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French Drain Gravel Calculator

Estimate the washed drainage stone for a french drain trench in cubic yards and tons. Subtracts the perforated pipe and adds a waste allowance.

French drain gravel
Enter the trench length, width, and depth, the pipe size, and the stone weight to get cubic yards and tons.

Starts with a common perforated pipe diameter. Set the trench and stone values to match your job.

Trench and pipe
ft

Total length of the trench run.

in

Width of the trench. A common french drain is 12 in wide.

in

Depth of the trench. Many drains run 18 in deep, but match your grade and outlet.

in

Outside diameter of the perforated pipe. Common sizes are 3, 4, and 6 in. Set to 0 if there is no pipe.

Stone and waste
lb/cu ft

Washed drainage stone weighs about 100 lb per cubic foot (roughly 1.35 tons per cubic yard). Use your supplier's figure if you have it.

%

A little extra for uneven trench walls and compaction. About 5 to 10 percent is common.

Stone to order

2.75 cu yd

2.1 m³

Weight: 3.71 tonsStone volume: 74.2 cu ft

What does a french drain gravel calculator tell me?

It turns the trench size and pipe into the cubic yards and tons of washed drainage stone to order.

A french drain is a gravel-filled trench, usually with a perforated pipe at the bottom, that carries water away from a wet spot or a foundation. To buy the stone you need to know the trench volume, minus the space the pipe takes up, converted to cubic yards and tons.

This calculator does that in one step: multiply the trench length, width, and depth for the total volume, subtract the pipe, add a small waste allowance, and convert to cubic yards and tons. Bulk stone is usually sold by the ton, so both numbers are shown.

The most important choice is the stone itself. Use clean, washed drainage stone such as #57, not crusher run or anything with fines, so water can move freely through the gaps. Washed stone weighs about 100 pounds per cubic foot, which is roughly 1.35 tons per cubic yard; use the figure from your supplier if you have it.

How does this calculator size french drain gravel?

Trench volume minus the pipe, plus a waste allowance, converted to cubic yards and tons.

Formula

Stone volume (cu ft) = (length x trench width x trench depth) - (3.14159 x pipe radius x pipe radius x length), then add the waste percent. Cubic yards = stone volume / 27. Tons = stone volume x stone weight per cubic foot / 2000.

Start with the trench as a box: length times width times depth. Then subtract the pipe, which is a cylinder running the length of the trench, so it does not get counted as stone. Add a small waste percent for uneven trench walls and settling, then convert the result to cubic yards (divide by 27) and to tons (multiply the cubic feet by the stone weight and divide by 2000). In metric mode you enter meters and millimeters and read cubic meters and tonnes alongside.

What stone should you use?

Use clean, washed drainage stone such as #57 stone. It has open gaps between the pieces so water drains through quickly. Avoid crusher run, quarry process, or anything with stone dust, because the fines pack tight and slowly clog the drain. Wrapping the trench in filter fabric helps keep surrounding soil from migrating into the stone.

How much extra should you buy?

Add about 5 to 10 percent. Hand-dug trenches are rarely perfectly straight, the stone compacts as you place and rake it, and running short in the middle of a run is a hassle. Since stone is delivered by the ton, round up to the supplier’s minimum load. The pipe subtraction keeps the estimate from overshooting on wide trenches with large pipe.

Example

A 50 ft trench, 12 in wide and 18 in deep, around a 4 in pipe, with washed stone at 100 lb per cubic foot and 5% waste.
  1. 1
    Trench volume50 x 1 ft x 1.5 ft = 75 cu ft.
  2. 2
    Pipe volumePipe radius = 4 in / 2 = 2 in = 0.167 ft; 3.14159 x 0.167 x 0.167 x 50 = 4.36 cu ft.
  3. 3
    Stone with 5% waste(75 - 4.36) x 1.05 = 74.2 cu ft.
  4. 4
    Cubic yards and tons74.2 / 27 = 2.75 cu yd; 74.2 x 100 / 2000 = 3.71 tons.
ResultAbout 2.75 cubic yards, or 3.71 tons, of washed drainage stone.

FAQ

What stone to use, why the pipe is subtracted, and how much to order.

How do I figure out how much gravel my french drain needs?

Measure the trench, subtract the pipe, add a little waste, and convert to cubic yards and tons. For a 50 ft trench 12 in wide and 18 in deep with a 4 in pipe and washed stone at about 100 lb per cubic foot, that comes to roughly 2.75 cubic yards, or about 3.71 tons, with a 5% waste allowance. The trench holds about 75 cubic feet; removing the pipe and adding waste leaves about 74 cubic feet of stone.

What kind of gravel should I use?

Use clean, washed drainage stone such as #57 stone, not crusher run or stone dust. Washed stone has open spaces between the pieces so water can move through it. Crusher run packs tight with fines and can clog the drain over time.

Why does it subtract the pipe?

The perforated pipe takes up space in the trench that stone would otherwise fill, so the calculator removes the pipe's volume for a closer estimate. If you are not using a pipe, set the pipe diameter to 0.

How much extra should I order?

Order about 5 to 10 percent over the calculated amount. Trench walls are rarely perfectly straight, the stone settles as you place it, and it is better to have a little left than to stop partway. Stone is usually sold by the ton, so round up to the supplier's minimum.

Can I plan the trench in meters?

Yes. Switch to Metric and enter the trench in meters and the pipe in millimeters. The results show cubic meters and tonnes alongside cubic yards and tons, so you can order in whichever unit your supplier uses.

Sources

High-trust standards and documented references.

  1. 1. Constructed Stone Underdrains (technical bulletin)

    Penn State Center for Dirt and Gravel Road Studies

    Design and construction guidance for stone underdrains, including washed aggregate and pipe placement in the trench.

    https://dirtandgravel.psu.edu/wp-content/uploads/2022/06/TB_Constructed_Stone_Underdrains.pdf

  2. 2. Underdrains (technical bulletin)

    Penn State Center for Dirt and Gravel Road Studies

    Underdrain practice for subsurface drainage; supports the washed-stone and perforated-pipe approach used here.

    https://dirtandgravel.psu.edu/wp-content/uploads/2022/06/TB_Underdrains.pdf

  3. 3. ASTM D448: Standard Sizes of Aggregate for Road and Bridge Construction

    ASTM International

    Defines standard aggregate size numbers such as #57 stone referenced for clean drainage gravel.

    https://www.astm.org/standards/d448

  4. 4. Aggregate Properties and Gradation (bridge/structures reference)

    U.S. Federal Highway Administration (FHWA)

    Background on aggregate gradation and drainage behavior behind the choice of open-graded washed stone.

    https://www.fhwa.dot.gov/publications/research/infrastructure/structures/bridge/15034/003.cfm

  5. 5. Aggregates industry resources

    National Stone, Sand and Gravel Association (NSSGA)

    Industry context for crushed stone products and typical unit weights used for ordering by the ton.

    https://www.nssga.org/

  6. 6. Gravel Tons by Area and Depth Calculator

    AnswerCalc

    Companion calculator for a flat gravel area when you are not subtracting a pipe.

    https://www.answercalc.com/en/construction/materials/gravel-tons-by-area-depth/

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