How to Estimate the Amount of Gravel Needed to Fill a French Drain

A French drain may sound like a fancy cooking tool, but it's an economical landscaping feature for directing water away from a structure. The technique uses a gravel-filled perimeter trench or line of tile to capture runoff or groundwater and redirect it to a location where it won't harm the structure. French drains are widely used in moist locations and where buildings are constructed on slopes. They are also often used behind retaining walls.

Stakes
50- or 100-foot tape measure
Paper and pencil
Calculator (optional)

Instructions

- 1
  Lay out the drain or drains needed to protect the structure. Mark each end of each section of the trench with a stake.
- 2
  Measure all sections of the planned drain with a 50- or 100-foot tape measure. Sum the lengths and record the total.
- 3
  Plan the width and depth of the trenches. Trenches for French drains are typically 6 to 12 inches wide and 8 to 12 inches deep. A drain that includes pipe or tile must be large enough to allow 1 to 2 inches of gravel on all sides of the pipe.
- 4
  Convert the planned width and depth from inches to feet by dividing the width and depth by 12. Multiply the width and depth by the total trench length. Example: For a trench 135 feet long, 9 inches deep and 6 inches wide: 135 * 0.75 * 0.5 = 50.6 cubic feet.
- 5
  Convert from cubic feet to cubic yards, the unit of measure in which gravel is typically sold. Divide the volume in cubic feet by 27. For example, 50.6 cubic feet / 27 = 1.87. Round up to the next whole number. This is the number of yards of gravel needed to fill the trench.

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