Retaining Wall Base Calculator
Size the gravel base trench for a segmental retaining wall — trench depth, base gravel in cubic yards and tons.
How it works
base gravel = length × ((block depth + overdig) ÷ 12) × (base depth ÷ 12) ÷ 27
A segmental retaining wall sits on a compacted crushed-stone leveling pad in a trench a few inches wider than the block, front and back. The base volume is that trench: wall length times base width times base depth, converted to feet. Use ¾ in minus (crusher run) that compacts, not clean drainage stone, and figure roughly 1.35 tons per cubic yard. Drainage stone behind the wall and buried courses are separate quantities.
Worked example
A 30 ft wall of 12 in deep block, base dug 6 in wider (18 in) and 6 in deep: 30 × 1.5 × 0.5 = 22.5 cu ft = 0.83 cubic yards, about 1.1 tons of compactable base.
Frequently asked questions
How deep should the base trench be?
Bury the first course so the trench is the base gravel depth (about 6 in) plus one buried block — roughly one-tenth of the wall height below grade. That anchors the toe against sliding and frost.
What gravel goes under a retaining wall?
A compactable base such as ¾ in minus / crusher run for the leveling pad, compacted in lifts. Behind the wall, use clean ¾ in drainage stone with a pipe — do not use the drainage stone as the base, as it will not compact.
How level does the base need to be?
Very. Every block above copies the first course, so a leveling pad off by even ¼ in over its length shows up magnified at the top. Screed and compact the pad, then set and re-check each block front-to-back and side-to-side.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.