Retaining Wall Calculator
Calculates segmental retaining wall (SRW) concrete blocks (e.g. Allan Block, Keystone, Anchor 12" or 16" wide blocks), wall cap units, buried bottom base course, and 3/4" clear drainage backfill gravel.
Formula
Total Height = Exposed + 0.5 ft buried; Blocks = (Length × Total Height × Blocks/Sq Ft) × (1 + Waste %)
1. Total Wall Height ft = Exposed Height + 0.5 ft (6" buried base course for stability) 2. Total Wall Face Area = Wall Length × Total Wall Height 3. Base Blocks = Total Face Area × Blocks per Sq Ft 4. Total Blocks with Waste = Ceil(Base Blocks × (1 + Waste %)) 5. Wall Cap Stones = Ceil((Wall Length ft × 12 in) ÷ Cap Width in) 6. Drainage Gravel (Tons) = (Wall Length × Exposed Height × 1 ft deep ÷ 27) × 1.4 tons/yd
Inputs
- Retaining Wall Length
- Exposed Wall Height
- Block System Format
Worked example
Building a 3-ft high retaining wall with a 6-inch buried base course and 1-ft gravel drainage backfill.
Outcome: Total height is 3.5 ft (3 ft exposed + 0.5 ft buried base course). Total face area is 105 sq ft = 157.5 base blocks (171 blocks with 8% waste). Cap stones: 23 twelve-inch caps. Drainage gravel: ~4.9 tons.
Why is a 1-foot column of clear gravel required directly behind retaining walls?
Water hydrostatic pressure behind a wall is the #1 cause of retaining wall failure. Clear 3/4" gravel backfill allows water to drain freely down to a perforated drain pipe rather than pushing against the back of the blocks.