Multiply the area in square feet by depth in feet and divide by 27 for cubic yards, then multiply by 1.45 for tons of washed #57 stone. A 10 × 10 ft area at 4 inches deep needs about 1.2 cubic yards, or roughly 1.8 tons.
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Order – tons (includes your waste allowance) so compaction and spillage don’t leave you short.
Drainage Gravel Calculator Guide
Drainage stone is the one aggregate you buy for its empty space. Washed, open-graded stone holds 35–40% of its volume as voids, and those voids are the plumbing: water drops through them at rates soil cannot approach. This calculator converts your drainage layer — a foundation strip, a soggy corner, an under-slab blanket — into cubic yards and tons at #57 stone’s 1.45 tons per cubic yard.
The estimate is only as good as the material choice behind it, so the calculator limits you to genuinely free-draining options. The instinct to grab whatever gravel is cheapest is how drains fail: material with fines packs tight, and a packed layer stores water instead of passing it.
Why Does Washed Stone Drain So Much Better?
Washing strips out everything smaller than the target stone size. What remains cannot pack into a dense mass — every stone bridges against its neighbors and the gaps stay open permanently. Dense-graded products do the opposite by design.
| Material | Tons per yd³ | Drainage | Best drainage role |
|---|---|---|---|
| #57 stone | 1.45 | Excellent | French drains, foundation perimeter, general-purpose |
| #67 stone | 1.45 | Excellent | Pipe bedding where a slightly smaller stone helps |
| Pea gravel | 1.35 | Good | Downspout splash zones, planter and pot bases |
| River rock | 1.50 | Good | Dry creeks, outfalls, visible drainage swales |
| Crusher run | 1.55 | Poor | None — its fines nearly seal it; keep it under pavement |
Where Drainage Gravel Earns Its Keep
- Foundation perimeter: a 12-inch-wide column of washed stone against the footing, wrapped in fabric.
- Downspout trenches: 4–6 inches of stone carrying roof water away from the house.
- Retaining wall backfill: a 12-inch stone zone behind the wall relieves the hydrostatic pressure that topples walls.
- Under slabs: a 4-inch layer as a capillary break — 81 square feet per cubic yard, about 56 square feet per ton, at that depth.
- Dry wells: a fabric-lined pit of stone that banks a storm surge and releases it slowly into the subsoil.
Wherever stone meets soil, separate the two with non-woven geotextile; silt migration is the slow death of every unwrapped drain. For piped trench systems specifically, size the stone with the French drain calculator, and when a project needs compactable base rock instead of drain rock, switch to the crushed stone calculator.
Frequently Asked Questions
What makes gravel good for drainage?
Washed, open-graded stone like #57 keeps 35–40% of its volume as open voids because the fines have been screened out, so water passes through almost unrestricted. Dense-graded material like crusher run compacts nearly watertight and belongs under pavement, not in a drain.
How much does a ton of drainage gravel cover?
At 1.45 tons per cubic yard, one ton of #57 stone covers about 56 square feet at a 4-inch depth. One cubic yard covers 81 square feet at the same depth.
Should drainage gravel be rounded or angular?
Both drain well as long as the stone is washed — angular #57 interlocks and stays put on slopes, while rounded pea gravel and river rock shift more but flow around pipes easily. The non-negotiable is the absence of fines, not the shape.
Can I use drainage gravel directly against a foundation?
Yes, washed stone against the footing is standard practice, but separate it from the surrounding soil with non-woven geotextile so silt cannot wash in and clog it. Slope the finished grade away from the wall so the stone handles subsurface water, not roof runoff.