French Drain Gravel Calculator
Calculate tons of gravel needed to fill a French drain trench by length, width, and depth, plus the total material cost.
How this calculator works
A French drain trench is filled with gravel by volume, so this calculator treats your trench’s length and width as the Length and Width fields, multiplies by the trench depth, and converts to cubic yards before applying gravel’s density of 1.4 tons per cubic yard to get the tons you need to order.
Formula: Cubic yards = (Trench length ft × Trench width ft × Depth ft) ÷ 27. Tons = Cubic yards × 1.4.
Worked example
A 30 ft trench, 1 ft (12 in) wide, dug 12 inches deep:
- Depth in feet: 12 ÷ 12 = 1.0 ft
- Cubic yards: (30 × 1 × 1.0) ÷ 27 ≈ 1.111 yd³
- Tons: 1.111 × 1.4 ≈ 1.56 tons
- At $45/ton, that’s $70.00 before waste.
Coverage reference
| Trench length | Tons needed (12 in wide × 12 in deep) |
|---|---|
| 20 ft | ~1.0 tons |
| 30 ft | ~1.6 tons |
| 50 ft | ~2.6 tons |
How to measure your trench
Run a tape measure along the trench’s planned path to get the total length, and measure the dug width straight across the top of the trench in feet (12 inches = 1 ft, 18 inches = 1.5 ft). For depth, measure straight down from grade to the trench bottom — the full dig depth, since gravel typically fills the entire trench around the wrapped pipe.
Frequently asked questions
What should I enter for Length and Width on a trench?
Length is the trench's total run in feet — how far it stretches from start to outlet. Width is the trench's dug width, typically 12-18 inches (1 to 1.5 ft) for a standard residential French drain. Depth is the trench's total dug depth in inches.
What kind of gravel should I use in a French drain?
Washed round stone or #57 stone is preferred over crusher run or road base. Washed gravel has no fines to clog the perforated pipe or reduce flow, while crusher run's packed fines are meant to compact tight — the opposite of what a drain needs.
How wide and deep should a French drain trench be?
A common residential spec is a 12-inch-wide trench dug 12-18 inches deep, with the perforated pipe wrapped in fabric sitting near the bottom and gravel filling around and above it. Deeper trenches (18-24 inches) are used where more water volume needs to move or the outlet point is farther away.
Do I need to subtract the pipe's volume from my gravel order?
Not really — a standard 4-inch perforated pipe takes up a small enough share of a 12-inch-wide trench that the difference is usually within your waste buffer. It only becomes worth subtracting on unusually narrow trenches or oversized pipe.
Does the trench need a slope for the water to drain?
Yes, a consistent downhill slope toward the outlet — commonly around 1% (roughly 1/8 inch drop per foot) — is standard so water moves by gravity rather than pooling partway along the trench.
Should I wrap the gravel and pipe in landscape fabric?
Yes, lining the trench with fabric before adding gravel is standard practice. It keeps surrounding soil from migrating into the gravel and clogging the drain over time, while still letting water pass through into the pipe.
Estimates only. Verify quantities with your supplier before purchasing.