French Drain Calculator

Calculate the gravel, perforated pipe and landscape fabric for a French drain from the trench length, width and depth, plus the soil to haul away and the minimum slope.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a French drain, multiply trench width by depth by length in feet for the trench volume, subtract the pipe volume, and divide by 27 for cubic yards of gravel. A 50 ft trench 12 in wide and 18 in deep with 4 in pipe takes 2.62 cubic yards of washed gravel (about 3.66 tons), 50 ft of pipe and 250 sq ft of fabric.

Your measurements

A length built from stock pieces: total length divided by piece length, rounded up, with the last piece cut.

Trench lengthftTrench widthin6 to 12 in is typical. A walk-behind trencher cuts a 6 in slot; a mini excavator's smallest bucket is 12 in. Leave 2 to 3 in of gravel each side of the pipe.Trench depthin18 to 24 in for a yard drain, and the outlet end must still be above the daylight point after the 1% fall. A foundation drain goes to the footing per IRC R405.1 and is a job for a contractor.Perforated pipeBuy unsocked pipe and wrap the trench instead. Sock pipe is for sandy soil; in clay or silt the sock blinds with fines within a few years.GravelAsk for #57 or 3/4 in washed (clean) stone. Never crusher run or anything with fines; they fill the voids and the drain stops carrying water.Landscape fabric4 oz non-woven geotextile (NW4 or equal), not woven weed barrier. Skip it only in clean sand or gravel soil.

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Gravel2.62 cu ydAll results

Results

Gravel: 2.62 cu yd; About 3.66 tons of 3/4 in washed gravel, after subtracting the 4 in pipe
Perforated pipe: 50 ft; 5 sections of 10 ft rigid pipe, or 1 roll of 100 ft corrugated pipe, plus a capped cleanout riser at the head and each bend, and a grated daylight outlet in solid pipe
Landscape fabric: 250 sq ft; 1 roll of 4 ft x 100 ft; the wrap needs 60 in of width (trench width plus both sides and a 12 in overlap)
Soil to remove: 2.78 cu yd; 75 cu ft in place; dug soil swells about 25%, so haul away about 3.47 cu yd
Minimum slope: 6 in total drop; about 1 in per 8 ft (a 1% grade) from inlet to outlet, and 2% is better; the outlet has to daylight on a slope, into a dry well or into a storm drain the city allows, never the sanitary sewer

French Drain formula

Trench volume (cu ft) = Width (in) / 12 x Depth (in) / 12 x Length (ft)
Pipe volume (cu ft) = 3.1416 x (pipe diameter (in) / 24)² x Length (ft)
Gravel (cu yd) = (Trench volume - Pipe volume) / 27;  Tons = cu yd x 1.4
Fabric width (in) = Trench width + 2 x depth + 12 overlap;  Fabric (sq ft) = width / 12 x Length

Worked example

An 80 ft drain along a fence line, 8 in wide and 24 in deep with 4 in pipe: trench volume is (8 / 12) x (24 / 12) x 80 = 106.67 cubic feet. The pipe takes 3.1416 x (4 / 24)² x 80 = 6.98 cubic feet, leaving 99.69 cubic feet of gravel, which is 3.69 cubic yards or 5.17 tons of 3/4 in washed gravel.

Pipe: 80 ft is 8 sections of 10 ft PVC or one 100 ft roll of corrugated pipe. Fabric: 8 + 48 + 12 = 68 in wide, so 68 / 12 x 80 = 453 sq ft, which is 2 rolls of 4 x 100 ft. Soil out of the trench is 106.67 cubic feet, or 3.95 cubic yards, before it swells. At a 1% grade the pipe drops 9.6 in from end to end.

Quick reference

Gravel and fabric per 100 ft of French drain with 4 in pipe (washed gravel, 1.4 tons per cu yd)
Trench (width x depth)	Gravel per 100 ft	Weight	Fabric width
6 x 12 in	1.53 cu yd	2.14 tons	42 in
8 x 18 in	3.38 cu yd	4.73 tons	56 in
12 x 12 in	3.38 cu yd	4.73 tons	48 in
12 x 18 in	5.23 cu yd	7.33 tons	60 in
12 x 24 in	7.08 cu yd	9.92 tons	72 in
18 x 24 in	10.79 cu yd	15.10 tons	78 in
24 x 24 in	14.49 cu yd	20.29 tons	84 in

How to calculate French drain materials

Start with the trench: width times depth times length, all in feet, gives the volume you dig out and nearly the volume of gravel you put back. The pipe displaces a little, about 0.087 cubic feet per foot for 4 inch pipe, and the calculator subtracts it. Divide by 27 for cubic yards and multiply by 1.4 for tons, which is how gravel yards price a delivery.

Fabric has to line the bottom and both walls of the trench and fold over the top of the gravel with a foot of overlap. That makes the width you need the trench width plus twice the depth plus 12 inches. For a 12 by 18 inch trench that is 60 inches, wider than a 4 foot roll. Either run two strips with an overlap or buy a 6 foot roll. Buy 4 oz non-woven geotextile, not woven weed barrier. Call 811 before digging; the free locate takes 2 to 3 business days and marks only utility lines, not your own sprinkler, gas grill or septic lines.

Slope, pipe and gravel

The drain only works if water can run downhill inside the pipe to somewhere lower. That means daylight on a slope with a rodent grate, a dry well sized for the storm, or a storm drain the city allows. Never the sanitary sewer, and never onto a neighbor's lot. Aim for a 1 percent grade, one inch of fall every 8 feet, and check it with a string line and line level as you dig. Set the pipe with its perforations facing down on 2 to 3 inches of gravel, so water enters from below once the gravel saturates. Put a capped riser at the head of the run and at every bend so the pipe can be jetted later.

Use rigid PVC where the trench crosses a driveway or where you may need to snake the pipe. Corrugated pipe is cheaper and bends around obstacles but sags and holds silt. In clay or silt, which is most yards, buy bare pipe and wrap the whole trench in fabric; a sock on the pipe blinds with fines in a few years. Downspouts go to the same outlet in separate solid pipe, not into the perforated run. Fill with washed gravel to within 3 or 4 inches of grade, fold the fabric over, then top with soil and sod. A foundation drain is a different job: IRC R405.1 puts the pipe at the footing under gravel and filter fabric, draining to daylight or a sump.

What this calculator leaves out

The result is gravel, pipe and fabric for the trench. Not included: the solid outlet run, grate or pop-up emitter, cleanout risers and caps, couplings, catch basins, sod repair, or the trencher rental. Spoil goes somewhere; 4 cubic yards of clay does not fit in a pickup, and dumpsters take dirt only at a dirt-only rate with a weight cap. Some cities require a permit and forbid discharge to the street.

Mistakes that cost money

No outlet: a French drain with nowhere to go is a bathtub. Find the daylight point and the fall before buying gravel.
Woven weed fabric instead of 4 oz non-woven: it blinds with silt in a year and the trench stops draining.
Sock pipe in clay: the sock clogs with fines. In clay, wrap the trench in non-woven fabric and use unsocked pipe.
Downspouts tied into the perforated pipe: roof water floods the trench. Run them in solid pipe to the same outlet.
Digging without 811: the locate is free and takes 2 to 3 business days, and a cut gas or fiber line is billed to you.
No cleanout at the head of the run: when it silts up you cannot jet it. Add a capped riser there and at each bend.

Key facts

A French drain is a gravel-filled trench with a perforated pipe at the bottom, sloped at least 1% (1 in per 8 ft) toward an outlet.
Yard drains are usually 6 to 12 in wide and 18 to 24 in deep; 4 in pipe carries far more water than a residential lawn ever sends it.
Washed 3/4 in to 1.5 in gravel is used because fines from pea gravel or unwashed rock clog the pipe over time.
Gravel weighs about 1.4 tons per cubic yard, so a pickup truck carries roughly half a yard.
Non-woven geotextile fabric lines the trench and wraps over the gravel to keep silt out; woven fabric is the wrong kind because it does not pass water fast enough.
Sock-wrapped pipe belongs in sandy soil only; in clay or silt the sock blinds with fines, so pros wrap the whole trench in non-woven fabric and use bare pipe.

Frequently asked questions

Where does the water from a French drain go?

To somewhere lower than the pipe: daylight on a slope with a grated outlet, a dry well sized for your storms, or a city-approved storm drain. It cannot go to the sanitary sewer or onto a neighbor's lot. If the yard has no low point, a dry well or a sump with a pump is the answer, and that is the first thing to settle before you dig.

What slope does a French drain need?

At least 1%, which is 1 in of drop for every 8 ft of run. A 50 ft drain needs about 6 in of fall from inlet to outlet.

How deep and wide should a French drain be?

Yard drains are usually 6 to 12 in wide and 18 to 24 in deep. The trench needs to reach below the wet soil and still have room for 2 to 3 in of gravel under the pipe.

Do I need fabric in a French drain?

Yes in silty or clay soil, which is most yards. Non-woven landscape fabric keeps fines out of the gravel so the drain keeps flowing for decades instead of clogging in a few years.

Sources and references

French Drain Installation guide (trench depth and width, gravel, 4 in perforated pipe, 1 to 2 percent slope, fabric) (NDS)
Single Wall corrugated HDPE drainage pipe (perforated, 3 to 15 in) (Advanced Drainage Systems)
Soil Drainage (French drain trench, gravel and 1 to 3 percent slope to an outlet) (Colorado State University Extension)
Chapter 4 Foundations, 2021 IRC (R405 foundation drainage) (ICC)

Next steps for this project

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.