French Drain Gravel Calculator

Why This One Is Different

This calculator tells you how much gravel to order for the trench you're about to dig around your drain pipe. A french drain is a trench, not a gravel pile: a perforated pipe wrapped in open-graded stone that lets water reach the pipe faster than it can move through the surrounding soil. The gravel envelope is typically run about 12 inches deep and however wide the trench was dug, so this tool treats your dig as a length-times-width-times-depth box and returns the stone volume that surrounds the pipe. Most installers line the trench with landscape fabric before backfilling: a low-cost step that keeps soil fines from migrating into the stone and clogging the drain over the years, a trade habit worth budgeting for even though it sits outside the volume math itself.

Where the Numbers Come From

• Trench volume is exact length × width × depth arithmetic once the dig is measured, a worked calculation, not a banded convention. Source: worked example, closed-form geometry
• A gravel envelope roughly 12 inches deep around the perforated pipe is a common french-drain sizing convention; actual depth and slope requirements depend on soil type and water volume, which is a drainage-design question, not a volume one. Source: trade convention, labeled as such (not codified in a standard)
• Lining the trench with non-woven landscape fabric before backfilling is standard trade practice to keep fines from migrating into the gravel and clogging the pipe. Source: trade convention, labeled as such (not codified in a standard)
• Gravel used in drain envelopes carries the same 1.2-1.6 ton-per-cubic-yard density band as general crushed stone, moving ±15% with moisture and gradation. Source: SME Mining Engineering Handbook density band (via construction-estimating canon)