French Drain Calculator

How to Calculate Gravel for a French Drain

Measure the stone-filled envelope along the planned drain. Multiply its length, width and depth, then subtract the outside volume of the pipe inside it. If the trench will be covered with soil or sod, that covering layer is not part of the gravel depth.

Use one length unit throughout the formula. Divide cubic feet by 27 to get cubic yards. The calculator assumes one constant cross-section and one pipe running the full entered length, entirely inside the gravel. For different trench sections, calculate each separately.

Example: a 50 ft run

A 50 ft run with a 12 in wide, 18 in deep gravel envelope holds 75 ft³ before pipe displacement. An illustrative 4 in actual outside diameter pipe displaces π × (4 ÷ 24)² × 50 = 4.36 ft³. The remaining gravel volume is about 70.64 ft³, or 2.62 yd³, before any extra allowance.

The 4 in figure above is an outside measurement for the example, not an assumption about every nominal 4 in drainpipe. Use your product's dimensional sheet. Do not subtract only the pipe-wall material: gravel cannot fill the pipe's internal water passage either.

Pipe Length and Filter Fabric Quantities

Pipe quantity starts with the measured run. If each connected section covers 10 ft, a 53 ft run needs six sections before allowing for a separate outlet connection or fittings. Enter the effective installed length, since a nominal cut length may include a socket or overlap.

For a continuous fabric wrap around a rectangular gravel envelope, the flat strip width is 2 × width + 2 × gravel depth + the specified top overlap. Multiply that strip width by the run length for the fabric area. A fabric sleeve around the pipe is a different shape and is not calculated by this setting.

That area excludes end closures, joins along the run and cut waste. Check both roll width and length: a narrow roll with enough total square footage may still need a different seam layout. The tool does not divide area by roll area and pretend that every roll shape will fit.

What the Drainage Gravel Estimate Does Not Decide

This drainage gravel calculator measures materials for a specified section. French drain slope, pipe capacity, gravel grading, filter compatibility and a suitable outlet must come from the site-specific design. Entering a larger trench does not prove that it will handle the water reaching it.

A French drain rock calculator gives bulk stone volume, which includes the spaces between the stones. Do not multiply the order by a solid-stone percentage. If your supplier sells by weight, use the gravel calculator's volume-to-weight option with a bulk density matching that drainage aggregate.

The optional gravel subtotal uses your own price. A French drain cost calculator that includes excavation, spoil removal, pipe, fabric and installation answers a broader budgeting question; those costs are not hidden inside this result. Gravel-free bundled systems also need their own product takeoff rather than this stone-envelope calculation.