| Project definition | 1% to 15% of full definition |
|---|---|
| Typical end usage | Feasibility and options studies, preliminary budget approval, approval to proceed to the next stage |
| Estimating method (general, RP 18R-97) | Equipment factored or parametric models |
| Estimating method (road and rail, RP 98R-18) | Adjusted cost per kilometre, approximate unit or assembly rates for major structures and systems, factored costs |
| Expected accuracy (80% confidence) | Low: −15% to −30% High: +20% to +50% |
| Other names | Feasibility, study, options analysis, preliminary, factored, planning, FEL 2 |
| Typical stage | FEL 2; Queensland PCEM: Concept (options analysis) |
AACE Estimate Classification: Study or Feasibility
AACE Class 4 Estimate
The estimate that chooses between options. A Class 4 estimate compares the alternatives that survived screening, and usually decides which single option is carried forward into detailed development.
Updated 2 October 2026
At a Glance
Class 4 Estimate Characteristics
The five attributes AACE uses to describe each class. Project definition maturity is the one that decides the class; the others follow from it.
Overview
What a Class 4 Estimate Is
A Class 4 estimate is prepared during feasibility or options analysis, when between 1% and 15% of the project is defined. For transport infrastructure that typically means preliminary road or rail geometry for each option, topographic mapping, the major structures identified, and the main environmental, community and land issues known.
The expected accuracy is -15% to -30% on the low side and +20% to +50% on the high side. That is narrow enough to rank options with confidence and to seek approval to proceed, but still too wide to set a firm budget.
Class 4 is where most option decisions are made, which is why the quality of a Class 4 estimate matters more than its precision. Options must be estimated on the same basis, or the comparison is meaningless.
What needs to be defined
Typical status of key deliverables for a road or rail project at Class 4. The class follows the weakest deliverable that materially affects cost, not the average.
- Preliminary road or rail design and geometry for each option
- Route corridors defined, with optimisation under way
- Major structures, intersections and systems identified and roughly sized
- Key environmental, regulatory, community and land acquisition constraints identified
How it is prepared
Class 4 estimates are still mostly top-down, but more refined than Class 5. The 98R-18 approach for road and rail is to adjust a cost-per-kilometre rate for the specific design elements of each option, and to price major structures and systems separately with approximate unit or assembly rates.
The estimator's main job is consistency: the same rate base, the same allowances and the same treatment of risk across every option, so that differences in the estimates reflect differences in the options.
Worked Example
What the Class 4 Range Means in Dollars
Illustrative figures for a preferred-option estimate for a new interchange, with preliminary geometry and structures: an estimate of $45M including contingency.
| Project risk profile | Accuracy range | 80% confidence range of final cost |
|---|---|---|
| Well understood, low risk (tight end of the band) | −15% / +20% | $38.25M to $54M |
| Complex or high risk (wide end of the band) | −30% / +50% | $31.5M to $67.5M |
Try it with your own estimate
The accuracy range calculator applies the published bands for every class to your figure, and shows where your project might sit within them.
Open the calculatorAACE stresses that these bands are typical outcomes, not targets. The range for a particular estimate should come from a quantitative risk analysis. See P50 and P90 explained.
Avoid These
Common Class 4 Mistakes
- Estimating options at different levels of detail, which biases the comparison toward the better-studied option
- Carrying the preferred option's Class 4 number forward as the approved budget
- Ignoring scope that is common to every option (and so 'cancels out') when the total cost is what matters to the funder
- Treating the accuracy range as symmetric when overrun risk is always larger than underrun risk
In Queensland, a Class 4 estimate broadly corresponds to the PCEM Concept (options analysis) phase. The mapping is approximate; see AACE vs PCEM for how the two frameworks line up.
FAQ
Frequently Asked Questions
What is an AACE Class 4 estimate?
A Class 4 estimate is prepared during feasibility or options analysis with 1% to 15% of project definition. It typically uses factored or parametric methods and has an expected accuracy of -15% to -30% on the low side and +20% to +50% on the high side at 80% confidence.
What is the difference between a Class 4 and a Class 3 estimate?
A Class 4 estimate compares options and supports approval to proceed. A Class 3 estimate is prepared for a single, defined option to support full funding approval, uses mostly unit-rate line items, and narrows the expected accuracy to -10% to -20% low and +10% to +30% high.
Which FEL stage does a Class 4 estimate belong to?
Class 4 estimates normally correspond to FEL 2 (select), where options are evaluated and one is chosen to carry forward.
Need AACE-Classified Cost Estimates?
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