WeekendCalc

French Drain Calculator

Enter the length of your French drain and the size of the trench. You get the tons of drain rock, the length of perforated pipe, the filter fabric, and the soil you will dig out.

Your project

%

Uneven trench walls and settling.

Results

Drain rock needed
4.0 tons
Clean washed stone, 3/4 to 1½ in
Drain rock, cubic yards
2.88 yd³

Pipe and fabric

Perforated pipe
50 linear ft
10 ft pipe sections
5 pieces
Filter fabric (wraps the stone, with overlap)
250 ft²

Digging

Soil to dig out
2.8 yd³
  • Lay the pipe with the holes down, on a bed of stone, sloping at least 1 percent (about 1/8 in per foot) to a daylight outlet or a dry well. Wrap the stone in non-woven filter fabric to keep soil out. Soil expands when dug: allow about 25% more volume to haul away.

How this calculator works

A French drain is a trench with perforated pipe, surrounded by clean stone and wrapped in filter fabric. The stone volume is the trench volume (length × width × depth) minus the pipe's volume, plus an allowance for settling. The fabric wraps around the trench, so its width is the trench width plus twice the depth plus an overlap.

stone = (length × width × depth − pipe volume) × (1 + waste %) fabric = length × (width + 2 × depth + 12 in)

Worked example: a 50 ft drain in a trench 12 in wide and 18 in deep has 75 ft³ of trench. The 4 in pipe takes up 4.4 ft³, leaving 70.6 ft³, or 77.7 ft³ with 10% extra: 2.9 yd³ or about 4 tons of drain rock. You also need 50 ft of pipe and 250 ft² of fabric, and dig out 2.8 yd³ of soil.

Tips before you buy

  • Slope is critical: a French drain only works if water can flow downhill to an outlet. Aim for at least 1 inch of drop per 8 feet.
  • Use clean, washed, angular stone with no fines, since fines clog the drain. Do not use sand or crusher run.
  • Always wrap the stone in non-woven geotextile fabric, so soil does not wash into the stone and block it.
  • Keep the drain away from foundations and septic fields, and make sure the outlet cannot be blocked or cause flooding for neighbours.
  • Cap the top with a few inches of soil or sod, or with decorative stone.

Tools & materials for this project

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Perforated drain pipeNon-woven geotextile fabricWashed drain rockTrenching shovelPipe fittings and couplers

Frequently asked questions

How much gravel do I need for a French drain?

Multiply length × width × depth, subtract the pipe volume, and add 10%. A 50 ft drain in a 12 × 18 in trench needs about 2.9 cubic yards (4 tons) of drain rock.

How deep should a French drain be?

Typically 12 to 24 inches deep, and deep enough to intercept the water: for a yard it can be shallow, while a foundation drain runs deeper at the footing.

Which way do the holes in the pipe face?

Down, in the standard instruction for rigid pipe, so water enters from below and flows along the bottom. Some manufacturers specify otherwise, so check the pipe.

Related calculators & guides

Gravel CalculatorTons and cubic yards of gravel, crushed stone or road base.Retaining Wall CalculatorBlocks, cap blocks, gravel backfill and pipe for a retaining wall.Rainwater Collection CalculatorGallons you can collect from a roof, and how many rain barrels.Cubic Yard CalculatorCubic yards, cubic feet and weight for any material.

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.