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Why the system exists
Every cost engineer has been asked for “a rough number” that later gets treated as a commitment. AACE's estimate classification system exists largely to stop that from happening. It gives everyone, from the sponsor to the estimator, a shared way of saying how mature an estimate is and how much it might move.
AACE sets out the general principles in Recommended Practice 17R-97, and RP 18R-97 applies them to engineering, procurement and construction in the process industries. Other RPs cover other sectors, such as building construction, so check which one fits your work.
What decides the class
The main driver is the maturity of project definition: how much of the engineering and scope is actually known. Not the time spent estimating, and not the size of the spreadsheet. The end use of the estimate, the method used to prepare it and its expected accuracy are treated as secondary characteristics that generally follow from that maturity.
The five classes in 18R-97
| Class | Project definition | Typical end use | Typical method | Expected accuracy (low / high) |
|---|---|---|---|---|
| Class 5 | 0% to 2% | Concept screening | Capacity factored, parametric models, judgement or analogy | -20% to -50% / +30% to +100% |
| Class 4 | 1% to 15% | Study or feasibility | Equipment factored or parametric models | -15% to -30% / +20% to +50% |
| Class 3 | 10% to 40% | Budget authorisation or control | Semi-detailed unit costs with assembly-level line items | -10% to -20% / +10% to +30% |
| Class 2 | 30% to 75% | Control or bid/tender | Detailed unit cost with forced detailed take-off | -5% to -15% / +5% to +20% |
| Class 1 | 65% to 100% | Check estimate or bid/tender | Detailed unit cost with detailed take-off | -3% to -10% / +3% to +15% |
Read the accuracy column carefully. These are ranges of ranges, and where a particular estimate lands depends on things like technical complexity, the quality of the cost data behind it and the team preparing it. AACE acknowledges the real range can be wider. For the exact wording, go to the RP itself; AACE members can download Recommended Practices free.
What each class looks like in practice
Class 5 is the number produced when a project is little more than an idea, a location and a capacity. Its job is to tell you whether the idea deserves a study at all. Class 4 follows a feasibility study and is used to compare options.
Class 3 is the one cost engineers spend the most energy on, because it's usually the estimate a budget is approved against and the baseline is set from. When a project later “overruns”, this is often the number it's being measured against.
Class 2 is built from detailed take-offs as engineering matures, and is used for control and for contractors' bids. Class 1 comes when the project is close to fully defined, typically to check bids or a contractor's estimate, or to support change orders and claims.
Four common misreadings
“Class 1 means plus or minus 3%.” It doesn't. The ranges are asymmetric and they're typical, not promised. “A longer estimate is a higher class.” Class follows definition, not effort. “The range already includes contingency.” It describes expected accuracy; contingency is a separate call, usually informed by risk analysis. And “18R-97 applies everywhere.” It's written for the process industries.
Where this shows up on the CCP exam
Estimating sits inside Domain 1, Managing Project Costs, which is the largest domain on the CCP at 43 of 119 questions. The May 2026 blueprint lists tasks such as preparing rough order of magnitude estimates, developing summary-level estimates, reviewing detailed estimates and verifying the basis of estimate. Understanding how definition drives accuracy helps with all of them. Our Domain 1 guide lists every task, and our TCM guide shows where estimating fits in the wider framework.
Frequently asked questions
What is a Class 5 estimate?+
What is a Class 3 estimate used for?+
What is AACE 18R-97?+
What determines the class of an estimate?+
Is this on the CCP exam?+
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Sources: AACE International CCP Candidate Handbook (rev. June 2026) and AACE's official CCP certification pages. CCP® is a registered mark of AACE International. More CCP articles →