Drainage gravel calculator

It is worth being precise about what this calculator is for, because the name invites confusion with a nearby but different job. Drainage gravel, in the sense this page handles, is clean, open graded stone used as backfill behind a retaining wall or as a general drainage layer somewhere on a site, with no perforated pipe running through it anywhere in the system, which keeps the installation simpler since there is no pipe to slope, connect or tie into an outlet. A french drain is a different structure built around a pipe that actually carries water away to a specific outlet, and if your project includes pipe, the french drain gravel calculator is the correct tool rather than this one, since it accounts for the pipe occupying part of the trench volume that would otherwise be filled entirely with stone.

Once that distinction is settled, the material requirement itself reduces to the same area and depth math used everywhere else on this site: measure the zone that needs backfilling, decide a depth, and the calculator converts that into volume and weight for ordering. What differs project to project is less the math than the zone itself and how it is shaped on the actual site. A retaining wall backfill is usually a vertical slice running the length of the wall and set back from it by some width, rather than a flat ground level area, so the figures you enter above should describe that backfill zone specifically, not the footprint of the wall or the yard in front of it, which would understate the real volume considerably.

Behind a retaining wall, the backfill zone is doing a specific job: letting water that would otherwise build up pressure behind the wall drain downward and away instead of pushing outward against the wall face with nowhere else to go. How wide that zone needs to be is a structural decision tied to the wall's height and design, which is outside what a materials calculator can determine on its own, but once that width is set by the wall's design or a contractor's spec, entering it above along with the wall's length and the chosen depth gives an accurate volume for ordering the right quantity the first time. Underbuilding the width of this zone is one of the more common causes of retaining wall failure, since trapped water behind a wall adds a load the wall was never engineered to resist over the long term.

Drainage stone and regular gravel are not interchangeable even though both can look similar sitting in a supply yard. Drainage gravel is specifically washed and screened to remove fines, the small particles that would otherwise clog the spaces between stones and defeat the point of using an open graded material in the first place. Ordinary crushed stone or crusher run, by contrast, often includes fines deliberately, because those fines help the material compact into a dense, stable structural layer suited to carrying weight. Using a fines heavy material for drainage backfill, or a clean open graded stone where compaction and load bearing matter more, each undermines the job the other was suited for, so it is worth confirming with your supplier that what you are buying is actually washed, open graded stone before it gets delivered to the site. A geotextile fabric wrapped around or placed over the drainage zone is a related decision worth raising with whoever is designing the wall, since fabric keeps surrounding soil from migrating into the open stone over time and clogging the very voids the drainage layer depends on to function.

If the drainage zone on your project sits alongside a structural base, a patio, or a slab rather than standing alone behind a wall, the crushed stone calculator and the gravel coverage calculator are useful companions for sizing the adjoining sections consistently, and the sand and gravel calculator is worth a look if the drainage system on your site also involves a sand layer working alongside the stone. A perimeter drain running along the full base of a wall is only as good as its weakest section, so it is worth measuring the full run length carefully rather than assuming one typical cross section applies evenly along a wall that changes height or direction partway across the site.