French Drain Calculator
Calculate materials needed for your french drain project.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator estimates gravel, perforated pipe, and filter fabric for a french drain trench. Enter your trench dimensions and the results update instantly. Here is how to get an accurate material estimate:
- Enter trench length. Measure the total run of your drainage trench in feet. A typical residential french drain runs 30 to 100 feet. Longer runs may need multiple outlet points.
- Set trench width. Standard french drain trenches are 12 inches wide. Wider trenches (18-24 inches) increase gravel capacity and are used in high-flow areas or around foundations.
- Set trench depth. Depth depends on the source of the water you are intercepting. Surface runoff drains are typically 18 inches deep. Drains meant to intercept a water table or foundation seepage are often 24 inches or deeper. The trench must slope at least 1% (about 1 inch per 8 feet) toward the outlet.
- Read your results. The calculator shows gravel in tons (the unit most suppliers use), cubic yards, pipe length, filter fabric square footage, and the number of catch basins recommended for the run.
Use the Share button to send your inputs to a contractor or supplier, or Copy to paste the result into an estimate.
About the French Drain Calculator
A french drain is a gravel-filled trench containing a perforated pipe that redirects surface water and groundwater away from a problem area. The trench is lined with filter fabric (geotextile) to prevent soil migration into the gravel, which would clog the drain over time.
Gravel volume equals the full trench volume: length times width times depth, converted to cubic feet and then to cubic yards (divide by 27). Gravel weighs approximately 1.4 tons per cubic yard for standard drainage stone (clean crushed stone or river rock). Pipe length matches trench length. Filter fabric wraps the sides and bottom of the trench, so the fabric width equals two times the depth plus the trench width, multiplied by the length. Catch basins are recommended every 50 feet to accept surface water inlets and allow for cleanout access. All calculations run in your browser. No data is stored or transmitted.
Frequently Asked Questions
How much gravel do I need for a 50-foot french drain?
A 50-foot french drain with standard dimensions of 12 inches wide by 18 inches deep requires approximately 2.78 cubic yards of gravel, which is about 3.9 tons. Wider or deeper trenches increase this significantly. Always order 5-10% extra to account for compaction and irregular trench walls.
What type of gravel should I use for a french drain?
Use clean, washed crushed stone or river rock in the 3/4-inch to 1.5-inch size range. This size allows water to flow freely while supporting the pipe and preventing large voids. Avoid pea gravel (too small, compacts and shifts) and limestone (degrades over time in wet conditions). The filter fabric around the gravel does most of the work keeping soil out, but clean angular stone lasts significantly longer than rounded or fine material.
How deep should a french drain be?
Depth depends on what you are draining. For surface runoff, 18 inches is typical and gives you enough depth to cover the pipe with 6 inches of gravel above it. To drain a soggy yard or intercept a shallow water table, aim for 24 inches. For foundation drainage and basement waterproofing, the drain typically runs at the base of the footing, which can be 4 to 8 feet deep. In cold climates, keep the pipe below the frost line to prevent heaving.
Do I need filter fabric for a french drain?
Filter fabric (geotextile) is strongly recommended for most french drain installations. Without it, fine soil particles migrate into the gravel over time, reducing drainage capacity and eventually clogging the system. In sandy soils the effect is slower, but in clay-heavy soils a drain without fabric can lose its effectiveness within a few years. Use a non-woven geotextile rated for drainage applications. Wrap it around the sides and bottom of the trench before adding gravel, then fold it over the top before backfilling.