAC</a>. When the two numbers diverge significantly, someone's wrong. Usually it's the manager.</p><p>IEAC is part of the <a href="/en/earned-value/definitions">earned value definitions glossary</a>. For full details on the four EAC formulas it cross-checks, see the <a href="/en/earned-value/eac-etc-tcpi">EAC, ETC and TCPI page</a>.</p><h2 id="the-core-formula">The Core Formula</h2><p><strong>IEAC = <a href="/en/earned-value/definitions/budget-at-completion">BAC</a> / <a href="/en/earned-value/definitions/cost-performance-index">CPI</a></strong></p><p>That's the simplest and most common form. It says: if cost efficiency continues at its current rate, this is what the project will cost. No adjustments, no management override, no wishful thinking. Just arithmetic.</p><p>On a £20M project with a CPI of 0.88: IEAC = £20M / 0.88 = £22.73M. The data says you're heading for a £2.73M overrun. The commercial manager might say £21.5M. The question is: who do you believe?</p><h2 id="the-five-ieac-variants">The Five IEAC Variants</h2><p>This is where it gets genuinely useful. Running a single IEAC is fine. Running all five and comparing them creates a forecast envelope that tells you far more than any single number.</p><div class="ge-table-wrap ge-anim"><table class="ge-table"><thead><tr><th>Variant</th><th>Formula</th><th>Assumption</th><th>Best Used When</th></tr></thead><tbody><tr><td><strong>IEAC-1</strong></td><td>BAC / CPI</td><td>Current cost efficiency continues</td><td>CPI stable for 3+ months</td></tr><tr><td><strong>IEAC-2</strong></td><td>AC + (BAC - EV)</td><td>Remaining work runs to budget</td><td>One-off cost spike, now resolved</td></tr><tr><td><strong>IEAC-3</strong></td><td>AC + (BAC - EV) / (CPI x SPI)</td><td>Both cost and schedule problems persist</td><td>Behind programme AND over budget</td></tr><tr><td><strong>IEAC-4</strong></td><td>AC + (BAC - EV) / CPI</td><td>Cost trend continues, schedule ignored</td><td>Schedule issues won't affect remaining costs</td></tr><tr><td><strong>IEAC-5</strong></td><td>AC + Bottom-up re-estimate</td><td>Independent re-estimate of remaining work</td><td>Fundamental scope or strategy change</td></tr></tbody></table></div><p>The power isn't in any one formula. It's in the spread. If all five variants cluster between £21M and £22M but the manager's EAC says £20.5M, you've got a credibility problem. If the variants spread from £19M to £24M, you've got an uncertainty problem. Both are useful signals.</p><h2 id="ieac-vs-managers-eac-the-gap-that-matters">IEAC vs Manager's EAC: The Gap That Matters</h2><pre class="ge-ascii-diagram ge-anim"> IEAC vs MANAGER'S EAC COMPARISON ================================== Project: £18M NEC4 Option C Water Treatment Upgrade Status Date: Month 10 of 24 Manager's EAC: £19.2M "We'll tighten up in Phase 2" | v ├──────────────────────────────────────────────┤ £17M £18M £19M £20M £21M £22M IEAC-1 (BAC/CPI): £20.45M ──┐ IEAC-2 (AC + remaining): £19.60M ──┤ IEAC-3 (CPI x SPI): £21.10M ──┤ IEAC Range IEAC-4 (AC + rem/CPI): £20.45M ──┘ | v ├─────────────────────────┤ £19.6M £21.1M GAP: Manager's EAC is £1.25M BELOW the IEAC midpoint ┌─────────────────────────────────────────────┐ │ VARIANCE ANALYSIS │ │ │ │ Manager's EAC: £19.20M │ │ IEAC Midpoint: £20.40M │ │ Gap: £1.20M (6.3% of BAC) │ │ │ │ DCMA threshold: &gt;10% gap = escalation │ │ This project: 6.3% = MONITOR │ │ │ │ Verdict: Manager may be optimistic. │ │ Recommend detailed review of Phase 2 │ │ cost assumptions. │ └─────────────────────────────────────────────┘ </pre><h2 id="why-the-gap-exists">Why the Gap Exists</h2><p>In my experience, the manager's EAC is almost always lower than the IEAC. Almost always. Three reasons:</p><p><strong>Optimism bias.</strong> Every commercial manager believes their team will improve. "We had a bad start but we'll hit our stride in Phase 2." Sometimes that's true. Usually it isn't. Research across US DoD programmes found that projects rarely improve their CPI by more than 10% from the cumulative figure established at the 20% completion point. The data says you are what your CPI says you are.</p><p><strong>Selective evidence.</strong> Managers know why costs are high, they can point to specific events. "The piling overrun was a one-off." Fair enough. But there's always another one-off next month. IEAC doesn't care about explanations. It cares about the cumulative trend.</p><p><strong>Fear of the message.</strong> Reporting a higher EAC means difficult conversations with the project director. IEAC doesn't have that problem. It's just maths.</p><h2 id="worked-example-whos-right">Worked Example: Who's Right?</h2><span class="ge-worked-label">Worked Example</span><div class="ge-callout ge-anim"><p><strong>Scenario:</strong> A £14M NEC4 Option C rail platform extension in South Yorkshire. At month 12 (of 20), the monthly progress report shows:</p><ul><li><a href="/en/earned-value/definitions/budget-at-completion">BAC</a> = £14,000,000</li><li><a href="/en/earned-value/definitions/earned-value">EV</a> = £7,560,000 (54% complete)</li><li><a href="/en/earned-value/definitions/actual-cost">AC</a> = £8,340,000</li><li>CPI = £7,560,000 / £8,340,000 = <strong>0.906</strong></li><li><a href="/en/earned-value/definitions/schedule-performance-index">SPI</a> = 0.94</li></ul><p><strong>The commercial manager's EAC:</strong> £15,050,000. His reasoning: the earthworks overspend (£420K) was due to contaminated material that's now been dealt with. He believes Phase 2 (M&amp;E and fit-out) will run to budget.</p><p><strong>The IEAC calculations:</strong></p><div class="ge-table-wrap ge-anim"><table class="ge-table"><thead><tr><th>Variant</th><th>Calculation</th><th>Result</th></tr></thead><tbody><tr><td>IEAC-1</td><td>£14M / 0.906</td><td><strong>£15,453,000</strong></td></tr><tr><td>IEAC-2</td><td>£8.34M + (£14M - £7.56M)</td><td><strong>£14,780,000</strong></td></tr><tr><td>IEAC-3</td><td>£8.34M + £6.44M / (0.906 x 0.94)</td><td><strong>£15,901,000</strong></td></tr><tr><td>IEAC-4</td><td>£8.34M + £6.44M / 0.906</td><td><strong>£15,447,000</strong></td></tr></tbody></table></div><p><strong>IEAC range:</strong> £14.78M to £15.90M <strong>IEAC midpoint:</strong> £15.40M <strong>Manager's EAC:</strong> £15.05M <strong>Gap:</strong> £350K below IEAC midpoint (2.5% of BAC)</p><p>The gap is small but consistent. Every IEAC variant except IEAC-2 (which assumes the overrun was purely one-off) comes in higher than the manager's figure. The commercial director asks the right question: "If the contamination is truly dealt with, why is CPI still at 0.906 and not recovering?"</p><p>Investigating further, the team discovers that plant hire rates on the second phase are 8% above the original estimate, a market cost increase the manager hadn't factored in. Adjusting for this, the revised manager's EAC becomes £15,380,000, now aligned with the IEAC range.</p></div><h2 id="when-to-use-ieac">When to Use IEAC</h2><p>Don't calculate IEAC for the sake of it. Use it at specific decision points:</p><p><strong>Monthly reporting.</strong> Include both manager's EAC and IEAC-1 in every progress report. Let the reader see the gap (or lack of one). Transparency builds credibility.</p><p><strong>Gate reviews.</strong> At project stage gates, run all five variants. Present the range. A narrow range means the forecast is solid. A wide range means the project has high forecast uncertainty, and that's a risk in itself.</p><p><strong>Dispute resolution.</strong> On NEC4 target cost contracts, the pain/gain share depends on final outturn cost. If the Contractor's forecast and the Client's forecast diverge, IEAC provides an objective third view. I've used this exact approach on a £35M highways scheme where the Contractor's EAC was £2.1M below the Client's independent assessment. The IEAC range fell squarely in the middle and gave both sides a basis for discussion.</p><h2 id="common-mistakes">Common Mistakes</h2><p><strong>Treating IEAC as "the right answer."</strong> It isn't. It's a mathematical cross-check. IEAC assumes the future will look like the past. If you're genuinely about to enter a different phase with different risk characteristics, the manager's judgement may be more appropriate, as long as they can substantiate it with evidence, not hope.</p><p><strong>Running IEAC too early.</strong> Before 15-20% physical completion, CPI is volatile and IEAC swings wildly. On a £20M project, don't put much weight on IEAC until you've spent at least £3M-£4M and measured three or more periods of stable data.</p><p><strong>Ignoring the range.</strong> Reporting a single IEAC (usually IEAC-1) is almost as misleading as reporting only the manager's EAC. The range across all variants tells the story. A £500K spread is good. A £3M spread means your forecast model isn't converging and needs investigation.</p><p><strong>Not updating BAC for compensation events.</strong> On NEC4, every implemented CE adjusts <a href="/en/earned-value/definitions/budget-at-completion">BAC</a>. If BAC is stale, IEAC is measuring against the wrong target. This is the most common error on UK projects running EVM alongside NEC4 contract administration.</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's the difference between IEAC and EAC?</h3><p><a href="/en/earned-value/definitions/estimate-at-completion">EAC</a> is the project team's forecast, which may include management judgement, bottom-up re-estimates, and risk adjustments. IEAC is a purely mathematical cross-check derived from cumulative performance data. Think of EAC as what the team believes, and IEAC as what the data says. When they agree, you've got a credible forecast. When they disagree, you've got a conversation to have.</p><h3>What IEAC-to-EAC gap triggers a formal review?</h3><p>The DCMA standard uses 10% variance as the escalation threshold. On UK construction projects, I'd set the trigger lower, around 5% of BAC. On a £20M project, that's a £1M gap. If your IEAC consistently comes in £1M above the manager's EAC, the manager needs to either explain why or revise their forecast.</p><h3>Which IEAC variant is most reliable?</h3><p>IEAC-1 (BAC/CPI) has the strongest track record in empirical studies. Research by Christensen and Heise found that cumulative CPI rarely changes by more than 10% from the value established at 20% completion. That means IEAC-1 becomes increasingly reliable as the project progresses. Use it as your primary cross-check and the other variants for context.</p><h3>Can IEAC be lower than the manager's EAC?</h3><p>Yes, though it's less common. It happens when the manager has built in risk allowances or contingency that the IEAC arithmetic doesn't see. In that case, the manager is being conservative, which is fine, as long as the contingency is explicit and justified. A manager's EAC above the IEAC range should be challenged just as much as one below it.</p></article></div>