French drain calculator

Trench gravel volume and tonnage, perforated pipe length, the filter-fabric width per linear foot for a full burrito wrap, and the slope check that decides whether the trench drains or just holds water.

01 How the math works

A french drain is a gravel-filled trench that carries water away from where it collects. Most carry a perforated pipe along the bottom; the gravel around it is the part you order by the yard. Four quantities come off the trench, and one check decides whether any of it works.

Gravel fills the trench cross-section minus the pipe it surrounds. Pipe length for a straight run is the trench length. Filter fabric wraps the gravel, so its width per linear foot is the trench width plus both walls plus the overlap that closes the top. And the slope is fall over run — the one number that turns a trench into a drain.

Clean drainage stone (ASTM D448 #57, 3/4 in) runs about 100 lb/ft³, so weight is volume in cubic feet times 100, and a cubic yard is about 2,700 lb (1.35 tons). The IPC sets the minimum slope for drainage pipe 3 in to 6 in at 1/8 in per foot — a 1% grade — and 4 in french-drain pipe sits in that band.

02 Worked example

Take a 50 ft run along the low side of a yard (15.24 m), trenched 12 in wide and 18 in deep (305 mm × 457 mm), with a 4 in (102 mm) perforated pipe and a 12 in (305 mm) fabric overlap. The yard gives up 9 in (229 mm) of fall across that run.

Gravel: the trench cross-section is 1.0 ft × 1.5 ft = 1.5 ft²; the 4 in pipe takes out 0.0873 ft², leaving 1.4127 ft². Over 50 ft that is 70.64 ft³ = 2.62 yd³ (2.00 m³), weighing 7,064 lb = 3.53 tons (3,204 kg). With a 10% overage, round up to the half-yard step: 3.0 yd³ (2.5 m³).

Fabric: 12 + 2 × 18 + 12 = 60 in = 5.0 ft (1.524 m) of width, times 50 ft is 250 ft² (23.23 m²). That 5 ft width is the catch — it is wider than a common 3 ft or 4 ft geotextile roll, so the trench depth, not its width, is what sets the roll you buy.

Slope: 9 in of fall over 50 ft is 0.75 ft ÷ 50 ft = 1.5%, or 0.18 in/ft (15 mm/m). The minimum fall to hit 1% over this run is 6 in (152 mm), so 9 in clears it with room to spare.

03 When this calculator is wrong

The result uses a flat 10% overage, which is fine for a long, straight, machine-cut trench with clean vertical walls. For a short residential run dug by hand, real overage runs closer to 15%: the trench bottom is never flat, the walls slough in, and the gravel settles into every over-dig. Bump the overage to 15% on a hand-dug job and order the extra half-yard — gravel keeps, and a second delivery for a quarter-yard shortfall does not pay.

• The slope check assumes the fall is usable end to end. It divides total fall by run, which is right only if the trench grades evenly. A dip in the middle ponds water even when the overall grade clears 1%. Shoot the line with a level every 10 ft (3 m), not just end to end, and keep the pipe falling the whole way — the IPC minimum of 1/8 in per foot (1%) is a floor, not an average.
• It sizes a rectangular trench. A real trench is dug a little wider at the top and the gravel heaps above the pipe, so the stone volume runs a few percent over the neat box. The overage factor absorbs it on a normal dig; a badly over-cut trench needs more.
• Pipe displacement is small. Subtracting the 4 in pipe trims about 0.16 yd³ off this example — real, but minor. On a gravel-only drain there is no pipe to subtract, which is why the "gravel only" mode fills the whole trench.
• Fabric width is a full burrito wrap. If you only cap the gravel with fabric instead of wrapping it, you need less — but an unwrapped drain silts up as fines migrate into the stone, which is the failure mode a french drain exists to avoid. The width + 2 × depth + overlap figure is for the wrap that lasts.

04 What to do with the result

Check the slope first. If the fall over your run comes in under 1% (1/8 in per foot), stop and regrade before you dig — more gravel will not fix a trench that does not fall, and a drain that holds water is worse than no drain. With the grade confirmed, order the rounded-up gravel by the ton or half-yard, buy the perforated pipe in the stick or coil length your yard stocks (rigid PVC comes in 10 ft sticks, corrugated HDPE in 100 ft or 250 ft coils), and match the geotextile roll width to the fabric width above — not to the trench width, which is the mistake that sends people back for a second roll.

05 Common questions

What slope does a french drain need?

A minimum of 1% — about 1/8 inch per foot, or 6 in of fall over 50 ft. That matches the IPC minimum slope for drainage pipe 3 in to 6 in in size. Where the grade allows it, 1–2% drains faster and silts up less. Below 1%, water sits in the pipe and the drain fails.

How much gravel do I need for a french drain?

Trench width × depth × length, minus the pipe, in cubic feet, divided by 27 for cubic yards. A 50 ft run at 12 in × 18 in with a 4 in pipe takes 2.62 yd³ (2.00 m³) of stone — about 3.5 tons. Order the rounded-up figure with an overage; clean 3/4 in #57 drainage stone is the standard fill.

How wide does the filter fabric need to be?

For a full wrap, trench width + 2 × trench depth + overlap. A 12 in wide, 18 in deep trench needs 12 + 36 + 12 = 60 in (1.52 m) of fabric width — wider than a 3 ft or 4 ft roll, so buy the roll to the depth of your trench, not its width.

Does a french drain need a pipe?

Not always. A gravel-only trench moves water by percolation and works for a short, shallow run on good grade. A perforated pipe carries far more flow for the same trench, which is why most modern french drains include one. The calculator's two modes cover both: "gravel only" fills the whole trench, "with pipe" subtracts the pipe's cross-section.

What size pipe goes in a french drain?

4 in (100 mm) perforated pipe is the residential standard — rigid PVC to ASTM D2729 or corrugated HDPE to ASTM F405. The 4 in size sits in the IPC's 3-to-6-inch band, so 1/8 in per foot is the minimum slope it needs to run.