Retaining wall gravel calculator

Estimate two different stone zones

A retaining wall estimate usually separates the leveling pad below the first course from the drainage zone behind the units. The products, compaction requirements and supplier prices may differ. This calculator keeps both quantities visible and lets you change the entered dimensions independently.

Length × width × depth gives a rectangular volume. Imperial dimensions are converted to feet first, then cubic feet are divided by 27. The metric view uses cubic metres. The output is a geometric takeoff with the percentage allowance you choose, not a laboratory compaction or drainage calculation.

Base = L × (block depth + extra trench width) × base depth
Drainage = L × drainage width × max(constructed height − soil cap, 0)

Include core fill when your block requires it

The hollow-core fill input is a volume per net installed unit. Multiply it by the net unit count, then add it to the drainage-stone total before the gravel allowance. Waste blocks are spare units and their cores are not filled in this model.

Use the manufacturer’s fill volume rather than guessing from the outside block dimensions. Internal cavities, webs and inter-unit spaces vary. Concrete grout in reinforced masonry is outside this tool and should not be ordered as drainage stone.

Order by volume or by weight

Ask your quarry whether its price is per cubic yard, per cubic metre, per US short ton, or per metric tonne. The weight output uses the density you enter. The starting 1.4 US tons/yd³ is an example and should be checked for your actual aggregate.

The pipe and fabric lines are planning allowances. They do not prove a working outlet, select a filter fabric, or specify a complete drainage detail. Keep drainage design and site water management separate from the arithmetic that counts materials.