Opnor
06 · Standards & compliance

AACE cost estimation classes (1–5)

Last updated 2026-04-21
Draft published
First-pass content live. Engineering review and Opnor-team validation in progress — see the "author backlog" callouts at the bottom.

Opnor's ECM cost estimates ship as AACE Class 4. That sounds like a contractual quirk — it isn't. AACE classes define how accurate a number is allowed to claim to be, and skipping a class because nobody asked you to specify is the most common way industrial projects come back with capex that's 50% over the audit number.

What AACE actually defines

AACE International (formerly the Association for the Advancement of Cost Engineering) publishes Recommended Practice 18R-97, which defines five cost estimate classes. Each class specifies:

  • Project definition maturity — how much engineering work has been done before estimating
  • Typical methodology — analogy / parametric / definitive
  • Expected accuracy range — the +/- band the estimate is allowed to claim
  • Use case — what business decision the estimate is supposed to support

It's a calibration framework, not a formula. The classes don't tell you what a VFD costs — they tell you what depth of pricing work the estimate is allowed to claim authority over.

The 5-class table

Class 5 — concept screening
Project definition: 0–2%. Methodology: stochastic / analogy. Accuracy: −50% / +100%. Used in: feasibility studies, very early roadmaps.
Class 4 — study / feasibility
Project definition: 1–15%. Methodology: parametric / equipment-factored. Accuracy: −30% / +50%. Used in: audit deliverables, capital screening.
Class 3 — budget authorization
Project definition: 10–40%. Methodology: semi-detailed / equipment-listed. Accuracy: −20% / +30%. Used in: project-go decisions, board approvals.
Class 2 — control
Project definition: 30–70%. Methodology: detailed unit-cost. Accuracy: −15% / +20%. Used in: project execution control, contractor bidding.
Class 1 — check / bid
Project definition: 65–100%. Methodology: full quantity take-off. Accuracy: −10% / +15%. Used in: final bid validation, contractor sign-off.
Why bands not points
Note the bands are asymmetric — Class 4 is −30% / +50%, not ±40%. This reflects reality: cost estimates miss high more often than they miss low. Industrial projects discover hidden site conditions, scope expansions, and contractor mark-ups more often than they find unexpected savings.

Why audits ship Class 4 (and what that means)

A 2-week audit can't produce a Class-3 estimate, full stop. Class 3 requires 10–40% project definition — that means an equipment list, vendor quotes for the major equipment, a preliminary contractor walk-through, and site-specific installation analysis. None of that fits inside a 2-week audit engagement.

What Opnor delivers in 2 weeks is Class 4: parametric cost estimates with equipment factored from a reference database, productivity multipliers for regional labour rates, and AACE-class contingency on top. That's accurate enough to make capital-screening decisions ("does this ECM clear our payback hurdle?") but not accurate enough to build a board AFE around without further refinement.

We're explicit about this in every audit deliverable: ECMs that pass the screening go to Class 3 estimation as a separate, paid engagement (typically 4–8 weeks per ECM, requires vendor engagement). Plants that bypass that step and try to commit capex against a Class-4 number are the ones that come back with overruns.

What Opnor includes in a Class-4 estimate

Every Class-4 ECM estimate includes the following lines:

Equipment
Major equipment (VFD, motor, heat exchanger, etc.) factored from the reference database by capacity
Material
Cable, piping, ducting, controls — quantity-factored from the equipment list
Labour
Hours estimated parametrically by component type, multiplied by regional labour rate × productivity factor
Subtotal — direct costs
Equipment + material + labour
Indirect costs (toggleable)
Commissioning, programming, surveillance, engineering, project management — each as a % of direct or as fixed hours
Subtotal — direct + indirect
Contingency
AACE Class-4 default: 25% on direct + indirect. Configurable to blanket %, BBA split, or custom per-line.
Overhead
User-adjustable, default 15%
Profit
User-adjustable, default 10%
Grand total
All-in installed cost (the number to use for payback math)

Moving from Class 4 to Class 3

For ECMs that clear the screening and need budget-authorization-grade pricing, the path is roughly:

  • Equipment-list refinement — get vendor quotes for the major equipment (typically the VFD, motor, heat exchanger, or compressor that drives the ECM)
  • Site survey — contractor walk-through to identify field-condition issues (cable routing, structural mounting, demolition, refractory removal)
  • Quantity refinement — actual cable runs, pipe lengths, demolition scope
  • Labour productivity assessment — adjust regional rates for plant-specific access constraints (confined spaces, lockout-tagout, off-shift work)
  • Schedule compression risk — Class 3 typically adds 5–15% premium for fast-track schedules

Opnor delivers Class 3 estimates as a separate engagement. We don't pad the 2-week audit deliverable to look Class-3 — it isn't, and pretending otherwise sets up the project for an overrun.

🚧 Author backlog (Opnor team to fill)
  • Confirm Opnor's contingency default (currently 25% AACE Class-4 — verify in cost_engine/)
  • Document the cost reference database — typical entry shape, refresh cadence, who maintains it
  • Add a worked example: same VFD ECM at Class 4 vs Class 3 to show the difference