Retaining wall block count calculator

Retaining wall — blocks visible, gravel hidden, fabric critical

A segmental retaining wall (SRW) is half visible block face and half hidden drainage system. Skip the drainage and the wall fails — sometimes year 2, often year 5.

Three layers of aggregate

1. Compacted base — 6 inches of crushed road base (¾ minus) in a trench under the bottom course. Sets level, prevents settling.
2. Core fill — drainage stone (clean #57 or pea gravel) inside the cores of hollow-core blocks for weight and drainage.
3. Backfill drainage chimney — 12 inches of clean drain stone behind the wall, ground to top, separated from native soil with filter fabric.

The fabric is non-negotiable. Without it, fines wash into the drain stone, clog it, build hydrostatic pressure, and push the wall over.

Height limits without engineering

Most jurisdictions require an engineered design above:

• 3-4 ft tall (varies — check local code)
• Any wall with a surcharge (driveway, structure, pool) above
• Tiered walls unless tier separation > 2× tier height

Below those limits, manufacturer's published prescriptive details (Allan Block, Versa-Lok, Belgard) are accepted.

What kills DIY walls

Skipped base, no batter (walls should lean back into the slope ~1 in per foot of height), no drain stone, no fabric, and stacking vertical instead of running-bond. Get the base right and the rest follows.