French Drain Calculator

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

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, so the width you need is 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, so either run two strips with an overlap or buy a 6 foot roll. Call 811 before digging so buried utilities are marked.

Slope, pipe and gravel

The drain only works if water can run downhill inside the pipe to a daylight outlet, dry well or storm drain. 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.

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. Fill with washed gravel to within 3 or 4 inches of grade, fold the fabric over, then top with soil and sod or leave the gravel exposed as a decorative channel. This calculator is for planning; a foundation or footing drain should be designed by a contractor or engineer.

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.

Frequently asked questions

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)