French Drain Gravel Calculator
Trenches for French drains typically run 12″–18″ wide and 18″–24″ deep, filled with clean, no-fines stone. Enter your trench dimensions below to get tons, cubic yards and cost.
Measure straight across the widest point.
Adjust depth below if needed.
Covers spillage and compaction. 5–10% is normal.
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Bags or bulk delivery?
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Coverage & weight
Material profile
A typical French drain trench — 12″ wide, 18″ deep — needs about 1 ton of clean 3/4″ drainage stone per 15 running feet, or roughly 2.8 tons for a 40-foot trench before adding a 10% waste allowance.
Drainage stone needed for common trench lengths
Cubic yards and tons for clean 3/4″ drainage stone at typical French drain trench widths and depths.
| Project | Depth | Cubic yards | Tons |
|---|---|---|---|
| 20 ft trench, 12″ wide | 18″ | 1.1 | 1.4 |
| 40 ft trench, 12″ wide | 18″ | 2.2 | 2.8 |
| 60 ft trench, 12″ wide | 18″ | 3.3 | 4.2 |
| 40 ft trench, 18″ wide | 24″ | 4.4 | 5.6 |
| 80 ft trench, 18″ wide | 24″ | 8.9 | 11.1 |
| 100 ft trench, 12″ wide | 18″ | 5.6 | 6.9 |
Drainage stone types, density and drainage rating
| Type | Weight per yd³ | Best for | Drainage |
|---|---|---|---|
| Clean 3/4″ drainage stone | 2,500 lb | French drain trenches | Excellent |
| Clean 1.5″ drainage stone | 2,450 lb | Larger trenches, culverts | Excellent |
| Washed round drain rock | 2,600 lb | French drains, dry wells | Excellent |
Figures are for dry material. Wet or freshly delivered material can weigh noticeably more.
How to calculate drainage gravel you need
- Measure the trench. Record the length and width in feet and the planned depth in inches — most French drains run 12″–18″ wide and 18″–24″ deep.
- Pick your stone. Choose clean 3/4″ stone for standard trenches, 1.5″ stone for larger trenches or culverts, or washed round rock for dry wells.
- Convert depth to feet. Divide the depth in inches by 12 so it matches the length and width units.
- Calculate volume and weight. Multiply length × width × depth (ft) and divide by 27 for cubic yards, then multiply by the stone’s density and divide by 2,000 for tons.
- Add waste and order. Add 10% for trench wall irregularities and settling, then round up to the supplier’s minimum load or nearest bag count.
Worked example
A 40-foot perimeter drain trench, 12″ wide (1 ft) and 18″ deep, filled with clean 3/4″ drainage stone at 2,500 lb/yd³, with a 10% waste allowance.
- Trench length40 ft
- Trench width1 ft (12″)
- Trench area40 × 1 = 40 sq ft
- Depth in feet18 ÷ 12 = 1.5 ft
- Raw volume40 × 1.5 ÷ 27 = 2.22 cubic yards
- Weight at 2,500 lb/yd³2.22 × 2,500 ÷ 2,000 = 2.78 tons
- Order with 10% waste2.78 × 1.10 = 3.06 tons (≈$234 incl. $75 delivery)
Recommended depth by project
| Project | Recommended depth | Notes |
|---|---|---|
| Project type | Depth | Notes |
| Downspout/surface drain line | 12″–15″ | Shallow line for roof runoff; still slope to daylight or a dry well. |
| Standard perimeter french drain | 18″ trench (12″ wide) | Common residential depth; keep below typical frost heave zones. |
| Foundation/basement drain | 24″ trench (18″ wide) | Deeper trench for full basement waterproofing systems. |
| Culvert or driveway crossing | 24″+ trench (18″ wide) | Use 1.5″ clean stone for larger voids under vehicle loads. |
| Dry well overflow bed | 18″–24″ around the well | Wrap in filter fabric to keep silt from filling voids. |
Bags or bulk delivery?
A 0.5 cu ft bag of clean drainage stone covers about 0.28 sq ft of trench at 18″ deep, so bagged stone only makes sense for short repair runs. A 40-foot trench needs roughly 120 bags versus a single bulk load, so anything past about a 10-foot section is cheaper and faster delivered by the ton. Most yards set a 2–3 ton minimum for delivery, so combine french drain stone with pipe bedding or another project if your trench is short.
- Small repair (under 10 ft of trench)Bags work fine — expect 15–20 bags of 0.5 cu ft stone.
- Medium trench (10–60 ft)Compare bag cost to a bulk minimum load; bulk usually wins past about 1 ton.
- Large or multi-run drain (60 ft+)Order bulk by the ton and have it dumped near the trench to limit wheelbarrow trips.
Common mistakes
- Using crushed stone with finesCrusher run and other stone with rock dust packs down and clogs the voids a French drain needs to move water — use clean, washed stone only.
- Skipping the filter fabricWithout fabric wrapping the stone (and pipe), surrounding soil migrates into the voids and the drain silts up within a few seasons.
- Making the trench too narrowA 12″ trench is standard, but tight digging equipment can leave less working room around the pipe than assumed — measure the actual dug width, not the pipe diameter.
- Ignoring the outlet slopeThe trench needs a consistent 1% minimum grade to daylight, a dry well, or a sump — flat sections turn into standing water.
- Not adding a waste allowanceTrench walls slough, stone settles as it’s placed, and pipe corrugations displace less volume than expected — a 10% buffer avoids a second delivery.
- Choosing stone that's too small3/8″ pea gravel compacts tighter than 3/4″ or 1.5″ stone and reduces the void space available for water flow — stick to sizes labeled for drainage use.
What this calculator assumes
Adjust your result if your project differs from these.
- Density figures are for clean, dry, settled stone at the yard — wet stone weighs more per yard.
- Trench is treated as a simple rectangular channel with vertical walls.
- Perforated pipe volume is not subtracted from the gravel volume.
- 10% waste covers trench wall irregularities, settling and pipe displacement.
- Bag estimates assume 0.5 cu ft bags; check your supplier’s actual bag size.
- Pricing assumes a single delivered load; multiple partial loads may cost more per ton.
Common questions
How deep should a French drain trench be?
Most residential French drains are 18″–24″ deep, below typical planting root zones and shallow frost lines. Deeper drains near foundations may go to 24″ or more.
What size stone is best for a French drain?
Clean, washed 3/4″ stone is the standard choice. Use 1.5″ stone for larger trenches or where flow volume is high, like culverts.
Can I use pea gravel in a French drain?
Not recommended. Pea gravel’s smaller, rounder profile packs tighter and leaves less void space for water than 3/4″ or larger drainage stone.
Do I need landscape fabric around the stone?
Yes. Filter fabric wrapped around the stone (and pipe, if used) keeps soil particles from washing in and clogging the drain over time.
How much stone do I need per foot of trench?
A 12″-wide, 18″-deep trench takes about 0.056 cubic yards (0.07 tons) per linear foot of clean 3/4″ stone before waste.
Should I order by the ton or by the yard?
Most quarries and landscape yards price and deliver drainage stone by the ton. Use the cubic yard figure to convert if a supplier only quotes by volume.
Does a French drain need pipe, or just gravel?
A gravel-only trench (a ‘blind drain’) works for light surface water. Adding perforated pipe at the bottom moves higher volumes farther and clears out easier if it clogs.
How much extra should I order for waste?
10% is a standard allowance for trench wall irregularities, settling, and stone displaced by the pipe.
What's the difference between 3/4″ and 1.5″ drainage stone?
1.5″ stone has slightly more void space per yard and moves water faster, but it’s harder to compact evenly around pipe — 3/4″ is the more common all-purpose choice.
Can I use bagged stone for a whole French drain?
Only for short repair sections. A 40-foot trench needs roughly 120 bags of 0.5 cu ft stone versus a single 3-ton bulk delivery, so bulk is far more practical past about 10 feet.
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Project
| Shape | — |
| Dimensions | — |
| Depth | — |
| Area | — |
| Material | — |
| Waste allowance | — |
Volume & weight (incl. waste)
| Cubic yards | — |
| Cubic feet | — |
| Weight | — |
| Tons (rounded up) | — |
Shopping list
| Bulk order | — |
| Bagged alternative (0.5 cu ft) | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes uniform depth and standard dry material densities. Confirm quantities with your supplier before ordering.