French drain gravel calculator

Trench mode exists because a drainage trench is not a flat bed, it is a long narrow channel with a pipe running through part of it, and the gravel only fills the space around and above that pipe rather than the full trench volume. That is a meaningfully different calculation from an area calculator, since the pipe diameter subtracts directly from how much stone you need per linear foot, and getting that subtraction wrong by even a little compounds over a long drain run. If your drainage project does not involve a pipe at all, such as a simple gravel filled trench for surface water, the drainage gravel calculator or a plain area calculation may be the simpler tool.

Trench width and depth are worth measuring carefully rather than estimating, because unlike a patio or a driveway where a slightly generous footprint just means a bit of extra stone at the edges, a drainage trench that is too narrow does not drain properly and one that is too deep wastes stone for no benefit. Dig to the dimensions the drain actually needs rather than rounding up for convenience, and enter those exact numbers above. If you are also deciding how wide to dig in the first place, the question is really about giving the pipe enough stone cover on every side to let water reach it from the surrounding soil, not about matching some separate target volume.

Round stone and crushed stone both show up in french drains, and the choice affects water flow more than it affects the volume calculation above. Crushed stone's angular faces lock together loosely and leave larger connected voids between pieces, which is generally considered better for moving water quickly through the trench, while round stone like pea gravel sits more densely and can slow that movement slightly. Either way, a washed stone with the fines removed is the usual choice for drainage work specifically, since fine particles clog the voids that water needs to pass through. The crushed stone calculator and the #57 gravel calculator both work from the same area style math if you want to check a specific numbered stone's volume for a non trench part of the same drainage project. Whatever size you settle on, buying washed rather than unwashed stone for a drain is usually worth the modest difference in price, since the fines an unwashed load carries end up exactly where you do not want them, filling the voids the pipe depends on.

Filter fabric is the other material a french drain needs alongside the stone itself, usually wrapped around the pipe and stone together before the trench is backfilled with soil. Its job is the same as it is under a driveway base, keeping soil from migrating into the stone over the years and gradually clogging the voids the water depends on. Fabric is typically ordered by the linear foot of trench plus enough width to wrap around the full perimeter, which is a separate quantity from the gravel tonnage above but scales with the same trench length, so it is worth sizing both from the same tape measure reading rather than two different ones.

Drainage trenches are also a project where it pays to think about where the water goes once it leaves the gravel, not just how much gravel fills the trench. A french drain that daylights into a yard with nowhere for water to run, or ties into a low spot that already floods, moves the standing water problem a few feet rather than solving it. That is outside what any calculator on this site can tell you, since it depends on your yard's grading, but it is worth walking the drain's full planned route before ordering stone, so the tonnage you buy here matches a layout that will actually work once it is in the ground. Once the route and dimensions are settled, the gravel cost calculator turns the trench tonnage into a delivered total, and it is worth pricing the stone and the fabric together in the same call rather than ordering the fabric separately later, since a supplier who sells both will often bundle them into a single delivery for less than two separate trips.