French Drain Calculator

Gravel, pipe, fabric and fall for a French drain trench.

Project Details
Enter your measurements and requirements
$
Results
Calculated estimates for your project
Drain rock2.88 cu yd (4.03 tons)
Perforated pipe, 10 ft5
Filter fabric6.0 ft wide x 50 ft
Total fall6.3 in end to end
Soil to remove2.8 cu yd
Rock cost$181

Turn these numbers into a build plan.

  • 4.0 tons rock
  • 5 sticks pipe
  • $326 est. cost
Materials List
Recommended materials and quantities
Drain rock4.0 tons
Perforated pipe, 10 ft5

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Project plan · built from your numbers

French drain, 50 ft

50 ft trench, 12 x 18 in with 4.0 tons of rock and 5 pipe sticks.

Materials
$326
Time
1 weekend
Level
Intermediate
5 steps 4 tools 3 materials
  1. 1

    Call 811 and plan the outlet

    Mark utilities and confirm where water will discharge.

    • Utilities marked
    • Outlet chosen
  2. 2

    Dig the trench

    Dig 12 x 18 in with 6.3 in of total fall.

    • Fall checked
  3. 3

    Line with fabric

    Line the trench and leave enough to fold over the top.

    • Fabric placed
01 · Your numbers
Rock
4.0 tons
Pipe
5 sticks
Est. cost
$326
02 · The plan

5 steps · 4 tools · 3 materials

2 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

Your plan is 60% built. Save it.

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French drain, 50 ft

Finished in 1 weekend · $326 DIY

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Borrow these 4 tools from neighbors

How to calculate gravel for a French drain

A drain is a sloped trench, perforated pipe and clean rock, all wrapped in fabric.

  1. Size the trench. 12 in wide by 18 in deep suits most yards.
  2. Find the rock. Trench area less the pipe area times the length gives cubic feet.
  3. Convert to tons. Divide by 27 for yards and multiply by about 1.4.
  4. Size the fabric. Wrap the full perimeter and add a foot of lap.
  5. Plan the fall. At least 1/8 in per foot to a daylight outlet or dry well.

Rock and pipe by length

LengthTons of rock10 ft pipesFall at 1/8 in
25 ft2.033.1 in
50 ft4.056.3 in
75 ft6.089.4 in
100 ft8.11012.5 in

See the basement waterproofing guide and the sump pump calculator if the water has nowhere to go by gravity.

Frequently asked questions

How deep should a French drain be?

Usually 18 to 24 in, or deep enough to intercept the water. Foundation drains sit near the footing and are a larger job.

What slope does it need?

At least 1/8 in per foot, and 1/4 in per foot if the site allows.

What gravel do I use?

Clean washed 3/4 in stone. Fines clog the drain and the pipe.

Do I need a permit?

Often not for a yard drain, but sending water to a neighbor or the street can be restricted. Ask your local authority and call 811 before digging.

Solid or perforated pipe?

Perforated within the drain, solid pipe once it leaves the trench for the outlet.

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