How Much Gravel for a French Drain? 2026 Calculator + Formula

Most residential French drains need 1 cubic yard of gravel for every 18 to 25 linear feet of trench. A standard 100-foot French drain in a 12-inch-wide, 24-inch-deep trench takes roughly 5 cubic yards (about 6.5 tons) of clean 3/4-inch washed stone. This guide gives you the one-line formula, a size chart for every common trench dimension, ton-to-yard conversions, the right gravel type to ask for, and 2026 delivery prices — plus the single mistake that clogs more French drains than every other cause combined.

Quick answer — how much gravel for a French drain?

• Per linear foot (12" × 24" trench) — about 0.05 cubic yard, or 1 yard per 20 ft
• 50 ft drain — 2.5 cubic yards (about 3.4 tons)
• 100 ft drain — 5.0 cubic yards (about 6.8 tons)
• 150 ft drain — 7.5 cubic yards (about 10.1 tons)
• 200 ft drain — 10.0 cubic yards (about 13.5 tons)
• Add 10% for settling, irregular trench walls, and the bedding layer under the pipe

The French drain gravel formula

Cubic yards of gravel = (Length ft × Width in × Depth in) ÷ 324 − Pipe volume

324 is the conversion constant from cubic-feet-with-inch-depths to cubic yards (27 cu ft per yard × 12 in per ft). If you're using a 4-inch perforated pipe, the pipe displaces about 0.087 cu ft per linear foot — small but worth subtracting on long runs. Multiply your final answer by 1.10 to add the 10% waste factor that every contractor builds in.

Simpler shortcut (when you're standing at the gravel yard)

Cubic yards ≈ Length ft ÷ 20 (for a standard 12" × 24" trench)

That shortcut already accounts for the pipe and the 10% waste factor. Use it for ballpark pricing; use the full formula when you're ordering bulk delivery.

Gravel needed by trench size (chart)

Cubic yards per 100 linear feet, by trench width and depth. Multiply by your length and divide by 100.

• 6" wide × 18" deep — 2.8 cu yd per 100 ft
• 8" wide × 18" deep — 3.7 cu yd per 100 ft
• 12" wide × 18" deep — 5.6 cu yd per 100 ft
• 6" wide × 24" deep — 3.7 cu yd per 100 ft
• 8" wide × 24" deep — 4.9 cu yd per 100 ft
• 12" wide × 24" deep — 7.4 cu yd per 100 ft (raw, before pipe and rounding)
• 12" wide × 36" deep — 11.1 cu yd per 100 ft
• 18" wide × 36" deep — 16.7 cu yd per 100 ft

Cubic yards to tons — what your supplier will quote

Gravel suppliers price by ton, not yard. Clean washed 3/4-inch drainage stone weighs roughly 1.35 to 1.45 tons per cubic yard dry. Use 1.4 as a safe planning number.

• 1 cubic yard = ~1.4 tons
• 5 cubic yards = ~7 tons
• 10 cubic yards = ~14 tons
• 1 ton covers ~21 linear feet of a 12" × 12" gravel envelope

Worked example — 80 ft drain, 10" wide, 24" deep

1. Trench volume in cu ft: (80 × 10 × 24) ÷ 144 = 133.3 cu ft
2. Convert to cu yd: 133.3 ÷ 27 = 4.94 cu yd
3. Subtract 4" pipe (0.087 cu ft per ft × 80 ft = 7 cu ft = 0.26 cu yd): 4.94 − 0.26 = 4.68 cu yd
4. Add 10% waste: 4.68 × 1.10 = 5.15 cu yd
5. Round up to delivery size: 5.5 cu yd (about 7.7 tons)

What kind of gravel goes in a French drain?

Use clean, washed, angular stone between 3/4 inch and 1.5 inches. Angular stones lock together and leave 30 to 40% void space for water to move through; rounded pea gravel and crusher run pack tight and choke the drain.

• Best — #57 stone (3/4" washed angular) — the contractor default
• Also great — #67 stone (3/4" minus, washed) for tighter trenches
• OK — 1" to 1.5" clean drainage rock for deep trenches and heavy flow
• Avoid — pea gravel (rounded, clogs), crusher run (has fines that cement together), and any "unwashed" stone (the fines plug your fabric inside 2 years)

How to layer a French drain (so you order the right amount)

1. Dig trench to depth — slope 1% (1/8" drop per foot) toward daylight or a dry well.
2. Line trench + extend 12" up each wall with non-woven geotextile filter fabric.
3. Add 2 to 3 inches of gravel as a bedding layer under the pipe.
4. Lay 4-inch perforated pipe (holes DOWN — counterintuitive but correct) on the bedding.
5. Backfill with gravel to within 4 to 6 inches of the surface.
6. Fold the fabric over the top of the gravel like a burrito to keep silt out.
7. Top with 2 to 4 inches of soil and sod, or river rock if it's a decorative trench.

2026 gravel prices for a French drain

• #57 washed stone (bulk) — $30 to $65 per ton, or $42 to $90 per cubic yard
• #67 washed stone (bulk) — $32 to $70 per ton
• Bagged 0.5 cu ft drainage stone — $5 to $8 per bag (only economical for runs under 20 ft)
• Delivery (10-mile radius) — $75 to $200 flat, plus $3 to $6 per mile beyond
• Geotextile fabric (4 ft × 100 ft non-woven) — $50 to $120 per roll
• 4" perforated PVC pipe — $0.80 to $1.50 per linear foot
• 4" perforated corrugated (sock-wrapped) — $1.00 to $2.00 per linear foot
• Pro install (turnkey French drain) — $25 to $60 per linear foot

Total cost example — DIY 100 ft French drain

• 5.5 cu yd #57 stone delivered — $385 (~7.7 tons × $50)
• Delivery fee — $125
• 100 ft of 4" perforated PVC — $110
• Geotextile fabric (one roll) — $80
• Couplers, end cap, pop-up emitter — $45
• Trenching (rental, 1 day, 24" trencher) — $250
• DIY total — about $995 (vs $2,500 to $6,000 for a pro install)

The #1 mistake that ruins French drains

Skipping the filter fabric — or using woven landscape fabric instead of non-woven geotextile. Soil silt washes into the gravel within 18 to 24 months, fills the void space, and the drain stops flowing. A $60 roll of non-woven fabric is what separates a 25-year drain from a 2-year drain. The second most common mistake: installing the perforated pipe with holes facing UP. Water enters from the bottom, so holes go DOWN — the pipe collects groundwater as it rises through the gravel envelope.

How long does a French drain last?

A properly built French drain — non-woven fabric, washed angular stone, 4-inch perforated pipe, 1% slope — lasts 30 to 50 years. Cutting any of those four corners typically drops lifespan to 5 to 10 years before clogging.

FAQ

How many tons of gravel do I need for a 100-foot French drain?

About 6.8 to 7.7 tons (5 to 5.5 cubic yards) for a standard 12-inch-wide, 24-inch-deep trench using clean 3/4-inch washed stone. Order 7 tons to be safe — leftover gravel works for downspout splash pads and walkway base.

How deep should a French drain be?

18 to 24 inches deep is standard for surface water and yard drainage. Go 36 to 48 inches deep for footing drains around a foundation, and 8 to 12 inches deep for shallow channel drains under a downspout. Always slope the trench at least 1% (1/8 inch per foot) toward the outlet.

How wide should a French drain trench be?

10 to 12 inches wide is the sweet spot — wide enough for a 4-inch pipe with at least 3 inches of gravel on each side, narrow enough to dig with a trencher rental. Go to 18 inches only for high-flow drains handling roof and downspout runoff together.

Can I use pea gravel in a French drain?

No. Pea gravel is rounded and packs tight, leaving very little void space for water. Use angular #57 or #67 washed stone instead — the sharp edges lock together and keep 30 to 40% of the volume open for drainage.

Do I really need filter fabric around the gravel?

Yes — non-woven geotextile is non-negotiable. Without it, silt washes into the gravel within 2 years and the drain stops working. Wrap fabric around the entire gravel envelope (trench walls and over the top) before placing the final soil cap.

How much does a 100-foot French drain cost in 2026?

Roughly $900 to $1,200 in materials and rentals for a DIY install, or $2,500 to $6,000 turnkey from a drainage contractor ($25 to $60 per linear foot). Rocky soil, tree roots, or hand-digging around utilities can push pro pricing to $80+ per foot.

Need a number for your own project? Plug your trench length, width, and depth into the free calculators on the home page and get cubic yards, tons, and total cost in under a minute.