Retaining wall calculator

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