French Drain Gravel Calculator

Calculate the gravel for a French drain trench, subtracting the pipe when it sits fully inside the gravel. A quantity tool, not a drainage design.

French drain gravel calculator

Units
Trench sections

Add a section for each run with different dimensions, or with and without pipe.

Trench section 1

The height of the trench that is filled with gravel, from the trench bottom.

Pipe
Advanced optionsExtra material / handling allowance

Adds the percentage you choose to the order volume for waste, handling or contingency.

Result

Your result will appear here

Enter your trench measurements and calculate to see the gravel volume in cubic yards, cubic feet, and cubic metres.

How this calculator works

For each trench section the calculator finds the volume of the gravel-filled part of the trench, then subtracts the space the pipe takes up.

Trench envelope = Length × Width × Gravel-filled height

Pipe displacement = Length × π × Pipe outside diameter² ÷ 4

Gravel = Trench envelope − Pipe displacement

Sections without a pipe subtract nothing. Section totals are added together.

The pipe is subtracted only when its whole cross-section sits inside the gravel-filled height. If the pipe would stick out above the gravel, the calculator stops and asks you to adjust the measurements — it does not estimate a partial overlap.

Drainage design limitations

What this calculator does not do

It estimates aggregate quantity only. It does not design drainage capacity, trench grade, the outlet, filter or geotextile compatibility with your soil, or code compliance. Check those separately, or with a professional.

No standard household trench

Published French drain examples come from specific applications. A USDA NRCS guide describes a spring-collection drain for livestock watering; Penn State describes a drain for sand-based sports fields. They use different pipe sizes, trench sizes and gravel layers because the jobs are different. None of them sets the size of a household French drain, so every dimension here is yours to enter.

Measuring

  • Gravel-filled height is the part of the trench filled with gravel — not necessarily the full trench depth if soil or another material goes on top.
  • Use the pipe's outside diameter.
  • Add a separate section wherever the trench size changes or the pipe stops.

Choosing a stone? The #57 Stone and Crushed Stone calculators cover those materials; for a simple area, use the Gravel Calculator.

Worked example

Example: 40 ft trench, 12 in wide, 18 in of gravel, 4 in pipe

  1. Envelope: 40 ft × 1 ft × 1.5 ft = 60.0 ft³
  2. Pipe: 40 ft × π × (0.3333 ft)² ÷ 4 = 3.49 ft³
  3. Gravel: 60.0 − 3.49 = 56.5 ft³ = 2.09 yd³

Result: 2.09 cubic yards of gravel (1.60 cubic metres). All dimensions are example inputs, not a recommended drain size.

Frequently asked questions

Why won’t the calculator subtract my pipe?

The pipe is subtracted only when its whole outside diameter fits within the gravel-filled height and trench width. If it doesn’t fit, check your measurements, or calculate that section without the pipe.

How wide and deep should a French drain be?

That depends on your site and what the drain must carry. This calculator does not recommend a size; use the dimensions from your plan.

Can I estimate the gravel weight?

Yes. After calculating, enter your supplier’s bulk density for the stone. The weight is shown as an estimate.

Sources & methodology

Volumes are trench geometry and pipe displacement from your measurements. The drain references below are context only — they set no dimension. Sources relevant to this page: