How much gravel do I need for a French drain?

Multiply trench length × trench width × stone depth (all in feet) to get cubic feet, then divide by 27 for cubic yards. A typical 50-ft drain in a 12"×18" trench needs about 2.8 yd³ or 3.8 tons of #57.

What gravel size is best for a French drain?

#57 washed crushed stone (about 3/4 inch). Open-graded (lots of voids), washed (no dust to clog the pipe), angular (locks together so it doesn't shift). #8 stone works for tight residential drains; #4 for higher-flow applications.

Can I use pea gravel for a French drain?

Not recommended. Pea gravel is too small and rounded — it has less void space than #57, water moves slower through it, and the small particles can pack tightly enough to defeat the drain. Use #57 washed.

How deep should a French drain trench be?

Minimum 18 inches deep total (4" of stone under the pipe + 4" pipe + at least 6" of stone over). 24 inches is more typical for yard drains. Footing drains go deeper — to the bottom of the foundation footing.

Do I need fabric for a French drain?

Yes — non-woven geotextile fabric, lining the trench in a 'burrito' wrap so the stone is fully enclosed. This stops soil from migrating into the gravel and clogging the system. Without fabric, the drain has a useful life of 3–5 years instead of 30+.

How much does a French drain cost in materials?

For a 50-ft drain: ~$230 in #57 washed stone, $50 in 4" perforated pipe with sock, $70 in non-woven geotextile, ~$30 in delivery, and ~$50 in fittings and outlet. Total around $400–$500 in DIY materials.

What slope does a French drain need?

A minimum 1% slope (1 inch of drop per 8 feet of run) is the standard. 2% is better if you have the grade for it. Without slope, water sits in the gravel and the drain becomes a bathtub. Use a string line and line level to confirm fall before you backfill.

Where should a French drain terminate?

To daylight on a downhill slope at least 10 feet from any structure, into a dry well, or into a storm drain (where local code permits). Never terminate into a sanitary sewer. The outlet must be lower than every point of the drain, and protected with a rodent-proof grate. Some municipalities require a permit for drains that discharge to the public right-of-way — check first.

French Drain Gravel Calculator

How this calculator works for French drains

A French drain in cross-section: a fabric-lined trench, 12+ inches of #57 stone, and a 4-inch perforated pipe at the bottom.

A French drain trench is a rectangle from the calculator's point of view: length × trench-width × stone-fill-depth. Most residential French drains run 12 inches of stone fill around a 4-inch perforated pipe, in a trench that's 12–18 inches wide.

Use #57 washed crushed stone — washed is non-negotiable, because dust-coated stone clogs the perforations in the pipe within a season. Wrap the trench in non-woven geotextile fabric (the "burrito" technique) so soil doesn't infiltrate the stone over time.

Don't subtract the pipe volume

A 4-inch perforated pipe takes up about 0.087 cuft per linear foot — roughly 3% of a 12"×18" trench's volume. Don't bother subtracting it. The 10% buffer absorbs the discrepancy.

Recommended depth & material

Use	Depth	Material	Notes
Standard yard French drain (4" perforated pipe)	12" – 18" stone fill	#57 washed crushed stone	Trench 12-18" wide, fabric wrap.
Footing drain (along foundation)	12" – 24" stone fill	#57 or #4 washed	Goes to footing depth; consult plans.
Curtain drain (intercepts surface water)	18" – 24" stone fill	#57 washed	Trench placed across slope upstream.
Trench drain (no pipe, gravel only)	12" stone fill	#57 or pea gravel	Less effective; only for very light water.

How to measure a French drain trench

A French drain calculation uses the trench dimensions, not the yard you're protecting. Measure the trench you're going to dig.

Mark the trench line. Run the trench from the wet area to a daylight or dry well. Slope the trench at least 1% (1 inch of fall per 8 ft of run) toward the outlet.
Measure the run length. Total linear feet of trench. Longer is fine — short drains often fail because they don't reach a real outlet.
Set the trench width. 12 inches is the minimum to fit a pipe with stone on each side. 18 inches is more comfortable to dig and gives the system more capacity.
Set the stone-fill depth. 12 inches is the standard around a 4-inch pipe. Footing drains go deeper (to the footing depth, often 24+ inches).
Use #57 washed. Material is set to #57 by default. Don't substitute pea gravel — it lacks the void space, and water moves slower through it.
Add 10-15% buffer. Trench walls collapse a little, you'll lose some stone backfilling around the pipe, and the geotextile fabric has overlaps to consider.

Worked example: 50-ft yard French drain

A typical residential drain solving a wet spot at the back of the yard: 50 ft long × 1.5 ft (18\") wide × 12\" deep of stone fill.

Convert depth to feet: 12 ÷ 12 = 1 ft
Volume: 50 × 1.5 × 1 = 75 cuft = 2.78 yd³
Weight: 2.78 × 2,750 ÷ 2,000 = 3.82 tons
With 10% buffer: ~3.05 yd³ / 4.2 tons of #57 washed.
Cost: at $55/ton (washed costs slightly more), ~$230 in stone, plus delivery, plus ~$50 in pipe and ~$70 in non-woven fabric.

For comparison, a 100-ft drain at the same trench size needs ~8.5 tons of stone — useful to know before you commit.

French Drain Gravel Calculator