French drain gravel
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A French drain moves water because the voids stay open. The pipe is the collector. The stone is the drain. This page is the size, the wrap, and how the bed and the cover share one trench. The cubic yards, the tons, and the bag line stay on the calculator, which already subtracts the pipe from the trench box.
Stone size
Maryland’s stormwater materials manual prints an AASHTO M43 size table and says the size names also follow ASTM C33. On the No. 57 row the nominal size is 1 inch to No. 4. The percentages finer than each sieve are 100 on the 1-1/2 inch, 95 to 100 on the 1 inch, 25 to 60 on the 1/2 inch, 0 to 10 on the No. 4, and 0 to 5 on the No. 8. The same manual calls #57 a combination of No. 5 and No. 7, and its prose says the particles are typically between 3/8 inch and 1 inch. That is the open stone this site’s calculator means by clean #57 or washed 3/4, at a planning density of 1,600 kg/m³, which the calculator page states as 1.35 short tons per cubic yard.
Crusher run, 304, Class 5, and quarry dust are bases. The fines clog a drain. Pea gravel drains, and the round pebbles are the weak choice beside a foundation, which is the comparison on pea gravel versus crushed stone. The sizes chart has the sieve row. Count known yards on the #57 calculator. The trench minus the pipe is the French drain tool.
| Product | What it is | In this drain |
|---|---|---|
| #57 or washed 3/4 | Open stone, 1 inch to No. 4, almost no dust. 1,600 kg/m³ on the calculator. | The envelope around the pipe. |
| AASHTO No. 1, 2 to 4 inch | The clean stone Penn State’s 2019 road-underdrain bulletin names. | A road underdrain, not the 12 inch yard trench. |
| Crusher run, 304, Class 5, quarry dust | Dense base with fines. | Leave it for a driveway or a shed pad. |
| Pea gravel | Rounded, one size, drains. | Paths and beds. A weak choice beside a foundation. |
Fabric wrap
Washington State University’s Clark County sheet, adapted in September 2005, describes a shallow trench lined with weed cloth or geotextile, a perforated plastic pipe surrounded by gravel, and the cloth wrapped over the top of the gravel before soil goes back. The cloth is there so soil does not fill the voids. The sheet’s figure labels a 4 inch perforated pipe. Penn State’s Center for Dirt and Gravel Road Studies, in the January 2019 constructed-stone underdrain bulletin, is the same idea for a road ditch: place non-woven geotextile in the trench with enough left for a double layer on top, fill with clean stone, and wrap the fabric so the stone is fully encased.
This site’s calculator page uses that order and names the fabric: nonwoven, folded over the top. A coated weed barrier or a plastic sheet stops water. A tight sock on the pipe, then dirty fill, is the detail that page tells you to skip when the design calls for a stone envelope. The sock can filter the perforations. It does not replace the stone. Use a nonwoven fabric that passes water. Not every product sold as weed cloth does.
Bedding and cover
Bedding here means the clean stone under the perforated pipe. Cover means the same stone over it. The calculator page says a few inches under the pipe, then the same stone to cover the pipe, then the fabric, then soil. None of the three sources opened for this page prints a single residential pair of inches, such as 3 under and 6 over, that I could read in the text. A project detail that names those inches is that detail. It is not a number this page will invent.
The worked trench on the calculator makes the share visible. The trench is 18 inches deep. The 4 inch pipe is taken as 4.5 inches outside diameter, which is the schedule-40 outside size that page uses. Eighteen minus 4.5 is 13.5 inches of stone, and that 13.5 inches is the bed plus the cover together. The page does not assign how many of those inches sit under the invert. If your detail says the pipe sits on 3 inches, the cover in that same 18 inch trench is the rest. Change the trench depth on the calculator when the detail changes. Do not keep 18 inches and pretend the cover grew.
Penn State’s 12 inches is a different measurement. The bulletin says to place at least 12 inches of fill over the wrapped underdrain so the finished road ditch can be shaped. That fill can be the soil that came out of the trench. It is not 12 inches of #57 inside the envelope, and it is not the bedding under the pipe. The labels on that sheet say “to bed pipe” and “to surround pipe” without a separate inch figure in the text.
A solid utility pipe is a different section. The pipe bedding calculator measures the bedding box and does not subtract the pipe. Its own example is a 50 foot trench, 2 feet wide, with 6 inches of bedding: 1.85 cubic yards and 2.50 short tons at 1,600 kg/m³. Use that page when the pipe is a solid line that needs a shaped bed. Use the French drain page when the stone around a perforated pipe is the drain.
The 50 foot example, and the slope
On the French drain calculator, 50 feet by 12 inches by 18 inches is a trench box of 2.78 cubic yards. The 4 inch pipe, 4.5 inches outside, along that 50 feet is 0.20 cubic yards. Gravel is 2.78 minus 0.20, which is 2.57 cubic yards, 1.97 cubic meters, 3.47 short tons, and 3.15 tonnes at 1,600 kg/m³. The formula does not add a caved-wall allowance. The page says to add about 10 percent yourself if the walls cave as you dig.
The same page says a residential trench is often 12 inches wide and 18 to 24 inches deep, sloped about 1 inch in 8 feet toward daylight or a dry well. One inch in 8 feet is 1 inch in 96 inches, about 1.04 percent. The WSU sheet states the band as 0.5 to 1 percent, and its example of 1 percent is 1 foot of fall in 100 feet. The calculator’s 1 inch in 8 feet sits at the top of that band. The WSU sheet also says these drains work best when they are not sitting in the groundwater table, and that heavy machinery should stay off the line so the pipe is not crushed.
Soil that goes back over the fabric is the backfill calculator. The cut, the swell, and the haul of spoil are the trench calculator. Wider and deeper moves more water and uses more stone. Type the trench you will dig. This guide does not keep a second copy of the widget.
Sources
- Maryland Department of the Environment, Guide to Common Construction Materials. The AASHTO M43 / ASTM C33 size table, including the No. 57 row, and the note that #57 is used around underdrains.
- WSU Clark County Extension, Improving Drainage (adapted September 2005). Fabric, a 4 inch perforated pipe, gravel around the pipe, and a slope of 0.5 to 1 percent.
- Penn State Center for Dirt and Gravel Road Studies, Constructed Stone Underdrains (January 2019). Non-woven wrap, 2 to 4 inch clean stone called AASHTO No. 1, and at least 12 inches of fill over a road-ditch underdrain.
Frequently asked questions
What size stone goes in a French drain?
This site’s French drain calculator uses clean #57 or washed 3/4 inch stone, at 1,600 kg/m³. Maryland’s stormwater materials manual prints the AASHTO M43 No. 57 row as a 1 inch to No. 4 stone: 100 percent passing 1-1/2 inch, 95 to 100 percent passing 1 inch, 25 to 60 percent passing 1/2 inch, 0 to 10 percent passing the No. 4, and 0 to 5 percent passing the No. 8. Crusher run, 304, Class 5, and quarry dust stay out of the envelope because the fines seal it.
How deep is the bedding under the pipe?
The calculator page sets the pipe on a few inches of the same clean stone, then covers the pipe with that stone. No source opened here prints a single split such as 3 inches under and 6 inches over. In the 50 foot example the trench is 18 inches deep and the 4 inch pipe is 4.5 inches outside, so 13.5 inches of stone is the bed and the cover together. A solid utility pipe is the pipe bedding calculator, which does not subtract the pipe.
Does the fabric wrap the pipe or the stone?
Wrap the stone. The WSU sheet lines the trench, surrounds the pipe with gravel, and folds the cloth over the gravel before the soil goes back. Penn State’s 2019 bulletin fully encases the stone in non-woven geotextile. A tight sock on the pipe plus dirty fill is the detail the calculator page tells you to skip when the design calls for a stone envelope. A plastic sheet holds the water.
How much slope does the trench need?
The WSU sheet says 0.5 to 1 percent. One percent is 1 foot of fall in 100 feet. This site’s calculator page uses about 1 inch in 8 feet, which is 1/96, or about 1.04 percent, at the top of that band. The same page’s residential trench is often 12 inches wide and 18 to 24 inches deep. Both figures are planning notes. A flat trench holds water in the pipe.
How much gravel is the 50 foot example?
Enter 50 feet, 12 inches wide, 18 inches deep, and a 4 inch pipe. The order line reads 2.57 cubic yards, 1.97 cubic meters, 3.47 short tons, and 3.15 tonnes at 1,600 kg/m³. The trench box before the pipe is 2.78 cubic yards and the pipe is 0.20 cubic yards. Add about 10 percent yourself if the walls cave. This guide does not repeat that widget.
Where do I count the soil that goes back?
Soil over the folded fabric is the backfill calculator, at that page’s 1,400 kg/m³. The cut, the swell, and the spoil that will not fit back are the trench calculator. Neither of those pages subtracts a perforated pipe from a stone envelope. The stone is only the French drain calculator.