Start with net wall area
Measure wall length and height, then subtract large openings that genuinely remove block area.
Calculate separate elevations when wall thickness or unit size changes.
Use the nominal installed module
Standard CMU layout is coordinated by nominal module including the mortar joint.
Verify the exact unit family because architectural and specialty units can differ.
Separate field blocks from special units
Corners, bond beams, lintels, jambs, halves, and knockouts should not be hidden inside a generic block count.
A plan-based takeoff is needed when special-unit locations are specified.
Add waste deliberately
Breakage and cuts can justify an allowance, but repetitive modular walls may need less than highly interrupted work.
Keep the allowance visible so it can be revised from supplier packaging and site conditions.
Do not infer reinforcement
Block quantity does not determine grout, rebar, anchors, or structural capacity.
Follow drawings, specifications, local requirements, and engineering for reinforced masonry.