YardCompute / Project Guides / French Drain Gravel Calculation

How to Calculate Gravel for a French Drain

A French drain gravel estimate starts with the dimensions of the gravel-filled space: trench length, width, and depth. Multiply those dimensions to find volume, convert cubic feet to cubic yards, and add an allowance when appropriate.

1. Measure the gravel-filled trench

Measure the length of the planned drain and the average width and gravel depth. If the trench changes size, divide it into sections and calculate each section separately.

2. Calculate cubic feet

With dimensions in feet:

Gravel volume = length × width × depth

For a 60 ft trench that averages 1 ft wide and 1.5 ft deep, the volume is 60 × 1 × 1.5 = 90 cubic feet.

3. Convert to cubic yards

Divide cubic feet by 27:

90 ÷ 27 = 3.33 cubic yards

If you use a 10% planning allowance, the estimate becomes about 3.67 cubic yards. The right allowance depends on trench shape, excavation conditions, delivery practices, and the material being installed.

Estimating weight

Gravel weight depends on the material's bulk density. A calculator can provide an approximate tonnage when you enter a density assumption, but density varies by rock type, grading, moisture, and supplier. Use the supplier's published density when available.

Do not confuse trench volume with gravel volume

A French drain may contain a perforated pipe, filter fabric, or other components. If your project intentionally leaves part of the trench unfilled or uses a different cross-section, the simple rectangular calculation can overestimate the gravel. Treat the calculated quantity as a planning estimate.

Common mistakes

• Mixing inches and feet.
• Using the entire excavation depth when only part will be filled with gravel.
• Assuming every gravel type weighs the same.
• Ignoring changes in trench width or depth.
• Using a generic density when the supplier provides a product-specific figure.

Use the French Drain Gravel Calculator

Use the YardCompute French Drain Gravel Calculator to estimate volume, cubic yards, and approximate tons from your trench dimensions and selected density.

FAQ

How many cubic feet are in a cubic yard?

There are 27 cubic feet in one cubic yard.

Can I calculate tons from cubic yards?

Yes, if you have a suitable bulk-density assumption. Multiply cubic yards by the density expressed in tons per cubic yard, or use a calculator that converts from a compatible density input.

What gravel should a French drain use?

The appropriate stone size and drainage assembly depend on the drainage design, pipe, soil, filter fabric, and site conditions. Follow the project specification rather than choosing material from the volume calculation alone.