Quantity takeoff is the structured measurement of quantities from construction drawings: linear feet of wall framing, square feet of drywall, cubic yards of concrete, count of doors, and so on. The output feeds the unit-cost lookup that produces the estimate. Takeoff can be done manually (digitizer pen on paper plans, paper calculator), semi-manually (PDF-based digital takeoff with on-screen measurement tools), or AI-assisted (machine reading drawings to detect rooms, walls, fixtures and produce quantities automatically).
Takeoff accuracy is the foundation of estimating accuracy. A 5% error on quantities cascades into a 5% error on the estimate even with perfect unit costs. The skilled estimator understands the drawings well enough to spot what is missing (interior soffits not visible on the floor plan, structural angles not detailed, fire-rated assemblies that change the wall type), which an automated tool can miss. Best practice on commercial work: AI or digital tool produces the first-pass quantities, the estimator reviews and adjusts, and the final takeoff log records the assumptions for field defense during construction.