Definition
In construction estimating, a deduction is an area or volume that is subtracted from a total measurement, typically for openings such as windows, doors, vents, and other penetrations. Deductions ensure accurate material quantities by accounting for areas that do not require material. Different rules apply for different materials-drywall typically requires full deduction, while painting may only deduct openings above a certain size.
Formula
For drywall: deduct all windows and doors completely. For paint: typically only deduct openings over 4 square feet. For flooring: deduct all fixed elements but add 10% waste. Always follow specific trade guidelines and local estimating standards.
Example
Drywall Deduction Example
A room has walls totaling 400 square feet. It has 4 windows (3x4 ft each = 48 sq ft total) and 1 door (3x7 ft = 21 sq ft). Net drywall area = 400 - 48 - 21 = 331 sq ft. With 10% waste factor, order 365 sq ft of drywall. Remember to deduct both sides of openings for full accuracy.