How much gravel do I need for a French drain?

Trench volume minus the pipe. Length × width × depth in feet gives cubic feet; take away π × (pipe diameter ÷ 2)² × length, then divide by 27 for cubic yards. A 50 ft trench 12 in wide and 18 in deep is 75 cu ft; the 4 in pipe takes 4.36 cu ft, leaving 2.62 cu yd, about 3.66 tons at 1.4 tons per cu yd. That is the trench volume with no waste, so add a few percent for spillage and settling if you want a margin.

How wide and deep should a French drain trench be?

Guidance from the sources listed below: about 12 to 18 in wide and 18 to 24 in deep for a yard drain, deeper for a foundation drain, with the pipe sitting on a few inches of stone and covered with stone. This is guidance, not a code, and what suits your site depends on the soil, how much water there is and local rules. A 100 ft trench 18 in wide and 24 in deep needs 10.79 cu yd of gravel, 15.10 tons.

What slope does a French drain need?

At least about 1%, which is 1/8 in of drop for each foot of pipe (1 in per 8 ft), toward the outlet, according to the sources below. A 50 ft drain therefore falls about 6.25 in from the high end to the outlet. More fall is fine; flat or backwards pipe holds water. This is guidance, so check any local rules. The calculator shows the drop for your length, but it does not change any quantity.

What size gravel is best for a French drain?

Clean, washed stone, roughly 3/4 to 1 1/2 in, according to the sources below (sources vary, and 3/4 in to 1 in is the most often named). The stone must be washed and free of fines, which settle and clog the drain; pea gravel and crusher run are not recommended. Ask for "washed drainage stone" at the yard, and use the gravel calculator for a different depth or the price.

How do I calculate how much landscape fabric a French drain needs?

The fabric lines the floor and both walls and folds over the top of the stone, so the strip is 2 × depth + 2 × width + overlap wide. For a 12 in wide, 18 in deep trench with a 12 in overlap that is 36 + 24 + 12 = 72 in = 6 ft; times 50 ft is 300 sq ft, 1 roll of 6 × 100 ft (600 sq ft), or 2 rolls of 3 × 50 ft. A 24 in overlap makes it 7 ft wide and 350 sq ft. Use non-woven drainage geotextile rather than woven weed barrier, which clogs. Rolls are by area, an estimate: a narrow roll needs seams, which use more fabric.

How many sections of perforated pipe do I need?

One 10 ft section per 10 ft of trench, rounded up: 5 for 50 ft, 3 for 30 ft. Lay the pipe with the holes facing down, and add couplings, end caps and solid (unperforated) pipe to carry the water to the outlet. Pipe is also sold in coils, so you can buy by length instead.

Is the pipe volume really worth subtracting?

A little. A 4 in pipe takes 4.36 cu ft out of 75 cu ft, about 5.80%, which is 0.16 cu yd of the 2.78 cu yd trench. A 6 in pipe in the same trench: 2.41 cu yd of gravel instead of 2.62. On a narrow trench the share is larger. Corrugated pipe is slightly bigger outside than its nominal size, so the calculator’s figure is a small overestimate.

How much dirt will a French drain trench produce?

The in-place volume is the trench volume: 2.78 cu yd for 50 ft × 12 in × 18 in. Dug soil swells, so use the excavation calculator for the loose volume and the truck loads, and keep some of the soil to cap the top if you want grass over the drain.

Do I need to call before I dig?

Yes. In the US, call 811 before any digging so buried gas, power, water and telecom lines are marked; the call is free and each state sets the waiting time. Trenches 5 ft (1.5 m) or deeper need a protective system under OSHA’s rules, which is why drains this size should stay shallow and deeper work goes to a professional. Also check that you may discharge water where the outlet goes: a storm drain, a dry well or daylight on a slope, per local rules.

French Drain Calculator: How Much Gravel, Pipe and Fabric Do I Need?

A French drain trench 50 ft long, 12 in wide and 18 in deep needs 2.62 cu yd of gravel, about 3.66 tons: the trench is 50 × 1 × 1.5 = 75 cu ft (2.78 cu yd to dig), less 4.36 cu ft for a 4 in pipe. You also need 5 sections of 10 ft perforated pipe and 300 sq ft of fabric (a 6 ft wide strip with a 12 in overlap), which is 1 roll of 6 × 100 ft. Enter the trench length, width and depth, the pipe size and the fabric overlap to get your own materials. Call 811 before you dig.

Figures based on the sources listed belowUpdated 2026-10-07How we calculate

Trench length

Trench width

Trench depth (in)

Guidance: yard drains are usually 18–24 in deep. Call 811 before you dig, and see the safety note below the calculator for deeper trenches.

Pipe diameter (in)

4 in perforated pipe is the usual size for a yard or foundation drain. Its volume is subtracted from the gravel, using the nominal size.

Fabric overlap on top (in)

Extra fabric folded over the top of the stone, added to the strip. 12 in is a common choice; follow the fabric maker’s instructions.

Fabric roll sizeRolls are counted by area, an estimate: a roll narrower than the strip needs seams, which use more fabric.

2.62 cu yd of gravel

Gravel by weight (approximate): 3.66 tons, at about 1.4 tons per cu yd
Trench to dig (excavation): 2.78 cu yd, in place, before the soil swells
Perforated pipe (10 ft sections): 5 sections (trench length 50 ft)
Fabric to wrap the trench: 300 sq ft (strip width 6 ft)
Fabric rolls (6 × 100 ft rolls): 1 roll
Fall to build in, high end to outlet (guidance minimum): 6.25 in, at 1/8 in per ft, about 1%

Shopping list

Perforated drain pipe, 4 in × 10 ft
Non-woven geotextile drainage fabric
Trenching shovel
Line level and mason line
Contractor wheelbarrow
Pipe fittings and end caps, 4 in drain

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Materials per 100 ft of trench by size

Worked by this calculator with a 4 in pipe and a 12 in fabric overlap. Gravel is the trench less the pipe, with no waste; tons at 1.4 per cu yd. Fabric is the area of the strip that wraps the trench.
Trench	Dig (cu yd)	Gravel (cu yd)	Gravel (tons)	Strip width (ft)	Fabric (sq ft)
12 in wide × 18 in deep	5.56	5.23	7.33	6	600
12 in wide × 24 in deep	7.41	7.08	9.92	7	700
18 in wide × 18 in deep	8.33	8.01	11.21	7	700
18 in wide × 24 in deep	11.11	10.79	15.10	8	800

Materials by trench length

A 12 in wide, 18 in deep trench with a 4 in pipe, a 12 in overlap and 6 × 100 ft fabric rolls. Pipe sections are 10 ft long.
Length	Pipe sections	Dig (cu yd)	Gravel (cu yd)	Gravel (tons)	Fabric (sq ft)	Rolls (6 × 100)
20 ft	2	1.11	1.05	1.47	120	1
30 ft	3	1.67	1.57	2.20	180	1
50 ft	5	2.78	2.62	3.66	300	1
75 ft	8	4.17	3.92	5.49	450	1
100 ft	10	5.56	5.23	7.33	600	1
150 ft	15	8.33	7.85	10.99	900	2

Fabric rolls for 100 ft of 12 in × 18 in trench

Counted by area, an estimate. A roll narrower than the strip needs seams, so you will use more than the area says.
Roll	Fabric (sq ft)	Rolls
3 × 50 ft roll (150 sq ft)	600	4
4 × 100 ft roll (400 sq ft)	600	2
6 × 100 ft roll (600 sq ft)	600	1

Guidance for a French drain (not calculated)

Typical figures from the sources listed below, shown as guidance. They are not results of this calculator, and local code and the pipe maker’s instructions come first.
Item	Typical guidance
Slope (fall) toward the outlet	At least 1%, about 1/8 in per foot (1 in per 8 ft)
Trench size for a yard drain	About 12 to 18 in wide and 18 to 24 in deep
Stone	Clean, washed stone about 3/4 to 1 1/2 in, not pea gravel or stone with fines
Pipe	4 in perforated pipe, holes down, in 10 ft sticks or a coil
Fabric	Non-woven drainage geotextile lining the trench and folded over the top of the stone

Methodology & assumptions

Trench volume (the excavation) is length × width × depth, divided by 27 for cubic yards. The pipe sits in the trench, so its volume, π × (diameter ÷ 2)² × length, is subtracted from the gravel; the calculator takes the pipe to run the whole length at its nominal diameter. A 4 in pipe is 4.36 cu ft in 50 ft, out of 75 cu ft. Corrugated pipe is a little bigger outside than its nominal inside diameter, so the gravel is a small overestimate, which is on the safe side. Gravel weight uses 1.4 tons per cubic yard, the planning figure the gravel calculator on this site uses (washed stone varies with the rock, size and moisture, so ask your supplier). There is no waste or settling allowance, so add a few percent if you want a margin, and note that you may stop the stone below grade and cap it with soil or sod. The strip of fabric that wraps the trench is 2 × depth + 2 × width + overlap: it lines the floor and both walls and folds over the top, so a 12 in wide, 18 in deep trench with a 12 in overlap is (36 + 24 + 12) in = 6 ft wide, and the area is that strip × the length (300 sq ft for 50 ft). Rolls are counted by area, an estimate: if your roll is narrower than the strip (a 3 ft roll against a 6 ft strip) you will need seams, with overlaps, and more fabric than the area says, so buy a roll as wide as the strip where you can. Perforated pipe is the trench length in 10 ft sections, rounded up; couplings, end caps and the solid pipe to the outlet are extra. The fall shown is the length × 1/8 in. The minimum slope, the trench size and the stone size are guidance from the sources below, not results of the calculator, and your local code and the pipe maker’s instructions come first. The metric toggle shows lengths in m, the depth, pipe and overlap in cm, and volumes in m³.

How to estimate the materials for a French drain

Call 811 and plan the route. Have buried utilities marked, then lay out the trench so it falls to an outlet you may use: daylight on a slope, a dry well or a storm drain, where local rules allow.
Measure the trench. Enter the trench length and width (in feet, so 12 in is 1 ft) and the depth in inches. Guidance is about 12 to 18 in wide and 18 to 24 in deep for a yard drain.
Enter the pipe and the fabric. Pick the pipe diameter (4 in is typical), the fabric overlap and the roll size you will buy.
Read the gravel, pipe and fabric. The calculator subtracts the pipe from the trench for the gravel in cu yd and tons, counts 10 ft pipe sections, and gives the fabric area and rolls. Order washed stone.
Build in the fall. Dig the floor so it drops at least about 1/8 in per foot toward the outlet, line the trench with fabric, lay the pipe holes down on a few inches of stone, fill with stone and fold the fabric over the top.

Worked examples

Project	Result
50 ft trench, 12 in wide × 18 in deep, 4 in pipe, 12 in overlap, 6 × 100 ft rolls	2.62 cu yd of gravel, 3.66 tons, 5 pipe sections, 300 sq ft of fabric, 1 roll
100 ft trench, 18 in wide × 24 in deep	10.79 cu yd of gravel, 15.10 tons, 10 pipe sections, 800 sq ft of fabric, 2 rolls of 6 × 100 ft
30 ft trench, 12 in wide × 18 in deep	1.57 cu yd of gravel, 2.20 tons, 3 pipe sections, 180 sq ft of fabric
The 50 ft trench with a 6 in pipe	2.41 cu yd of gravel, 3.38 tons, against 2.62 cu yd with a 4 in pipe
The 50 ft trench with 3 × 50 ft fabric rolls	2.62 cu yd of gravel, 300 sq ft of fabric, 2 rolls of 3 × 50 ft
The 50 ft trench with a 24 in fabric overlap	2.62 cu yd of gravel, 350 sq ft of fabric in a 7 ft wide strip, 1 roll

Frequently asked questions

How much gravel do I need for a French drain?: Trench volume minus the pipe. Length × width × depth in feet gives cubic feet; take away π × (pipe diameter ÷ 2)² × length, then divide by 27 for cubic yards. A 50 ft trench 12 in wide and 18 in deep is 75 cu ft; the 4 in pipe takes 4.36 cu ft, leaving 2.62 cu yd, about 3.66 tons at 1.4 tons per cu yd. That is the trench volume with no waste, so add a few percent for spillage and settling if you want a margin.
How wide and deep should a French drain trench be?: Guidance from the sources listed below: about 12 to 18 in wide and 18 to 24 in deep for a yard drain, deeper for a foundation drain, with the pipe sitting on a few inches of stone and covered with stone. This is guidance, not a code, and what suits your site depends on the soil, how much water there is and local rules. A 100 ft trench 18 in wide and 24 in deep needs 10.79 cu yd of gravel, 15.10 tons.
What slope does a French drain need?: At least about 1%, which is 1/8 in of drop for each foot of pipe (1 in per 8 ft), toward the outlet, according to the sources below. A 50 ft drain therefore falls about 6.25 in from the high end to the outlet. More fall is fine; flat or backwards pipe holds water. This is guidance, so check any local rules. The calculator shows the drop for your length, but it does not change any quantity.
What size gravel is best for a French drain?: Clean, washed stone, roughly 3/4 to 1 1/2 in, according to the sources below (sources vary, and 3/4 in to 1 in is the most often named). The stone must be washed and free of fines, which settle and clog the drain; pea gravel and crusher run are not recommended. Ask for "washed drainage stone" at the yard, and use the gravel calculator for a different depth or the price.
How do I calculate how much landscape fabric a French drain needs?: The fabric lines the floor and both walls and folds over the top of the stone, so the strip is 2 × depth + 2 × width + overlap wide. For a 12 in wide, 18 in deep trench with a 12 in overlap that is 36 + 24 + 12 = 72 in = 6 ft; times 50 ft is 300 sq ft, 1 roll of 6 × 100 ft (600 sq ft), or 2 rolls of 3 × 50 ft. A 24 in overlap makes it 7 ft wide and 350 sq ft. Use non-woven drainage geotextile rather than woven weed barrier, which clogs. Rolls are by area, an estimate: a narrow roll needs seams, which use more fabric.
How many sections of perforated pipe do I need?: One 10 ft section per 10 ft of trench, rounded up: 5 for 50 ft, 3 for 30 ft. Lay the pipe with the holes facing down, and add couplings, end caps and solid (unperforated) pipe to carry the water to the outlet. Pipe is also sold in coils, so you can buy by length instead.
Is the pipe volume really worth subtracting?: A little. A 4 in pipe takes 4.36 cu ft out of 75 cu ft, about 5.80%, which is 0.16 cu yd of the 2.78 cu yd trench. A 6 in pipe in the same trench: 2.41 cu yd of gravel instead of 2.62. On a narrow trench the share is larger. Corrugated pipe is slightly bigger outside than its nominal size, so the calculator’s figure is a small overestimate.
How much dirt will a French drain trench produce?: The in-place volume is the trench volume: 2.78 cu yd for 50 ft × 12 in × 18 in. Dug soil swells, so use the excavation calculator for the loose volume and the truck loads, and keep some of the soil to cap the top if you want grass over the drain.
Do I need to call before I dig?: Yes. In the US, call 811 before any digging so buried gas, power, water and telecom lines are marked; the call is free and each state sets the waiting time. Trenches 5 ft (1.5 m) or deeper need a protective system under OSHA’s rules, which is why drains this size should stay shallow and deeper work goes to a professional. Also check that you may discharge water where the outlet goes: a storm drain, a dry well or daylight on a slope, per local rules.

Handy tools

Feet and Inches Calculator: Turn a tape reading in feet and inches into the decimal feet the length and width fields take.
Cubic Yards to Tons: Convert the cubic yards of gravel to tons, or back, for the yard’s ticket.