This page explains the measurement or planning assumption behind the result. When you need the project quantity itself, use French Drain Gravel Calculator.
Measure the physical scope before choosing a factor
Measure trench segments with consistent width/depth and use only the depth that will actually contain aggregate. Keep a sketch or takeoff that names each segment, surface or dimension so the number can be traced back to a physical location. French-drain aggregate takeoff Recheck the dimensions that drive the largest area, run or volume, because a small measurement error there propagates through every later allowance and package rounding decision.
Keep the project assumption explicit
Pipe outside diameter is the correct displacement dimension; fabric, soil cover and bedding zones may use different geometry. Treat that value as a visible input rather than a hidden constant. Product labels, manufacturer instructions, project drawings and applicable requirements should override generic examples. When two zones use different products, dimensions or design assumptions, calculate them separately instead of blending them into an average that is hard to audit.
Separate measured quantity from purchase allowance
Add supplier/order allowance after net gravel volume, then convert to cubic yards or supplier units. Geometry answers how much surface, run or volume exists; waste, overlap, spares and handling reserve answer how much extra material may be prudent. Keeping those layers separate makes the estimate easier to revise when the product, layout or supplier package changes.
Round only when the supplier forces a package boundary
Carry useful precision through the measurement and formula steps, then round at the bag, box, coil, slab, pallet or stock-piece boundary. Early rounding can accumulate across multiple rooms or wall segments. After rounding, compare purchased coverage with the actual target so the surplus is visible instead of disappearing inside the estimate.
Verify the decisions this calculator cannot make
Hydraulic capacity, outlet, slope, fabric and groundwater behavior require separate drainage design. A trustworthy quantity calculator stays narrow: it exposes the arithmetic while leaving design, code, compatibility and manufacturer-specific choices to the sources that govern them. Record those decisions with the takeoff so the same measurements are not later reused with an incompatible assumption.
Planning workflow
Measure trench segments
Split dimension changes.
Define gravel-filled depth
Exclude soil cover.
Subtract pipe displacement
Use OD.
Add order allowance
Confirm supplier units.
Example: changing trench width
A French drain widened from 12 to 18 inches over half its length should be calculated as two trench segments; averaging can hide a large volume difference.
Reference table
| Planning item | Useful starting point | What to verify |
|---|---|---|
| Trench | L × W × depth | Use gravel envelope. |
| Pipe | outside diameter | Subtract physical volume. |
| Aggregate | bulk order | Density/supplier units vary. |
Frequently asked questions
Should fabric volume be subtracted?
No; fabric thickness is usually negligible for aggregate volume, but its overlap and roll quantity are a separate takeoff.
Does pipe perforation change displacement?
Not materially for bulk gravel volume; use outside diameter as the physical envelope.
How do I order gravel by tons?
Use supplier-specific bulk density to convert volume to weight, because aggregate density varies.
Planning note: Product coverage, yield, package size, installation instructions and project requirements take priority over generic examples. Verify the exact product and project before purchasing.