How the brick count is calculated
Each brick plus its mortar joint takes up (brick length + joint) × (brick height + joint). A US modular brick is 7⅝ × 2¼ in; with ⅜ in joints that is 8 × 2⅝ = 21 sq in, so one square foot (144 sq in) holds 144 ÷ 21 = 6.86 bricks. Multiply by the net wall area (wall minus openings) and add 5–10% for breakage and cuts.
Worked example: a 25 × 8 ft garden wall
The wall is 200 sq ft. 200 × 6.857 = 1,371.4 bricks; with 5% waste that is 1,440 bricks. If the wall has a 40 sq ft opening, the net area is 160 sq ft and you need 1,098 bricks before waste.
Common US brick sizes
Use the actual size, not the nominal size, together with your joint width. With ⅜ in joints: modular (7⅝ × 2¼ in face) is about 6.86 bricks per sq ft; king size (about 9⅝ × 2⅝ in) about 4.8 per sq ft; utility (11⅝ × 3⅝ in) about 3.0 per sq ft. Sizes vary between manufacturers, so check the product sheet.
Single or double wythe?
The count above is for one layer of brick (single wythe), as used for veneer and most garden walls. A solid double-wythe wall needs twice as many bricks. For structural walls, retaining walls and anything over a few feet tall, follow local building codes and an engineer's design.