Estimating Practice FAQ
Practitioner Q&A drawn from wiki content. Each answer is sourced from a specific wiki page — follow the link for full detail. Updated on each /faq run.
Each entry uses a <!-- slug: --> marker so the skill can find and merge duplicates.
What are the accuracy ranges for AACE Class 1 through Class 5 estimates?
Section titled “What are the accuracy ranges for AACE Class 1 through Class 5 estimates?”Class 5 (0–2% design complete): -50%/+100% — concept screening only, never used for budget commitment. Class 3 (10–40% design): -20%/+30% — the typical GMP candidate in design-build. Class 1 (65–100% design): -10%/+15% — check estimate or full bid/tender basis. Always label estimates with their class and accuracy range; the BOE makes this explicit.
→ Estimate Classification and BOE
Updated: 2026-05-18
What contingency percentage should I apply at each estimate class?
Section titled “What contingency percentage should I apply at each estimate class?”Flat-percentage guidelines from AACE: Class 5 = 30–50%, Class 4 = 20–30%, Class 3 = 15–20%, Class 2 = 8–15%, Class 1 = 5–10%. For a Class 3 design-build GMP, best practice breaks contingency into three buckets: design development (8–12%), scope gap (3–5%), execution/risk (3–5%), totaling 15–25%. Using flat percentages alone is discouraged for Class 2 and above — use the expected-value method or Monte Carlo at P70–P80.
→ Risk Contingency and Escalation , GMP Contingency Structure
Updated: 2026-05-18
What are Lang factors and what are the typical values for manufacturing plants?
Section titled “What are Lang factors and what are the typical values for manufacturing plants?”A Lang factor multiplies the total FOB equipment purchase cost to estimate Total Installed Cost (TIC), capturing installation labor, bulk materials (piping, electrical, instrumentation, insulation), civil/structural, and indirects. Original 1948 Lang values: solid processing = 3.10, mixed solid-fluid = 3.63, fluid processing = 4.74. AACE-updated values (Wijoseno 2023) are higher — fluid processing now 6.21 — because they reflect modern safety, environmental, and automation requirements. Lang factors do NOT include building shell, site work, or contingency; those are priced separately.
→ Parametric Estimating Models
Updated: 2026-05-18
How does the six-tenths rule work for scaling process plant costs?
Section titled “How does the six-tenths rule work for scaling process plant costs?”The formula is C₂ = C₁ × (Q₂/Q₁)ⁿ, where C₁ is the known cost, Q₁ and Q₂ are capacities, and n is the scaling exponent (0.60 default for generic chemical/process plants). F&B processing typically uses n = 0.55–0.65; pharma/specialty chemical with high automation uses n = 0.40–0.50. The rule degrades outside 0.1×–10× the reference plant scale — do not use it for very large or very small departures from the reference.
→ Parametric Estimating Models
Updated: 2026-05-18
What are the $/SF benchmarks for manufacturing facilities?
Section titled “What are the $/SF benchmarks for manufacturing facilities?”2026 all-in TIC benchmarks by complexity tier: Tier 1 (shell + basic fit-out, pre-engineered/tilt-up) = $80–$150/SF; Tier 2 (standard manufacturing) = $150–$250/SF; Tier 3 (process-integrated, automation/compressed air) = $200–$350/SF; Tier 4 (F&B sanitary, heavy MEP) = $300–$600/SF; Tier 5 (pharma, paint shop, heavy chemical) = $600–$1,500+/SF. Assign a tier before selecting a $/SF range — never pick a number from the middle without documented rationale.
→ Parametric Estimating Models
Updated: 2026-05-18
What are the three contingency layers in a GMP contract and who controls each?
Section titled “What are the three contingency layers in a GMP contract and who controls each?”Layer 1 (Design Development Contingency): contractor-held, covers cost growth as design matures from GMP basis to IFC — not a change order. Layer 2 (Execution/Risk Contingency): contractor-held with owner notification, covers risk register items that materialize during construction. Layer 3 (Owner Contingency / Management Reserve): owner-only, covers scope changes and program evolution — the contractor never sees this number. Collapsing all three into a single “contingency” line is a common source of closeout disputes; best practice names and sizes each layer explicitly in the contract.
Updated: 2026-05-18
At what point in design is the GMP typically set?
Section titled “At what point in design is the GMP typically set?”Most GMP contracts are set at approximately 30–60% design completion (end of FEED or schematic design). The earlier it is set, the more design development contingency is required: at 15–30% design complete, DD contingency norms are 12–18%; at 50–75% design complete, 5–8%. At Class 3 basis (10–40% design), total contractor-held contingency of 15–25% is industry standard.
→ GMP Contingency Structure , Delivery Methods and Contracts
Updated: 2026-05-18
What is the key estimating risk difference between design-build and design-bid-build?
Section titled “What is the key estimating risk difference between design-build and design-bid-build?”In design-bid-build, the contractor prices completed drawings — design risk is the owner’s. In design-build, the contractor takes on design risk: scope gaps, design development cost growth, and specification uncertainties must all be priced into the estimate even though the drawings are incomplete. This means a DB estimator must explicitly price scope not yet shown on drawings, which a DBB estimator does not. This risk must be reflected in the contingency structure, not hidden in a flat fee.
→ Delivery Methods and Contracts , GMP Contingency Structure
Updated: 2026-05-18
What sections must a Basis of Estimate include for a manufacturing plant?
Section titled “What sections must a Basis of Estimate include for a manufacturing plant?”A complete BOE requires: (1) project identification with revision number and pricing date; (2) AACE class and accuracy range; (3) scope inclusions — explicitly stated; (4) scope exclusions — owner-furnished equipment, validation, IT, escalation beyond pricing date; (5) estimating methodology by cost category; (6) pricing basis — location factor, labor rates, material sources; (7) escalation methodology; (8) contingency basis and composition; (9) allowances for known-but-undefined items. The BOE often becomes a contract exhibit — scope exclusions in the BOE define what the contractor is not responsible for.
→ Estimate Classification and BOE
Updated: 2026-05-18
What percentage of direct cost should general conditions run on a manufacturing plant project?
Section titled “What percentage of direct cost should general conditions run on a manufacturing plant project?”General conditions (indirect project costs: superintendent, PM time, field office, temporary utilities, safety, insurance, permits, testing) typically run 8–15% of direct cost for a manufacturing plant expansion, with longer and more complex projects at the higher end. Design fees in design-build add 5–10% of construction cost depending on scope. Contractor fee (overhead and profit) on top of cost-of-work + gen cons runs 3–8% for design-build manufacturing.
Updated: 2026-05-18
Should escalation be included in contingency?
Section titled “Should escalation be included in contingency?”No — escalation and contingency must always be separate line items. Escalation is a certainty: costs will be higher at construction midpoint than at the pricing date. Contingency is a reserve for uncertain events. Combining them masks both and leads to under-funded contingency when escalation is high. The formula: Escalation $ = Base Estimate × Annual Rate % × Years from pricing date to construction midpoint, applied separately to labor (3–6%/yr) and materials (2–8%/yr). On projects with 18+ month durations, negotiate an escalation clause in the GMP contract.
→ Risk Contingency and Escalation , Pricing and Cost Assembly
Updated: 2026-05-18
How many subcontractor quotes are required before putting a number in the GMP?
Section titled “How many subcontractor quotes are required before putting a number in the GMP?”Seek a minimum of three quotes per major scope (mechanical, electrical, structural steel, roofing, civil). Settle for two if the market is tight, but document why. Allow minimum 10 working days for mechanical and electrical scopes, 5 days for concrete and steel. If no sub quotes are available, use RSMeans unit prices × your quantities, add a 5–10% scope gap contingency on that scope, and flag in the BOE as “unit price estimate, no sub quote obtained.”
→ Subcontractor Bidding and Bid Leveling
Updated: 2026-05-18
What is bid leveling and why does it matter?
Section titled “What is bid leveling and why does it matter?”Bid leveling adjusts every sub bid to the same scope basis so you are comparing leveled totals, not raw bid numbers. For each bid, you list every exclusion and add back your estimate of the excluded scope; the resulting leveled total is what goes into your GMP comparison. The leveled total is what you use in the estimate — never the raw bid. Sub quotes represent 60–80% of total GMP cost on a manufacturing plant project, so a scope gap in a selected sub bid creates a direct loss.
→ Subcontractor Bidding and Bid Leveling
Updated: 2026-05-18
How do FEL stages map to AACE estimate classes?
Section titled “How do FEL stages map to AACE estimate classes?”FEL-1 (business opportunity / conceptual) = Class 5; FEL-2 (feasibility / preliminary engineering) = Class 4; FEL-3 / FEED (front-end engineering design) = Class 3; Detailed Design through IFC drawings = Class 2 → Class 1. The GMP in design-build is almost always set at FEL-3 / Class 3 basis — which means 15–25% contingency is appropriate and should be stated explicitly in the BOE.
→ Estimate Classification and BOE , Phase-by-Phase Workflow
Updated: 2026-05-18
What confidence level (P-value) should I use when setting contingency via Monte Carlo?
Section titled “What confidence level (P-value) should I use when setting contingency via Monte Carlo?”P50 is not suitable for contractor contingency — it means a 50% chance of overrun. Industry norm for contractor contingency setting is P70–P80. Use P90 for large, complex, or first-of-kind projects. Tools: @RISK by Lumivero, Crystal Ball, Primavera Risk Analysis. Summing expected values from a risk register (probability × impact) systematically underestimates total contingency because it assumes risk independence — Monte Carlo captures correlation effects.
→ Risk Contingency and Escalation
Updated: 2026-05-18