Retaining Wall Calculator
Block counts, base gravel, and cap pieces for segmental block walls.
- Wall blocks
- 80
- Cap blocks
- 20
- Base gravel (cy)
- 0.74
- Drainage backfill (cy)
- 1.48
4 courses of 20
6 in deep by 24 in wide leveling pad
12 in of #57 stone behind wall
Sizing a segmental retaining wall
Segmental retaining wall (SRW) blocks are sold by the piece, not the pallet. A standard 6 inch face by 12 inch wide block is what you see at every home center. Larger walls use 8 inch face blocks, and small garden walls use 4 inch face blocks. The math is straight geometry: total blocks equals courses times blocks per course.
For a 20 foot long, 24 inch tall wall using 6 by 12 blocks, you need 4 courses tall by 20 blocks per course, or 80 wall blocks plus 20 caps. Plus half-blocks at every other course end for staggering, which most manufacturers include in a pallet count.
When SRW works and when it does not
Segmental block walls are engineered for heights up to about 3 to 4 feet without geogrid reinforcement. Anything taller requires geogrid layers extending horizontally into the retained soil, and any wall over 4 feet in Texas typically requires an engineered design and a permit. Homeowners underestimate this constantly.
For steep grades or retaining more than about a car-load of soil above the wall, walk away from block and look at cast-in-place concrete or a wood cribbing system. The failure mode of an under-engineered block wall is not a slow lean; it is a catastrophic collapse after a hard rain.
Base preparation and drainage
The base is what makes a block wall last. Excavate 6 inches deep and 24 inches wide (twice the wall thickness) along the entire footprint. Fill with 6 inches of compacted road base or #57 stone, screed level, and set the first course dead flat. If the first course is off by an eighth of an inch, the top course is off by an inch.
Drainage behind the wall is non-negotiable. Place a 12 inch layer of #57 washed stone directly behind the wall from base to within 6 inches of the top. Install 4 inch perforated pipe at the base of the drainage stone, sloped to daylight. Cap the stone with woven fabric to prevent silt from clogging the drainage over time.
Setback, cap installation, and finish
Most SRW systems have a built-in setback of about one inch per course, either through a lip on the back of the block or a pin system. The setback is what creates the batter that resists overturning. Do not stack blocks vertically without setback; it looks fine on day one and leans within a year.
Cap blocks are attached to the top course with masonry construction adhesive. Cut caps with a masonry saw for curves and end pieces. Finish grade behind the wall should slope away from the wall to prevent water from ponding at the top.
For hardscape and landscape contractors
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Frequently asked.
- How many blocks do I need for a 20 ft long, 2 ft tall wall?
- Using 6 by 12 blocks, that is 4 courses by 20 blocks per course = 80 wall blocks plus 20 caps.
- Do I need a permit for a retaining wall?
- Most Texas jurisdictions require a permit and engineered plans for walls over 4 feet tall (measured from bottom of base to top of wall). Under 4 feet, check with the local building department.
- How deep should the base be for a block wall?
- 6 inches of compacted #57 stone or road base is standard, extended 6 inches in front and 6 inches behind the wall footprint.
- What goes behind a retaining wall?
- Twelve inches of #57 washed stone with a 4 inch perforated drain pipe at the base, wrapped in woven fabric at the top to keep silt out. Backfill the rest with the excavated soil.
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