How this planner works
- Slope. Fall in inches = length in feet × 12 × slope percent ÷ 100. Depth increases by that much on level ground. Use a level or survey to check actual grade and outlet. The trapezoidal volume uses the average depth.
- Gravel. Trench width × length × (average depth minus any soil cap), minus the space the pipe takes based on its outside diameter, plus 5%. About 1.4 tons per cubic yard for washed drain stone.
- Fabric. Enough to line the bottom and both sides, fold over the top and overlap by 12": twice the width plus twice the gravel depth at the deepest point, plus 1 ft. The cost uses the next whole-foot roll width as an estimate. Check available rolls and allow extra for seams if using more than one piece.
- Pipe. The run plus 2 ft, in 10 ft pieces, with a coupler at each joint.
- Spoil. The soil you dig out takes about 25% more room than it did in the ground. Plan where it will go.
Reference guidance:NDS exterior drainage principles. Confirm material coverage and specifications with your supplier.
Worked example: a 50 ft drain
- 8" wide, 18" deep at the start, 1% slope: the trench drops 6" and is 24" deep at the outlet.
- With 3" of soil on top: 2.48 tons of drain stone (1.77 cu yd, or 96 half-cubic-foot bags).
- Fabric at least 5.84 ft wide and 50 ft long (a 6 ft roll if available); 6 ten-foot pieces of pipe and 5 couplers.
- About 2.7 cubic yards of soil to dig out and move.
Frequently asked questions
How much slope does a French drain need?
A common minimum is a 1% fall, about 1/8 inch per foot or 1 inch every 8 feet. More slope moves water faster. Over 50 feet, a 1% slope means the trench is 6 inches deeper at the outlet end than at the start.
How deep and wide should a French drain be?
Many yard drains are about 6 to 12 inches wide and 18 to 24 inches deep at the shallow end, getting deeper toward the outlet. Drains that protect a foundation are often deeper, down near the footing. The right size depends on how much water you need to move and where it can go.
What gravel goes in a French drain?
Clean, washed angular stone about ¾ to 1½ inches, such as #57 stone, is common. It has no fine dust to clog the pipe or fabric. Avoid crusher run or road base here: the fines that make them compact well also block water.
Do I need fabric around a French drain?
Lining the trench with a non-woven drainage fabric and folding it over the top of the gravel, sometimes called a burrito wrap, keeps soil from washing into the stone. Without it, many drains slowly silt up. A pipe with a sock adds another layer of protection.
Where should a French drain empty?
Somewhere lower than the start of the drain, away from your house and not onto a neighbor's property: a daylight outlet on a slope, a pop-up emitter in the lawn, a dry well or a storm drain where allowed. Check local rules before connecting to a street or storm system.