Retaining Wall Drainage Calculator
Size everything behind the wall — drainage gravel in yards, tons or bags, block-core fill, perforated pipe with outlets, and filter fabric. Bad drainage is what fails most walls; this is what to order so it doesn't.
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Why drainage decides whether a wall lasts
Retaining walls almost never fail because the blocks weren't strong enough. They fail because water builds up in the soil behind them and pushes them over. A dry wall carries the weight of the soil; a saturated wall carries the soil plus the water pressure, which can be several times larger. The whole job of the gravel, pipe and fabric behind the wall is to give that water a fast way out before it can build up. Getting the quantities right — and actually installing all three parts — is the cheapest insurance in the entire project, and it's exactly what most block-count calculators leave out.
How each quantity is worked out
- Chimney drainage stone: a vertical zone of clean crushed stone behind the blocks — length × width × the full wall height including the buried base course. This is the number the full takeoff also gives; it's here in yards, tons and bags so you can order it either way.
- Block core fill: hollow-core units (Allan Block, Keystone and similar) are filled with the same clean stone. That's real gravel most estimates forget — modelled here as an editable equivalent stone thickness across the wall face. Turn it off for solid units.
- Perforated pipe & outlets: a perforated pipe runs the full length at the base of the stone, then each outlet needs a length of solid pipe to daylight. We count one outlet per length interval you set (default every 50 ft), minimum one, plus its run.
- Filter fabric: non-woven geotextile wraps the back and bottom of the stone zone with an overlap, so length × (wall height + stone width) × overlap. It stops native soil from silting up the stone.
- Bulk vs bagged: total stone volume is also shown as the number of bags at your bag size, and priced both ways — bulk by the ton and bagged per bag — with the cheaper option flagged.
Volumes convert to weight at about 1.4 tons per cubic yard (≈1,680 kg/m³, roughly 105 lb/ft³) for clean crushed stone — ask your supplier for their exact figure. Every field is editable; the defaults follow common segmental-wall practice, not a specific engineered design. Order 5–10% extra for compaction and spillage.
Practical drainage tips
- Slope the pipe. Even a little fall toward the outlet matters — a flat pipe holds water instead of shedding it. Daylight it somewhere lower, or run it to a drain.
- Keep the stone clean. Use washed 3/4 in crushed stone (angular, not rounded pea gravel), and never use the excavated soil as backfill against the wall.
- Wrap, don't skip, the fabric. Non-woven geotextile between soil and stone is what keeps the drain working ten years later.
- Cap the top. A few inches of low-permeability soil or a swale at the top keeps surface water from pouring straight down into the drainage zone.
Frequently asked questions
How much gravel goes behind a retaining wall?
Plan on a clean-stone zone about 12 in wide for the full height behind the blocks, plus core fill for hollow units. For a 40 ft wall exposed 3 ft that's roughly 5–7 cubic yards. Enter your wall to get tons, yards and bags.
Can I use the dirt I dug out as backfill?
No. Native soil holds water and silts up the drain. The zone right behind the wall must be clean crushed stone, separated from the soil by filter fabric.
What size and type of drain pipe?
A 4 in perforated corrugated or PVC pipe, holes down, at the base of the stone, wrapped in fabric or a sock, sloped to a daylight outlet. This tool counts the perforated run plus solid pipe to each outlet.
Is bagged gravel ever worth it?
Only on small or hard-to-reach walls. Bagged stone costs several times more per yard, but avoids delivery minimums and lets you carry it to a backyard a truck can't reach. The calculator flags whichever is cheaper for your quantity.