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Retaining wall calculator

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135 blocks5 courses × 27 per course · 27 caps
Blocks
135 blocks (5 × 27)
Caps
27 caps
Buried below grade
3"
Base rock (18" × 4", compacted)
0.74 CY · 1.22 tons
Drainage rock (12" behind)
4.94 CY · 6.40 tons
Wall face
120.00 SF

These sketches are here to help you order material and price the work quickly. They are not engineered or shop drawings — always build from your approved project drawings.

Block

Face size from the block cut sheet. Set cap length to 0 for no caps. Setback, corners, steps and core fill are not counted.

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  1. Buried block
    1" per foot of wall height
    3' 0" × 1" per ft = 3"
  2. Courses
    (height + buried) ÷ block height, rounded up
    ( 3' 0" + 3" ) ÷ 8" = 5
  3. Blocks per course
    wall length ÷ block length, rounded up
    40' 0" ÷ 1' 6" = 27
  4. Blocks
    courses × blocks per course
    5 × 27 = 135
  5. Caps
    wall length ÷ cap length, rounded up
    40' 0" ÷ 1' 6" = 27
  6. Base rock
    length × trench width × base depth = CY; × 3,300 lb/CY ÷ 2,000 = tons
    40' 0" × 1' 6" × 4" = 0.74 CY × 3,300 ÷ 2,000 = 1.22
  7. Drainage rock
    length × 12" × stack height (courses × block height) = CY; × 2,590 lb/CY ÷ 2,000 = tons
    40' 0" × 12" × 3' 4" = 4.94 CY × 2,590 ÷ 2,000 = 6.40

Where the numbers come from

  • Base, buried block and drainage (Allan Block): Base trench 24" wide × 6" of rock (18" × 4" under 4 ft); bury 1" per foot of height; 12" of wall rock behind the block; drain pipe over 4 ftAllan Block residential installation guide. Other brands are similar but check their guide. Rock weights: base compacted 3,300 lb/CY, drainage rock loose 2,590 lb/CY (crushed stone, MoDOT — same table as the cubic-yard calculator). Rock to fill hollow block cores is not included — it depends on the block. Block setback and corners are not counted.Source
  • When an engineer is needed: IRC R404.4: walls retaining more than 48" of unbalanced fill, or over 24" with loads besides soil (a driveway, a slope, a building), must be engineeredMany towns also need a permit at a lower height. Geogrid, setback and drainage are part of the design.Source

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This is a community calculator for the construction trades. We check every number against its source, but mistakes happen. If a number looks wrong, tell us — and tell us which calculator you want next.

Blocks, caps, base rock and drainage rock for a straight segmental (dry-stack) block wall.

The formula

Buried block = 1" per foot of wall height. Courses = (height + buried) ÷ block height, rounded up. Blocks = courses × (wall length ÷ block length, rounded up). Base = length × 24" × 6" of compacted rock (18" × 4" for walls under 4 ft). Drainage rock = length × 12" × the stack height.

Worked example

A 40' × 3' wall in 18×8 block: bury 3", (36 + 3) ÷ 8 = 4.9 → 5 courses; 480 ÷ 18 = 26.7 → 27 per course; 135 blocks and 27 caps. Base 40 × 1.5 × 0.33 = 20 ft³ = 0.74 CY = 1.22 tons; drain rock 40 × 1 × 3.33 = 133 ft³ = 4.94 CY = 6.40 tons.

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