Garden & Landscaping calculator
French Drain Calculator
Calculate French-drain trench volume and aggregate fill from trench length, width, depth, pipe diameter and allowance.
Calculate French-drain trench volume and aggregate fill from trench length, width, depth, pipe diameter and allowance. Before calculating, confirm the measured geometry and any product specification entered into the form. Small errors in trench section and pipe displacement can become significant across a full project. French drains are not designed by volume alone. Confirm pipe fall, geotextile, outlet, aggregate grading and local drainage requirements separately.
What you need to enter
The French Drain Calculator uses the following project information to calculate the result.
- Drain length
- Trench width
- Trench depth
- Pipe diameter
- Extra allowance (%)
Calculator result
Trench volume and aggregate fill
How the French Drain Calculator works
The French Drain Calculator estimates trench volume and aggregate fill from this relationship: trench volume less optional pipe volume, plus allowance. Because the page is designed for aggregate fill volume for a French drain trench, the calculation depends especially on trench section and pipe displacement. Metric and imperial entries are normalised before the arithmetic so equivalent measurements produce equivalent results.
French Drain Calculator worked example
French Drain Calculator example: For a 20 m trench 400 mm wide and 600 mm deep, first calculate gross trench volume, then deduct the pipe volume if the configuration calls for it. The remaining volume is the theoretical aggregate fill before the selected allowance. The important sense-check is trench section and pipe displacement, because that can move the real requirement away from the neat theoretical result.
Important calculation assumptions
For aggregate fill volume for a French drain trench, french drains are not designed by volume alone. Confirm pipe fall, geotextile, outlet, aggregate grading and local drainage requirements separately.
Frequently asked questions
Which starting measurements make the French Drain Calculator more reliable?
The quality of the result starts with the inputs. Measure trench length, width and depth plus the pipe diameter. Use the actual trench section rather than only the nominal pipe size. They are used here to estimate aggregate fill volume for a French drain trench.
What if my assumption for trench section and pipe displacement is wrong in the French Drain Calculator?
For the French Drain Calculator, use project-specific values for trench section and pipe displacement. French drains are not designed by volume alone. Confirm pipe fall, geotextile, outlet, aggregate grading and local drainage requirements separately.
What should I verify after using the French Drain Calculator?
For the French Drain Calculator, use the result in context: use the aggregate volume for ordering only after confirming the intended trench section and pipe displacement. Drainage fall, outlet, fabric and aggregate specification are separate design decisions.