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French Drain Materials Calculator

Calculate the gravel and pipe needed for a French drain trench.

Result

Gravel Needed
2.78 cu yd
Approx. Weight
3.89 tons
Perforated Pipe Needed
50 ft

Assumes the trench is filled almost entirely with drainage gravel surrounding a central perforated pipe. Always maintain a consistent downhill slope (typically at least 1% grade, about 1/8 inch drop per foot) along the entire trench length so water flows to the outlet rather than pooling.

About the French Drain Calculator

The French Drain Materials Calculator estimates the gravel volume and perforated pipe length needed to build a drainage trench that carries water away from a wet yard or foundation. It is meant for anyone planning a French drain project who needs to know how much stone to order and how much pipe to buy before digging starts.

How It Works

You enter the trench's length, width, and depth. The calculator converts width and depth from inches to feet, multiplies by length to get the trench's total volume in cubic feet, then converts that to cubic yards and to an approximate weight in tons using a gravel density factor. It also reports the perforated pipe length needed, which matches the trench length directly since the pipe runs the full length of the trench.

Volume (cu ft) = Length x (Width / 12) x (Depth / 12); Volume (cu yd) = Volume (cu ft) / 27; Gravel Weight (tons) = Volume (cu yd) x 1.4; Pipe Needed = Trench Length

Formula & Methodology

Because trench width and depth are usually specified in inches while length is measured in feet, the calculator first divides both by 12 to bring every dimension into feet, then multiplies length by width by depth for a raw cubic-foot volume. Dividing by 27 (the number of cubic feet in a cubic yard) gives the volume in the unit most gravel suppliers quote by, and multiplying that by 1.4 tons per cubic yard, a typical bulk density for crushed drainage stone, gives an approximate delivery weight.

Examples

Standard Residential Trench

With the default 50 ft trench, 12 in width, and 18 in depth, the trench volume works out to 75 cubic feet, or about 2.78 cubic yards, which comes to roughly 3.89 tons of gravel plus 50 feet of perforated pipe.

Shorter, Narrower Run

For a 30 ft trench that is 8 in wide and 12 in deep, the volume is 20 cubic feet (about 0.74 cubic yards), which comes to roughly 1.04 tons of gravel and 30 feet of pipe.

Advantages

  • Calculates gravel and pipe quantities together in one pass, since both materials scale directly with trench length.
  • Converts the raw volume into tons, the unit most bulk gravel suppliers actually sell and deliver by.
  • Adjusts automatically for any trench cross-section, so narrow shallow trenches and wide deep ones both get an accurate volume.

Common Mistakes

  • Ordering gravel without separately budgeting for landscape fabric, which lines the trench to keep surrounding soil from silting up the gravel over time.
  • Digging the trench level instead of maintaining a consistent downhill slope toward the outlet, which lets water pool in the trench instead of draining away.
  • Assuming the calculated tonnage is exact when actual stone density can vary meaningfully from the 1.4 tons-per-cubic-yard estimate used here.

Edge Cases to Watch For

  • The calculation treats the entire trench volume as filled with gravel and does not subtract the volume the pipe itself occupies, so the true gravel need is very slightly lower than the reported figure.
  • The 1.4 tons-per-cubic-yard factor is a general estimate for crushed drainage stone; actual gravel density varies by stone type and size, so heavier or lighter aggregate will shift the real delivery weight.
  • The result does not account for the downhill slope needed along the trench run (commonly at least a 1% grade, about 1/8 inch drop per foot), which affects how the trench depth should vary from start to outlet.

Common Use Cases

  • Homeowners addressing a chronically wet yard area or water pooling near the foundation.
  • Landscapers installing perimeter French drains around a home before backfilling and regrading.
  • DIYers ordering bulk gravel delivery who need a reasonably accurate tonnage figure ahead of time.
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why is a French drain wrapped in landscape fabric?

Landscape fabric lining the trench (between the soil and the gravel) prevents surrounding dirt and silt from migrating into the gravel and clogging the drain's pore space over time, which would otherwise gradually reduce the drain's effectiveness at moving water away from the area it's protecting.

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Conclusion

This calculator gives a practical starting point for ordering French drain materials based on trench dimensions alone. Because it assumes a standard gravel density and doesn't account for slope grading, it works best paired with a site-specific plan for drain depth and outlet placement.