How to use the french drain gravel and pipe calculator
- Enter the length of the drain in feet, and the trench width and depth in inches.
- Enter the soil cap, the depth at the top of the trench that is filled with soil instead of gravel, and the pipe outside diameter.
- Check the fabric overlap and the fall per 10 feet, and open the advanced options to change the gravel allowance, weight and pipe length.
- Read the gravel to order in cubic yards and tons, the pipe, the filter fabric and the total fall.
Formula
Gravel depth = trench depth - soil cap
Trench volume = length x width x gravel depth
Pipe volume = pi x (pipe diameter / 2)^2 x length
Gravel = (trench volume - pipe volume) x (1 + allowance)
Fabric width = trench width + 2 x trench depth + overlap
Fall = length / 10 x fall per 10 feet
Volumes are in cubic feet (inches divided by 12), and 27 cubic feet make a cubic yard. The pipe is treated as solid, because water and gravel fines will not fill the bore of a perforated pipe in practice.
A 50 foot drain
A 50 ft trench 12 inches wide and 18 inches deep, with a 4 inch soil cap, has 14 inches of gravel. The volume to fill is 50 x 1 x 14/12 = 58.3 cubic feet. A 4.5 inch pipe takes up 5.5 cubic feet, leaving 52.8 cubic feet of gravel.
With a 10% allowance that is 58.1 cubic feet, or 2.15 cubic yards, which is about 3.01 tons at 1.4 tons per cubic yard. The fabric is 12 + 2 x 18 + 12 = 60 inches wide, so 250 square feet is needed. The pipe, with 5% extra, is 52.5 ft or six 10 ft lengths, and a fall of 1 inch per 10 feet drops the far end by 5 inches.
What the numbers mean
A French drain is a gravel-filled trench with a perforated pipe that carries water away from a wall or a wet patch. The trench is lined with filter fabric so soil does not wash into the gravel and clog it. The gravel surrounds the pipe on all sides, and a cap of soil or sod finishes the top.
The gravel amount is the volume of the filled part of the trench minus the space taken by the pipe, plus an allowance for settling and spillage. Stone is sold by weight or by the cubic yard, so both are shown. The weight of gravel varies with the stone and with moisture, so the 1.4 tons per cubic yard default is an example to replace with your supplier's figure.
Planning the slope and the rest
Water moves along the pipe only if it falls. The fall field lets you check how much lower the far end must be. Pipe makers and local codes give minimum slopes, so check those rather than relying on an example. For related materials see the mulch and gravel calculator, the raised bed soil calculator for the soil cap, and the ramp slope calculator for converting a fall into a percent grade.
The page covers geometry only. Where the water can be discharged, how much water the drain must carry and whether permits are needed depend on your site and local rules.
Assumptions and limits
- The trench has straight vertical sides and a flat bottom, with the pipe lying on the bottom surrounded by gravel.
- The gravel fills the trench up to the soil cap. A cap of 0 fills it to the top.
- The gravel allowance, weight per cubic yard, extra pipe and fall are example values you can change.
- The pipe diameter is the outside diameter. Check the label because nominal sizes are smaller.
Frequently asked questions
Why is the fabric wider than the trench?
The fabric lines the bottom and both walls and then folds back over the top of the gravel. Its width is the trench width plus twice the depth, plus the overlap on top, which keeps soil out of the stone.
How much extra gravel should I order?
A small allowance of about 10% covers settling and spillage, and the field lets you change it. Rough or uneven trench walls call for more, so check the trench size after digging.
What slope does a drain pipe need?
Many guides suggest a gentle slope, such as 1 inch per 10 feet or more, but the right figure depends on the pipe and local rules. The page shows the fall for any value you enter, so you can compare.
Does the pipe size affect the amount of gravel much?
A little. A larger pipe displaces more gravel, so the net amount goes down slightly. The effect is small compared with the trench width and depth, which drive the volume.