How much gravel for a French drain?
Enter the trench and the pipe, get gravel, pipe and fabric instantly.
Trench size and pipe are examples. WSU Extension suggests a 0.5–1% slope; ask your supplier for the stone weight.
How much gravel do I need for a French drain?
Multiply the trench length by its width and the depth of the gravel layer, then take out the space the pipe fills: π ÷ 4 × diameter² × length. Example: a 50 ft trench 12 inches wide and 18 inches deep holds 75 ft³; a 4-inch pipe takes 4.4 ft³, leaving 70.6 ft³, or 2.62 cubic yards of gravel — about 3.53 tons at 1.35 tons per yard.
How much slope does a French drain need?
Washington State University Extension recommends sloping the trench between 0.5% and 1% — a 1-foot drop over 100 feet is 1%. The calculator shows the drop for your length: a 50 ft run at 1% falls 6 inches. Enter your own slope if your site or local guidance calls for a different one.
How much filter fabric do I need?
In the WSU Extension design the fabric lines the trench and wraps over the top of the gravel, so the strip must be as wide as both sides, the bottom and the top of the gravel layer plus the overlap: 2 × depth + 2 × width + overlap. For the 12 × 18-inch trench with a 12-inch overlap that is 72 inches — a 6-foot-wide strip, 300 ft² for 50 feet.
How much does drain gravel weigh?
It depends on the stone, the quarry and the moisture. The calculator starts at 1.35 tons per cubic yard — the Minnesota DOT figure for one-size stone such as #57, and the top of Caltrans' 2,550 to 2,700 lb for loose crushed stone. Your supplier's number is better — enter it in the stone weight field.
Does the pipe size matter for the gravel?
A little: the pipe takes the place of some stone, so a bigger pipe means less gravel. Use the outside diameter of the pipe you will buy. The calculator also shows how much soil comes out of the trench, which is the full length × width × depth.
Worked example: an 80-foot drain with soil on top
Take a trench 80 feet long, 12 inches wide and 24 inches deep, with 6 inches of soil planned over the fabric, so the stone layer is 24 − 6 = 18 inches deep. The trench volume is 80 × 1 × 2 = 160 ft³ (5.93 yd³ of soil to dig out). The stone layer is 80 × 1 × 1.5 = 120 ft³, and a 4 inch pipe takes π ÷ 4 × (4 ÷ 12)² × 80 = 7.0 ft³ of it, leaving 113.0 ft³, or 4.19 cubic yards, which weighs about 5.7 tons at 1.35 tons per yard. The fabric strip is 2 × 18 + 2 × 12 + 12 = 72 inches wide, 480 square feet in all. At a 1% slope the pipe falls 9.6 inches over the run, and at 0.5% it falls 4.8. Without the soil on top the same trench would hold 5.67 cubic yards of stone.
What does each layer of a French drain do?
In the design the WSU Extension describes, a shallow trench is lined with geotextile fabric, holds a perforated plastic pipe surrounded by gravel, and the fabric is folded over the top of the gravel and covered with soil. The gravel gives water an easy path to the pipe, the pipe carries it along the slope, and the fabric keeps soil out of the spaces between the stones so the flow is not blocked. Follow the pipe maker's instructions for which way the perforations face.
Common French drain mistakes
Leaving out the soil layer when counting depth: stone should stop below the surface, and the calculator takes the soil depth off the stone layer. Using the nominal pipe size when the outside diameter is larger, which makes the gravel figure slightly too high; the effect is small. Ordering fabric by the trench length alone, when the strip must also wrap the sides, the bottom and the overlap. Setting the slope on paper but not on the trench bottom: a drop of only 0.6 inch per 10 feet at 0.5% is easy to lose with a shovel, so check the grade with a string line or level. Using the stone weight from a different material: crushed stone, washed gravel and pea gravel weigh differently, so use your supplier's figure.
Sources: Washington State University Extension, Clark County Small Acreage Program, “Improving Drainage”; stone weight — Minnesota DOT, Grading and Base Manual, 5-692.430, and Caltrans Maintenance Manual, Vol. 1, Appendix. The construction layers and the 0.5 to 1 percent slope come from the same WSU page. Drainage near a foundation or on a slope may need an engineer or a local permit. Related: gravel calculator for a patio or path, trench volume calculator, pipe slope calculator and retaining wall calculator. How we check our calculators.