TL;DR: Most EAC articles hand you three formulas and leave the interpretation to you. This one shows IT project managers which formula fits their project's current health, what the resulting number is actually telling you, and what to do before the budget breaks — not after.
What EAC means and how it differs from BAC and AC
EAC (Estimate at Completion) is a forward-looking forecast of what your project will cost in total, given everything you know right now. That single distinction separates it from the two metrics it's most often confused with.
BAC (Budget at Completion) is the original approved budget, set before work starts. It doesn't move unless you formally change scope. AC (Actual Cost) is a snapshot of money spent to date, nothing more. Neither tells you where the project is headed.
EAC does. It takes your current performance data and projects it forward to answer one question: if things continue this way, what will the final bill be? That makes it the core predictive metric in earned value management (EVM), the framework PMI's PMBOK formalizes for tracking cost and schedule together.
Here's why the distinction matters in practice. A project can show a healthy BAC and a modest AC while quietly heading toward a 30% overrun, because neither metric accounts for how efficiently the team is burning through the remaining budget. EAC catches that drift early. For context on what "healthy" looks like at the start, understanding how Budget at Completion is calculated is worth reviewing before you run your first EAC.
The next section covers the two formulas that produce that forecast, and when to use each one.
Two EAC calculation methods and when to use each
The choice between the two primary EAC formulas comes down to one question: do you expect past performance to predict the future, or has something changed?
Formula 1: EAC = BAC / CPI
This version of the estimate at completion formula assumes your current cost efficiency will hold for the rest of the project. If your CPI and SPI formulas show a CPI of 0.85, you divide your Budget at Completion (BAC) by 0.85 to get the new projected total. The assumption is baked in: the team will keep performing at the same efficiency rate, no better, no worse.
Use this formula when the project is past its early stages, the cost variance is consistent across multiple reporting periods, and there is no specific corrective action in place. It is the more conservative estimate, and in CPI project management practice, it tends to be more accurate once a project is 20% or more complete.
Formula 2: EAC = AC + ETC
This formula replaces the assumption with a fresh estimate. AC is what you have already spent. ETC (Estimate to Complete) is a new bottom-up forecast of remaining work. Add them together and you get EAC.
Use this formula when something material has changed: a scope revision, a team restructure, a resolved technical blocker, or a deliberate re-plan. It is the right choice when past performance is no longer a reliable guide to future cost.
The practical difference matters for EAC in project management because the two formulas can produce meaningfully different numbers on the same project. Before running either calculation, check your earned value analysis data to confirm whether your CPI is stable or trending. A stable CPI points to Formula 1. A shift in conditions points to Formula 2, and likely to corrective cost control actions as well.
Worked example: calculating and reading EAC on a real IT project
Take a mid-sized IT infrastructure rollout: 20-server deployment, BAC = $120,000, scheduled over 12 weeks.
At week 6, your earned value management numbers look like this:
Planned Value (PV): $60,000
Earned Value (EV): $48,000
Actual Cost (AC): $54,000
From these, CPI = EV / AC = 48,000 / 54,000 = 0.89. You're getting $0.89 of value for every dollar spent.
Method 1: EAC = BAC / CPI
This estimate at completion formula assumes your current cost inefficiency holds for the rest of the project.
EAC = $120,000 / 0.89 = $134,831
That's $14,831 over budget if nothing changes. This formula is the right call when your CPI has been consistently below 1.0 for three or more consecutive periods, meaning the overrun is structural, not a one-time spike.
Method 2: EAC = AC + ETC
This version assumes the team corrects course from here. ETC (Estimate to Complete) is your re-estimated cost for remaining work. If your team re-forecasts the remaining 50% of scope at $58,000:
EAC = $54,000 + $58,000 = $112,000
That's actually under the original BAC, but only if the corrective plan holds.
What the variance tells you
The gap between the two results, $22,831, is your decision range. Method 1 reflects reality if nothing changes. Method 2 reflects the best case if the recovery plan executes cleanly. A project manager using project budget forecasting should present both numbers to the sponsor, not just the optimistic one.
With CPI at 0.89 and schedule variance negative ($8,000 behind), this project needs a corrective action conversation now, not at week 10.
The EAC Formula Selection Matrix: matching method to project health
Choosing the wrong EAC formula doesn't just produce a bad number — it produces a confidently wrong forecast that delays corrective action. The matrix below maps each project health scenario to the formula that fits it, the index reading that signals it, and the action your team should take.
Project health | CPI / SPI signal | EAC formula to use | Management action |
|---|---|---|---|
On track | CPI ≥ 1.0, SPI ≥ 1.0 | EAC = BAC / CPI | Monitor; no change needed |
Cost overrun only | CPI < 1.0, SPI ≥ 1.0 | EAC = AC + (BAC − EV) / CPI | Review scope; cut low-value work |
Schedule slip only | CPI ≥ 1.0, SPI < 1.0 | EAC = AC + (BAC − EV) | Re-sequence tasks; protect budget |
Both overrun and slip | CPI < 1.0, SPI < 1.0 | EAC = AC + [(BAC − EV) / (CPI × SPI)] | Escalate; rebaseline likely |
A few things to read alongside this table.
CPI in project management tells you how much completed work you're getting per dollar spent. A CPI of 0.85 means you're delivering 85 cents of value for every dollar burned — and that ratio tends to be sticky. PMI's PMBOK recognizes that early CPI readings are reliable predictors of final cost performance, which is why the BAC/CPI formula carries weight even when it feels pessimistic.
TCPI in project management is the efficiency rate your team must sustain on all remaining work to hit a target — either your Budget at Completion (BAC) or a revised EAC. When TCPI climbs above 1.1, the math is telling you the target is no longer realistic without a scope or resource change.
EAC vs BAC is the comparison that matters most in a status meeting. BAC is the plan; EAC is the current forecast. A gap between them isn't automatically a crisis, but it is always a question that needs an answer.
Use the CPI and SPI formulas to populate the signal column first, then let the matrix tell you which formula applies. That sequence keeps the forecast grounded in earned value analysis rather than gut feel.
How to use EAC to trigger corrective action before project completion
EAC becomes useful only when it's wired to a response, not just a status report.
Three thresholds should be on every project manager's radar. First, when EAC exceeds your Budget at Completion (BAC) by more than 10%, treat that as a formal escalation trigger, not a note for the next status meeting. Second, a CPI below 0.9 signals that for every dollar spent, the team is delivering less than 90 cents of planned value. At that point, the trajectory rarely self-corrects without intervention. Third, a TCPI above 1.1 in project management terms means the team must outperform its current efficiency by more than 10% on all remaining work just to finish on budget. That's statistically unlikely without a scope or resource change.
The corrective actions tied to each signal differ. A CPI between 0.85 and 0.9 typically warrants a cost audit and resource reallocation. A TCPI above 1.1 paired with a CPI below 0.85 usually means the original budget is no longer recoverable, and the right move is a formal re-baseline with stakeholder sign-off. Waiting for project completion to surface these numbers defeats the purpose of earned value analysis entirely.
Build the thresholds into your project budget forecasting cadence. Review CPI and SPI formulas weekly on high-risk phases, and document which threshold triggered each corrective cost control action so the decision trail is auditable.
Common mistakes that make EAC unreliable
Four errors show up repeatedly when teams apply EAC in project management.
Stale actuals: Plugging last month's AC into this week's formula produces a number that reflects the past, not the present. Update actuals at least weekly on active IT projects.
Wrong formula for the scenario: If your CPI has been drifting, using EAC = BAC / CPI assumes that trend holds. If you've course-corrected, it doesn't. Match the formula to reality, as the CPI and SPI formulas guide explains.
Ignoring schedule variance: Cost and schedule interact. Skipping earned value analysis on both dimensions hides scope compression risk.
Treating EAC as a one-time calculation: A single EAC at project midpoint is a snapshot, not a control tool. Recalculate every reporting cycle or after any significant scope change.
How to integrate EAC into your project management workflow
Treating EAC in project management as a weekly ritual rather than a one-time calculation is what separates teams that catch overruns early from those that discover them at project close. Set a fixed recalculation cadence, weekly on active projects, bi-weekly on stable ones, and tie each update to your CPI and SPI formulas so the number reflects current performance, not last month's snapshot. Feed each new EAC back against your Budget at Completion (BAC) immediately. If the gap widens two cycles in a row, that is your trigger for corrective cost control actions, not a reason to wait for the next status meeting.
Closing
EAC is only useful if you're reading it correctly and acting on it early. The formula you choose matters less than matching it to your project's actual health — on-track projects use one approach, overruns use another, and slips use a third. The real risk isn't picking the wrong formula once; it's letting stale data drive your forecast. When your actual cost, earned value, and time logs update automatically, your EAC updates with them, and you catch budget drift the day it happens, not the day before the board meeting. Start by pulling your current CPI and SPI, then use the matrix to pick your formula. What does your EAC say about the next 30 days of work?
FAQ
What is EAC in project management and how is it calculated?
EAC (Estimate at Completion) is a forward-looking forecast of total project cost based on current performance. Use either EAC = BAC / CPI (assumes past efficiency continues) or EAC = AC + ETC (fresh estimate of remaining work).
What is the difference between EAC and BAC in project management?
BAC is your original approved budget, set before work starts and unchanged unless scope formally changes. EAC is your current forecast of what the project will actually cost, updated as you learn more.
Which EAC formula should I use when my project is over budget?
If cost overrun is consistent across multiple periods, use EAC = BAC / CPI. If something material changed (scope, team, blockers resolved), use EAC = AC + ETC with a fresh estimate of remaining work.
What does it mean when EAC is higher than BAC?
Your project is forecasted to exceed the original budget. The gap signals cost overrun risk and typically triggers a corrective action conversation with your sponsor about scope, resources, or timeline.
How do CPI and TCPI relate to EAC forecasting?
CPI (Cost Performance Index) drives the BAC/CPI formula — a CPI below 1.0 predicts overrun. TCPI is the efficiency rate your team must sustain on remaining work to hit a target; when TCPI exceeds 1.1, the target is unrealistic without change.
What are the most common project management challenges when using EAC?
Choosing the wrong formula for your project's health, using stale earned value data, and presenting only the optimistic EAC number instead of the decision range between both methods.
How often should you recalculate EAC during a project?
At minimum, every reporting period (weekly or bi-weekly). Recalculate immediately after scope changes, team restructures, or when CPI or SPI trends shift, so your forecast stays current.
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Lauren Brooks is a Project Delivery Lead & Business Operations expert who has managed complex, multi-team projects across agencies, SaaS companies, and service firms. She writes about what separates projects that deliver on time from those that spiral; and how smart systems make the difference before problems even appear.
