French Drain Gravel Calculator

Measure your trench's length in feet and its width and depth in inches, then enter all three. The french drain gravel calculator instantly returns how much gravel you need in cubic yards (and cubic feet), the weight in tons, and how many pieces of perforated pipe to buy. For most backyard drains those three measurements are all you need. Open More options to switch to metric, pick a gravel type, add a waste allowance for compaction, change the pipe piece length, or enter a price per cubic yard to estimate cost.

A french drain is just a gravel-filled trench with a perforated pipe at the bottom that carries water away from where it collects. The gravel you order is simply the volume of the trench: length × width × depth. Because gravel is sold by the cubic yard (or by the ton) and there are 27 cubic feet in a cubic yard, this calculator does the conversion for you so you know exactly how many cubic yards of gravel for a french drain to have delivered. The perforated pipe running along the bottom displaces only a few percent of the space, so most people fill the whole trench with gravel and treat that as the amount needed — you always want a little extra on site rather than running short mid-project. Typical DIY drains run 12 inches wide and 18–24 inches deep, which is the default this tool assumes.

Trench volume (cu ft) = Length(ft) × (Width in ÷ 12) × (Depth in ÷ 12)
Cubic yards of gravel = Trench volume ÷ 27
Gravel weight (tons) = Trench volume × density (≈105 lb/ft³) ÷ 2000
Pipe pieces = Trench length ÷ piece length (rounded up)

Example: a 40 ft trench that is 12 in wide and 18 in deep is 40 × 1 × 1.5 = 60 cubic feet, or 60 ÷ 27 = 2.22 cubic yards of gravel — about 3.15 US tons.

How much gravel for a 40-foot french drain? At 12 in wide and 18 in deep that is 60 cubic feet, or 2.22 cubic yards (≈3.15 tons) — plus four 10-ft sticks of perforated pipe.

How much gravel for a 20-foot french drain? Same 12 in × 18 in trench gives 30 cubic feet, or 1.11 cubic yards (≈1.58 tons).

A wider, deeper drain: a 50 ft trench dug 18 in wide and 24 in deep works out to 50 × 1.5 × 2 = 150 cubic feet, or 5.56 cubic yards (≈7.9 tons) of stone.

- Round up your order. Gravel settles and compacts, so add 5–10% in *More options* rather than ordering the exact trench volume.
- Use the right stone. Washed #57 or 3/4-inch crushed stone drains best; avoid fines and dust that clog the drain over time.
- Wrap it in fabric. Line the trench with landscape/filter fabric so soil doesn't migrate into the gravel and choke the pipe.
- Slope the pipe. Aim for about 1 inch of fall per 10 feet so water actually moves toward the outlet.
- Buying by the ton? Divide pounds by 2,000 — the calculator already shows the tonnage so you can compare a per-yard vs per-ton quote.