What is a Class 5 estimate?

A Class 5 estimate is a Rough Order of Magnitude (ROM) estimate prepared when project scope is 0–2% defined. It relies on parametric models, capacity-factored analogies, and judgment. Typical accuracy is −20% to −50% on the low side and +30% to +100% on the high side.

When is a Class 3 estimate used?

Class 3 estimates are typically used for budget authorization and to establish a project control baseline once scope definition is between 10% and 40%. They are detailed enough to support funding decisions but still rely on a mix of deterministic and parametric methods.

AACE Estimate Classes: A Guide to Construction Cost Estimate Classes 1–5

Q: What are AACE estimate classes?

AACE International defines five estimate classes (Class 1 through Class 5) that describe how mature a project's scope definition is and the expected accuracy of the resulting cost estimate. Class 5 is the earliest, roughest order-of-magnitude estimate; Class 1 is the most detailed, near-final estimate.

AACE International's recommended practice (RP 17R-97 / 18R-97) defines five estimate classes that align estimate accuracy with how well a project's scope is defined. This guide compares Class 1 through Class 5 — what each is for, how accurate it should be, and when to use it.

What are AACE estimate classes?

The Association for the Advancement of Cost Engineering (AACE) classifies cost estimates by the maturity of project scope definition. The lower the class number, the more defined the scope and the tighter the expected accuracy range. Five primary characteristics drive the class: degree of project definition, end usage, estimating methodology, expected accuracy range, and preparation effort.

Comparison: Class 1 to Class 5

Class	Scope Maturity	Typical Purpose	Methodology	Accuracy (Low)	Accuracy (High)	Effort
Class 5	0% – 2%	Concept screening, feasibility, business case	Stochastic / parametric, capacity factored, judgment	−20% to −50%	+30% to +100%	Lowest (hours to a few days)
Class 4	1% – 15%	Study or feasibility, concept selection	Primarily stochastic, equipment factored, parametric models	−15% to −30%	+20% to +50%	Low
Class 3	10% – 40%	Budget authorization, funding, project control baseline	Mixed, but more deterministic — semi-detailed unit costs with assembly line items	−10% to −20%	+10% to +30%	Moderate
Class 2	30% – 75%	Control or bid/tender, change order pricing	Primarily deterministic, detailed unit cost with forced detailed take-off	−5% to −15%	+5% to +20%	High
Class 1	65% – 100%	Check estimate, bid/tender, definitive estimate	Deterministic, detailed unit cost with detailed take-off	−3% to −10%	+3% to +15%	Highest

Accuracy ranges are typical industry values from AACE RP 18R-97 (process industries) and 17R-97 (general). Actual ranges vary by sector, contingency allowances, and team experience.

Class 5 estimate (ROM)

A Class 5 estimate — often called a Rough Order of Magnitude (ROM) — is prepared when scope is essentially conceptual (0–2% defined). It is used for screening between alternatives, business-case development, and very early feasibility work. Methods rely on parametric models, capacity factors, historical analogies, and engineering judgment rather than line-item take-off. Expect a wide accuracy band (often −50% / +100%) and treat the number as a range, not a price.

Class 4 estimate

A Class 4 estimate supports concept-study and feasibility decisions with 1–15% scope definition. It still leans on parametric methods but introduces some equipment-factored estimating and early major-quantity take-offs. Useful for narrowing options before committing engineering hours.

Class 3 estimate

A Class 3 estimate is the budget-authorization estimate. With 10–40% scope definition, it blends deterministic line-item pricing with parametric fill-ins for undefined work. It typically becomes the project control baseline once funding is approved.

Class 2 estimate

A Class 2 estimate is a control or bid/tender estimate built on 30–75% scope definition. It is largely deterministic, with detailed unit costs and forced detailed take-off across most disciplines. It is the contractor's working estimate during construction.

Class 1 estimate

A Class 1 estimate is the most detailed: 65–100% scope definition, fully deterministic, with detailed take-off of every discipline. It is used as a check estimate, definitive bid, or change-order baseline. Only parts of a project are usually estimated to Class 1 because of the effort involved.

How to pick the right class

Match the class to the decision you need to make, not to how much time you have.
Document assumptions, exclusions, and basis-of-estimate alongside every class.
Carry explicit contingency that matches the class accuracy range.
Re-baseline as scope matures — a Class 5 ROM should not be reused as a Class 3 budget.

Build a structured Class 5 ROM

Precon ROM Builder turns rough early-stage scope notes into a structured AACE Class 5 planning ROM with assumptions, RFIs, exclusions, and risk flags.

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