How to use the retaining wall calculator
- Enter the wall length and exposed height.
- Enter the block face size from the manufacturer.
- Keep the first course buried unless the manufacturer says otherwise.
- Adjust base and drainage dimensions to the manufacturer's installation guide.
Formula
Courses = ⌈height ÷ block height⌉ (+1 buried). Blocks = courses × ⌈length ÷ block length⌉ × (1 + waste). Base (yd³) = length × trench width × base depth ÷ 27. Drainage (yd³) = length × drainage width × total height ÷ 27.
Worked example
A 30 ft wall, 3 ft exposed, with 12 × 4 in blocks: 9 + 1 = 10 courses × 30 = 300, or 315 blocks with 5% waste, plus 30 caps, 1.11 yd³ of base and 3.70 yd³ of drainage stone (12 in wide behind the full 40 in of block).
Tips for using the retaining wall calculator
- Bury the first course and set it on a compacted gravel base for a stable wall.
- Backfill with clean drainage stone and add a perforated pipe behind taller walls.
- Walls over about 4 ft, or walls holding loads, usually need an engineer and a permit.
Retaining Wall Calculator FAQs
How much does a retaining wall cost?
National cost data show most walls at $20–$53 per sq ft of face (average $33), with interlocking block $15–$35 and natural stone $20–$90 (updated October 2026).
Do retaining walls need drainage?
Yes. Water pressure behind a wall is a leading cause of failure; manufacturers specify clean stone backfill and often a perforated drain pipe at the base.
When do I need an engineer?
Under IRC R404.4, walls retaining more than 48 in of unbalanced fill (not supported at the top) must be designed; walls with surcharges such as driveways or slopes above often need design at lower heights.
