How Much Gravel Do I Need for a French Drain?
For a typical 100 ft residential French drain (12″ wide, 6″ of gravel), plan on about 1.5 cubic yards — roughly 2 tons or 3 bulk bags — of clean ¾″ crushed stone. Wider, deeper, or longer trenches scale linearly.
Gravel needed by trench size
| Trench | Cubic yards | Tons | Bulk bags* |
|---|---|---|---|
| 50 ft · 12″ wide · 6″ gravel | 0.8 | 1.1 | 2 |
| 100 ft · 12″ wide · 6″ gravel | 1.5 | 2.1 | 3 |
| 100 ft · 12″ wide · 12″ gravel | 3.4 | 4.8 | 7 |
| 100 ft · 18″ wide · 12″ gravel (path) | 5.2 | 7.3 | 10 |
| 100 ft · 24″ wide · 18″ gravel (driveway) | 11.0 | 15.4 | 22 |
| 100 ft · 24″ wide · 36″ gravel (foundation) | 22.0 | 30.8 | 44 |
Assumes ¾″ clean crushed stone at ~1.4 tons per cubic yard. Pipe displacement (~0.3 cu yd per 100 ft of 4″ pipe) already subtracted.
The formula behind those numbers
Gravel volume for a French drain is a rectangle minus the pipe — length × width × gravel-depth in feet, divided by 27 to get cubic yards, then subtract the cylindrical displacement of the pipe.
For a 100 ft trench that's 12 inches wide with 6 inches of gravel: 100 × 1 × 0.5 = 50 cubic feet ÷ 27 ≈ 1.85 cu yd. Subtract about 0.3 cu yd for a 100 ft × 4″ pipe and you get roughly 1.5 cu yd — matching the table above.
What kind of gravel to buy
Use clean, washed ¾″ crushed stone — often labeled #57 stone or drainage stone at the supplier. The angular shape locks together so the trench doesn't shift, and the void space lets water move freely.
Avoid pea gravel (too small, packs down and clogs) and rounded river rock (too smooth, doesn't lock together, more expensive). Whether the stone is granite, limestone, or basalt matters less than that it's clean and angular.
Bags, bulk, or by the ton?
Under 1 cubic yard: bagged at a home center, roughly $5–8 per 0.5 cu ft bag. Convenient but expensive — a yard works out to 54 bags.
1 to 5 cubic yards: bulk delivery from a landscape supplier, typically $50–80 per cubic yard delivered. Have a flat dump area cleared.
5+ cubic yards: order by the ton from a stone yard or quarry, often $30–50/ton. A cubic yard of drainage gravel weighs about 1.4 tons.
Variations that change the math
Driveway drains
Wider trench (typically 24″) with a deeper gravel base (12–18″) to take the load. Easily 3–4× the gravel of a yard drain at the same length.
Foundation / footer drains
Trench goes 4–6 ft deep next to the wall. Gravel often surrounds the pipe on three sides, so the volume balloons — a 100 ft footer drain can need 20+ cu yd.
Gravel-free French drains
These use a wrapped pipe and skip the gravel layer entirely. Useful when access is tight or speed matters. Capacity is lower, so they aren't right for heavy-flow situations.
Buy 10–15% extra
Gravel compacts and the trench is rarely perfectly uniform. Order a buffer so you don't end up one wheelbarrow short on installation day.
Frequently asked questions
How many bags of gravel are in a cubic yard?
A cubic yard is 27 cubic feet. At 0.5 cu ft per bag, that's 54 bags per yard. With larger 1 cu ft bags, 27 per yard. Bulk bags (often called 'mini-supersacks') hold about 0.5 cu yd each.
What does a yard of drainage gravel weigh?
About 1.4 tons (2,800 lbs) — the conservative end of the 1.4–1.7 t/yd³ range. Heavier stones like basalt run higher; lighter limestone runs at the low end.
Do I really need to subtract the pipe volume?
For small trenches it barely matters (~0.3 cu yd per 100 ft of 4″ pipe). For long driveway or foundation drains with larger pipes, the pipe displacement adds up — the calculator subtracts it automatically.
Is gravel cheaper by the ton or by the yard?
At small volumes (under a yard), bags are simplest and the per-unit cost doesn't matter much. Past a few yards, by the ton is almost always cheaper — quarries sell to professional buyers and don't mark up like a home center.
Can I reuse the excavated soil instead of buying more gravel?
No — gravel and soil play different roles. Gravel goes around the pipe to move water; soil goes on top to restore the surface. The calculator's 'leave a little room' slider lets you reserve depth at the top for re-using excavated soil there.