Introduction to Cost Estimating
Learning Objectives
- Understand the purpose and importance of cost estimating in civil engineering projects.
- Differentiate between the various types of estimates across project phases.
- Familiarize with the AACE International Estimate Classifications.
- Learn about the Work Breakdown Structure (WBS) and its role in organizing an estimate.
- Compare CSI MasterFormat and UniFormat classification systems.
An overview of engineering cost estimating, covering its purpose, different types of estimates, and the general estimating process.
Overview of Cost Estimating
Cost estimating is a fundamental aspect of civil engineering and construction management. It involves forecasting the cost of completing a project within a defined scope. Accurate estimates are critical for project feasibility analysis, budgeting, bidding, and cost control. The reliability of any estimate relies heavily on the quality of the project information available and the experience of the estimator.
Purpose of Cost Estimating
The primary purposes of cost estimating vary depending on the project phase.
Estimating Goals by Phase
The goals and required accuracy of an estimate evolve as a project moves from an initial idea to a fully constructed reality.
- Concept Phase: To determine if a project is economically viable (feasibility). Owners use these estimates to decide whether to proceed with or abandon a project.
- Planning/Design Phase: To establish a preliminary budget and compare the financial impacts of different design alternatives. This helps keep the project within the owner's financial limits.
- Bidding Phase: For contractors to submit competitive, accurate bids while ensuring the company makes a reasonable profit margin.
- Construction Phase: To control project costs, track performance against the baseline budget, and evaluate the financial impact of scope changes or claims.
Rough Order of Magnitude (ROM) Estimate
A highly preliminary conceptual estimate prepared with minimal design information (often 0% to 5% complete). Used for initial feasibility studies with very wide accuracy bounds (e.g., -25% to +75%).
Parametric Estimate
An estimate that uses statistical relationships between historical data and other variables (e.g., cost per square meter, cost per hospital bed) to calculate a cost for a new project. More accurate than a ROM but still preliminary.
Types of Estimates
Estimates become progressively more detailed and accurate as the project design advances.
- Conceptual Estimate (Rough Order of Magnitude - ROM): Used for initial feasibility studies.
- Parametric Estimate: Uses statistical relationships between historical data and other variables.
- Preliminary Estimate (Design Development Estimate): Prepared when the design is partially complete (e.g., 30% to 60%). Used to verify the budget as the design progresses and major systems are selected.
- Detailed Estimate (Definitive or Bid Estimate): Prepared from 100% complete plans and specifications. It involves a detailed quantity takeoff and unit pricing. This is the most accurate estimate, used for bidding and contract formation, with an accuracy typically between -5% to +10%.
General Estimating Process
- Review the Scope: Thoroughly read and understand the project plans, specifications, and scope of work.
- Establish a WBS: Create a Work Breakdown Structure to systematically organize the work into manageable pieces.
- Perform Quantity Takeoff: Measure and quantify all materials, labor, and equipment needed for each WBS element.
- Determine Unit Costs: Assign a unit cost (labor, materials, equipment, subcontractors) to each quantified item.
- Calculate Direct Costs: Multiply quantities by unit costs to determine the direct cost of the work.
- Add Indirect Costs and Markup: Apply overhead, profit margin, contingency, and taxes to reach the final estimated cost.
- Review and Finalize: Double-check calculations, verify assumptions, and finalize the estimate report.
The Cone of Uncertainty
The "Cone of Uncertainty" is a concept illustrating how estimate accuracy improves as project definition increases. Early in a project, the uncertainty (and thus the estimating error margin) is very high. As design progresses and more information becomes available, the cone narrows, leading to much tighter, more accurate estimates.
AACE International Estimate Classifications
Industry-standard classification system for estimate accuracy based on project maturity.
AACE Estimate Classes
The Association for the Advancement of Cost Engineering (AACE) International defines a widely accepted classification system. This provides a standard framework for the "Cone of Uncertainty."
- Class 5 (Concept/Screening): 0% to 2% design maturity. Used for strategic planning. Expected accuracy: -50% to +100%.
- Class 4 (Study/Feasibility): 1% to 15% design maturity. Used to determine project feasibility. Expected accuracy: -30% to +50%.
- Class 3 (Budget/Authorization): 10% to 40% design maturity. Used for budget authorization and appropriation. Expected accuracy: -20% to +30%.
- Class 2 (Control/Bid/Tender): 30% to 75% design maturity. Used to establish the baseline for project control. Expected accuracy: -15% to +20%.
- Class 1 (Check/Estimate): 65% to 100% design maturity. Used for detailed bid verification or negotiation. Expected accuracy: -10% to +15%.
Work Breakdown Structure (WBS) & Classification Systems
The foundation of organizing any professional cost estimate is a structured classification system.
Work Breakdown Structure (WBS)
A deliverable-oriented, hierarchical decomposition of the work to be executed by the project team. It breaks down the entire project scope into manageable, trackable, and estimable work packages.
Organizing the Estimate
A fundamental step in preparing any estimate is organizing the scope of work into a logical, hierarchical format. This organization prevents omissions, avoids duplications, and allows for clear communication among stakeholders.
CSI MasterFormat
The industry standard for organizing construction data, specifications, and estimates in North America.
CSI MasterFormat
A trade-based classification system for organizing construction requirements, products, and activities. It uses a multi-level numbering system to group specifications and cost estimates by trades and materials.
CSI MasterFormat Organization
The Construction Specifications Institute (CSI) MasterFormat is the most widely adopted classification system for commercial and institutional building projects.
- Structure: It uses a multi-level numbering system (e.g., 50 Divisions in the current format).
- Common Divisions: Division 01 - General Requirements, Division 02 - Existing Conditions, Division 03 - Concrete, Division 04 - Masonry, Division 05 - Metals, Division 09 - Finishes, Division 31 - Earthwork.
- Usage: Estimators use MasterFormat to logically group their quantity takeoffs and unit prices, matching the way specifications are written and the way subcontracts are typically awarded (e.g., all concrete work grouped under Division 03).
UniFormat
A systems-based approach useful for early-stage estimating.
UniFormat
A classification system that organizes building elements by functional systems (e.g., Substructure, Shell, Interiors) rather than by materials or trades.
UniFormat Organization
While MasterFormat is trade-based, UniFormat organizes building elements by functional systems (e.g., Substructure, Shell, Interiors, Services). It is particularly useful during the schematic design phase when the exact materials (MasterFormat) might not be known, but the functional systems are defined.
- Cost estimating is the structured process of forecasting the cost of a project based on available information.
- The primary purpose of an estimate depends heavily on the current project phase, ranging from initial feasibility checking to final bidding and budget control.
- Estimates follow the "Cone of Uncertainty": starting as high-level Conceptual (ROM) estimates with very wide error margins, and narrowing down to highly accurate Detailed (Bid) estimates as the project design solidifies.
- A Work Breakdown Structure (WBS) is essential for systematically organizing an estimate to prevent omissions.
- CSI MasterFormat is the dominant trade-based classification system for detailed estimating, while UniFormat is useful for systems-based preliminary estimating.