AACE estimate classes

The classification, set out in AACE's recommended practices for cost estimate classification, ties three things together: the maturity of the engineering deliverables (from a few percent defined at Class 5 to essentially complete at Class 1), the estimating methods appropriate at that maturity (factored and parametric early, detailed take-off later), and the expected accuracy range, which narrows from tens of percent either way at Class 5 to around ten percent or better at Class 1. The exact ranges vary by industry and by each organisation's own history, which the practice explicitly acknowledges.

Its value is shared vocabulary. "A Class 4 estimate" tells an owner, a lender and a reviewer the same thing about what the number is built on and how far it can be trusted, without anyone inspecting the take-offs. Most large owners and EPC firms key their gate decisions and contingency policy to estimate class.

The abuse is class inflation: labelling an estimate by the accuracy the project wants rather than the definition it has. The class is determined by the deliverables that exist, and an estimate cannot be promoted a class by wishing — only by engineering.