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Your wall
Buried courses

Auto buries about 1 in for every foot of wall height (at least one course), a common minimum for segmental walls. Your block maker’s guide may ask for more.

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You need

111 wall blocks

7 courses × 15 blocks · 15 caps · 4.48 tons of gravel

grade20 ft3 ftburied
Wall20 ft long × 3 ft high
Courses7 (6 above ground + 1 buried course)
Blocks per course15 (16 in long)
Wall blocks111 (incl. 5% extra)
Cap blocks (16 in long)15
Base gravel (6 in deep)0.68 cu yd · 0.98 tons
Drainage stone behind the wall (12 in wide)2.59 cu yd · 3.5 tons
Perforated drain pipe20 ft

Blocks = courses (above ground + buried) × ceil(wall length ÷ block length) × (1 + extra %) · Caps = ceil(wall length ÷ cap length) · Gravel: 6 in base, 12 in drainage zone