French Drain Calculator
Find how much gravel, drain pipe, and filter fabric you need for a French drain. Enter the length of each run of trench, its width and depth, and the pipe size. The calculator takes the pipe out of the gravel volume, and gives the gravel in cubic yards and tons or cubic meters and tonnes, the bags, the pipes to buy, and the fabric.
- Choose US or Metric. You can still change the unit of any single field.
- Enter the length of each straight run of the trench, measured along its center, and use Add run for the next one. Include the run to the outlet if it gets the same pipe and gravel.
- Enter the Trench width and the Trench depth. A French drain gets deeper toward the outlet, so enter the average depth. If parts of the drain are dug to different widths, work out each width separately.
- If you'll stop the gravel short of the top and cover it with soil or sod, enter the Gravel depth, measured from the bottom of the trench. Leave it blank to fill the trench with gravel.
- Choose the Drain pipe. For corrugated pipe, or any pipe not listed, choose Custom size and enter its outside diameter. Choose No pipe for a trench filled with gravel only. Change the Pipe length to the length of the pipes or coils you'll buy, such as a 100 ft or 50 m coil.
- Under Fabric, keep Wrap the gravel to get the width and length of filter fabric, or choose No fabric, such as when the pipe comes with a filter sock.
- Choose the Gravel type. If your supplier gives a density, enter it in Density. If your bag isn't listed, enter its volume or weight in Custom bag size.
- Optionally, enter prices for the gravel, pipe, and fabric to estimate the cost. For fabric sold in rolls, divide the roll price by the roll length to get a price per ft or per m.
Length fields accept whole numbers, decimals, fractions, and mixed
numbers, such as 12, 10.5, 3/4, or
4 1/2. Choose ft + in as the unit of a run
to enter a length such as 32 ft 6 in in two fields.
How to calculate gravel for a French drain
The calculator treats the trench as a rectangle in cross section, the same along its whole length. The gravel fills the trench up to the gravel depth, and the pipe takes up a cylinder of its outside diameter:
Pipe volume = π × outside diameter² ÷ 4 × trench length
Gravel = (trench length × width × gravel depth − pipe volume) × (1 + waste % ÷ 100)
Weight = gravel × density
The trench length is the total of all runs. It doesn't matter how high the pipe sits in the gravel, so you don't enter the gravel under the pipe. Measure each run along the center of the trench: at a corner, the center lines of the two runs each count half of the square where they meet, so the corner is counted once. Measuring along the outside edge counts the corners twice and gives a little more gravel.
Pipes
Pipes to buy = trench length ÷ pipe length, rounded up
The calculator divides the total length, so the piece left over from one run is used on the next. Cutting the pipe for each run separately can take one or two more pipes.
Filter fabric
Fabric width = 2 × trench width + 2 × gravel depth + overlap
The fabric lines the bottom and both sides of the trench, then folds over the top of the gravel, with the two edges overlapping on top. Its length is the trench length. If you stop the gravel below the top, the fabric wraps the gravel only, not the soil above it.
Trench size, slope, pipe, and fabric
Trench size and slope
The calculator has no default trench size. As a general guide, a Bob Vila guide suggests a trench at least 12 in wide and 18 to 24 in deep. UK drainage supplier JD Pipes suggests a trench wider than the pipe and at least 450 mm deep. The size you need depends on how much water the drain collects and on your soil, which the calculator doesn't work out.
The trench slopes toward the outlet. The same Bob Vila article gives about 1 in of fall for every 8 ft, with some contractors using 1 to 2%, and drainage maker NDS calls for at least 1% away from the house. The slope doesn't change the amount of gravel, because the average depth gives the same volume. To find how much deeper the trench gets at the outlet, multiply the run length by the slope: 100 ft at 1% is 12 in deeper.
Drain pipe
The calculator subtracts these outside diameters. The US sizes are PVC sewer and drain pipe to ASTM D2729 from Westlake Pipe & Fittings, and the Metric sizes are perforated land drain coils from UK supplier JD Pipes.
| Pipe | Outside diameter |
|---|---|
| 3 in | 3.25 in |
| 4 in | 4.215 in |
| 6 in | 6.275 in |
| 80 mm | 80 mm |
| 100 mm | 100 mm |
| 160 mm | 160 mm |
US pipes are named by their nominal size, so the calculator subtracts the outside diameter of PVC drain pipe. Corrugated pipe is larger on the outside: ADS lists an average outside diameter of about 3.6 in for 3 in, 4.6 in for 4 in, and 7.0 in for 6 in single wall pipe. For corrugated pipe, choose Custom size and enter its outside diameter. In a 12 × 18 in trench, a 4 in corrugated pipe needs about 1.3% less gravel than the PVC size, well within the waste allowance, so the PVC size doesn't leave you short with either pipe.
The pipe length starts at 10 ft, the length of straight PVC drain pipe in the US, and at 25 m, the shortest land drain coil. Corrugated pipe is also sold in 10 and 20 ft lengths and in coils of 100 ft or more, and 100 mm land drain comes in 25, 50, and 100 m coils. Enter the length you buy.
Gravel
The calculator starts with crushed stone, such as #57 or 20 mm stone. Bob Vila's article lists #57 (3/4 in) gravel, and the City of Brampton's landscape specification for drainage pipe calls for 19 mm crushed stone around the pipe. If you use rounded gravel, such as the 10 mm pea shingle in the DRAINTEX fabric installation guide, choose Pea gravel or river rock.
The weight uses the loose density of the gravel, 96 lb/ft³ (1.538 t/m³) for crushed stone, the same as the Gravel Calculator. The 10% waste allowance covers the gravel settling and a trench that's dug a little wider or deeper than planned. It applies to the gravel only: the pipe and fabric follow the trench length.
Filter fabric
The DRAINTEX installation guide lines the bottom and sides of the trench and leaves enough fabric at the top to fold over the gravel. Alaska's Department of Transportation specification for subsurface drainage overlaps the fabric at least 12 in on top in trenches wider than 12 in, and the City of Brampton's specification closes it over the gravel with at least 300 mm of overlap. The overlap starts at 12 in or 300 mm.
Filter fabric comes in rolls of different widths, such as 1, 2, and 4.5 m for DRAINTEX, or 4 and 6 ft for DeWitt filter fabric. Choose a roll at least as wide as the fabric width, or lay narrower strips side by side, overlapping each one by the overlap. Where one roll ends and the next begins, overlap the ends, at least 300 mm in the DRAINTEX guide.
A common finish stops the gravel a few inches below the ground and covers it with soil or sod. Enter that as the Gravel depth.
Examples
A 100 ft drain
A trench 100 ft long, 12 in wide, and 18 in deep is 150 ft³. A 4 in PVC pipe takes up 9.69 ft³, leaving 140.31 ft³, and 10% waste makes 154.34 ft³, or 5.72 yd³. Of crushed stone, that's 7.41 US tons (14,817 lb), or 309 bags of 0.5 cu ft. You need 10 pipes of 10 ft and 100 ft of fabric at least 6 ft wide. At $45 per US ton, $12.50 per pipe, and $0.60 per ft of fabric, the materials come to $518.38.
If you stop the gravel at 12 in and cover it with 6 in of soil, the gravel drops to 3.68 yd³ (4.77 US tons), and the fabric only needs to be 5 ft wide.
A Metric drain
A trench 20 m long, 300 mm wide, and 450 mm deep is 2.70 m³. A 100 mm pipe takes up 0.16 m³, and with 10% waste the gravel is 2.80 m³, or 4.30 tonnes of crushed stone: 5 bulk bags of 1 tonne, 6 of 800 kg, or 173 bags of 25 kg. One 25 m coil of pipe covers the drain, and the fabric is 20 m long and at least 1.80 m wide. At £60 per tonne, £45 per coil, and £1.20 per m of fabric, the materials come to £327.09.
FAQ
About 5.7 yd³, or 7.4 US tons of crushed stone, for 100 ft of trench 12 in wide and 18 in deep with a 4 in pipe and 10% waste. Enter your trench in the calculator for your own length, size, and pipe.
About 1.54 ft³, or about 148 lb of crushed stone, per foot of a trench 12 in wide and 18 in deep with a 4 in pipe, including 10% waste. A 300 × 450 mm trench with a 100 mm pipe takes about 0.14 m³ per meter.
As a general guide, at least 12 in wide and 18 to 24 in deep: a Bob Vila guide suggests that size, and a UK drainage supplier suggests at least 450 mm deep. The size you need depends on how much water the drain has to carry and on your soil.
Drainage guides commonly call for at least 1%, or about 1 in for every 8 ft, toward the outlet: a drainage product maker calls for at least 1%, a Bob Vila guide suggests about 1 in per 8 ft, and a contractor it quotes uses 1 to 2%. Over a 100 ft run, 1% makes the trench 12 in deeper at the outlet end. Enter the average depth in the calculator.
About $518 for 100 ft in the example above, at $45 per US ton of gravel, $12.50 per 10 ft pipe, and $0.60 per ft of fabric. Enter your local prices to estimate yours. Labor, fittings, and the outlet are extra.
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