Quantity Takeoff
Learning Objectives
- Define quantity takeoff and its critical role in accurate cost estimating.
- Apply general principles of systematic takeoff, including in-place quantities and waste factors.
- Calculate basic quantities such as concrete volume, linear measurements, and steel weights.
- Understand the benefits and limitations of using Building Information Modeling (BIM) for automated takeoffs.
Quantity Takeoff (QTO)
The comprehensive and detailed measurement of physical materials and labor units needed to complete a construction project, derived directly from the contract drawings and specifications. It forms the undeniable mathematical basis of any detailed cost estimate.
Overview of Quantity Takeoff
Quantity takeoff is the detailed measurement of materials, labor, and equipment required to complete a construction project. It involves systematically reviewing drawings and specifications to compile an exhaustive list of all required items. Without an accurate quantity takeoff, a detailed cost estimate is impossible.
Importance of Quantity Takeoffs
Significance of QTO
The quantity takeoff is arguably the most critical step in the entire estimating process because every subsequent calculation (material cost, labor hours, equipment duration) relies on it.
Impacts of Accurate Takeoffs
- Cost Accuracy: Accurate cost estimation depends directly on accurate quantities. A 10% error in concrete volume results in a 10% error in concrete material costs, forming a cascading failure.
- Material Procurement: Ordering materials correctly based on the takeoff helps avoid costly shortages (which halt production) or excess surplus (which wastes money on unneeded material and disposal fees).
- Resource Scheduling: Accurately calculating the volume of work is necessary for efficiently scheduling labor crews and renting equipment for the correct duration.
- Project Management Baseline: The takeoff establishes a quantitative baseline for evaluating project progress, approving subcontractor payment applications (percent complete), and managing change orders fairly.
General Principles of Takeoff
Standardized Principles
To ensure accuracy and consistency across different estimators and projects, professional estimators adhere to strict procedures:
Standard Takeoff Principles
- Follow a Systematic Approach: Take off items in a logical sequence, universally following the order of construction (e.g., clearing, site work, excavation, foundations, framing, exterior finishes, interior finishes).
- Use Standard Units: Strictly utilize standard units of measurement for specific trades to match pricing databases (e.g., cubic meters () for concrete/excavation, square meters () for finishes/formwork, linear meters () for piping/baseboards).
- Calculate Net Quantities First ("In Place"): Measure the exact, mathematical quantities required by the drawings (the "in-place" quantity) before applying any subjective adjustments or waste factors.
- Apply Waste Factors: Add a calculated percentage to the net quantities to account for unavoidable material waste during handling, shipping, cutting to fit, and installation errors. This percentage varies widely depending on the material value and task complexity.
- Organize by Trade (WBS): Group similar items together (e.g., all concrete work under CSI Division 3, all framing lumber under Division 6) using a Work Breakdown Structure (WBS) to simplify pricing and subcontractor bid comparisons.
Common Takeoff Calculations
Geometric Property Calculations
The vast majority of a quantity takeoff involves calculating geometric properties based on the plan dimensions.
Concrete Volumes
Example: Rectangular Slab Volume
The basic formula for volume is Area times Depth.
Volume of Rectangular Prism
Calculates the volume of a rectangular shape like a concrete slab.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Total volume of the material | ||
| Length of the element | m | |
| Width of the element | m | |
| Thickness or depth of the element | m |
Slab Volume Example
For a concrete slab that is 10 m long, 5 m wide, and 0.2 m thick:
Example Concrete Slab Volume
Example calculation of concrete volume for a rectangular slab.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Volume |
Linear Measurements and Counts
Common Measurement Types
- Linear Meters (): Used for items like baseboards, piping, electrical conduit, and structural steel beams (often converted to weight based on a factor).
- Each (): Used for counting discrete, individual items like doors, windows, plumbing fixtures (toilets, sinks), and light fixtures.
Steel Weight Takeoff
Weight-Based Estimation
Unlike concrete (volume) or timber (board feet or linear meters), structural steel is predominantly estimated and purchased by weight, specifically metric tons () or kilograms () in the SI system, or pounds () and tons in the US Customary system.
Estimating Steel Tonnage
- The Process: The estimator determines the total length of each specific steel shape (e.g., W-shapes, HSS columns, angles) from the structural drawings.
- The Unit Weight: They then multiply the total length by the standard unit weight (mass per linear meter) of that specific shape, found in steel construction manuals (like the AISC or CISC manuals).
- Waste and Accessories: A percentage (often 5% to 10%) is added for connection materials (bolts, weld metal, small gusset plates) if they are not detailed explicitly, plus a waste factor for offcuts.
Total Steel Weight
Calculates the total weight of structural steel members based on their lengths and unit weights.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Total accumulated weight of steel | kg | |
| Length of a specific steel member | m | |
| Standard mass per linear length of the steel shape | kg/m |
Reinforcing Bar (Rebar) Schedules
Rebar Schedules
Rebar is also estimated by weight, but the process is unique because it is embedded within concrete and bent into complex shapes. The estimator creates a Rebar Schedule.
- Detailed Extraction: The estimator extracts the bar size, number of bars, spacing, and bent length of each piece from the structural plans.
- Conversion to Weight: The total length of each bar size is calculated and multiplied by its specific unit weight (e.g., a #5 bar weighs 1.55 kg/m).
- Lap Splices and Waste: Crucially, estimators must add lap splice lengths (where two bars overlap to transfer stress) and a significant waste factor (often 10%) due to cutting standard 6m or 12m stock lengths to fit.
Building Information Modeling (BIM) and Automated Takeoffs
BIM for Automated Takeoffs
Traditional 2D takeoff is incredibly time-consuming and prone to human error (missed plan sheets, incorrect scale). Building Information Modeling (BIM) has revolutionized the estimating profession.
BIM for Estimating (5D BIM)
A BIM model is not just a 3D picture; it is a database. Every element (a wall, a door, a beam) contains intelligent data. When an estimator links the 3D model (the 3rd dimension) with the project schedule (4th dimension) and cost data (5th dimension), it creates a powerful tool for rapid, automated takeoff and cost feedback during design.
- Automated Extraction: Estimating software (like Navisworks, Vico, or Bluebeam Revu linked with Revit) can instantly extract the exact volume of concrete, the square footage of drywall, or the number of doors directly from the designer's model properties.
- Clash Detection: BIM identifies physical conflicts (e.g., a plumbing pipe running through a structural steel beam) before construction begins, preventing costly change orders and rework that a 2D estimator might miss entirely.
- Visual Verification: The estimator can visually isolate all the "concrete foundation" elements in 3D to verify that nothing was missed and the quantities "look right."
- Limitations of BIM Takeoff: The automated takeoff is only as accurate as the model. If the architect did not model the baseboards, the software cannot count them. Estimators must still manually check for un-modeled items and apply their own waste factors and temporary construction items (like scaffolding) that are rarely modeled.
- Quantity takeoff is the foundational, indispensable step of detailed cost estimating. Mathematical or omission errors here multiply throughout the entire estimate.
- A precise takeoff is essential not only for bidding but also for procurement, scheduling, and project control during construction.
- Estimators must work systematically, following the sequence of construction to avoid omitting elements.
- Net quantities must be calculated strictly and purely from the drawings first.
- Waste factors are applied after the net calculation to account for real-world, physical material losses.
- The three primary dimensions of a quantity takeoff are volume (cubic meters), area (square meters), and length (linear meters), supplemented by discrete counts (each).
- Concrete and earthwork are almost exclusively measured by volume.
- Surface treatments (paint, drywall, flooring) and concrete formwork are measured by area.
- When performing takeoffs for elements with openings (like doors or windows), always calculate the gross quantity first and then subtract the openings to find the net quantity.
- BIM (5D Estimating) allows for rapid, automated quantity extraction directly from intelligent 3D models, vastly increasing speed and reducing mathematical errors.
- Estimators must still manually verify BIM quantities, account for un-modeled elements (like temporary work), and apply real-world waste factors.