How this calculator works
It finds the trench volume from its length, width and depth, converts that to cubic yards and tons of drain rock (at about 1.4 tons per cubic yard), and adds 10% to the trench length for the perforated pipe to cover fittings and waste.
Worked example — a 50 ft trench, 12 in wide, 18 in deep:
50 × 1 × 1.5 = 75 cubic feet → ÷ 27 = 2.78 cubic yards → × 1.4 = 3.89 tons of gravel, plus 55 ft of perforated pipe.
50 × 1 × 1.5 = 75 cubic feet → ÷ 27 = 2.78 cubic yards → × 1.4 = 3.89 tons of gravel, plus 55 ft of perforated pipe.
How deep and wide should a French drain trench be?
Most residential French drains use a trench 12–18 inches wide and 18–24 inches deep, sloped at roughly a 1% grade (about 1 inch of drop per 8 ft) toward the outlet so water keeps moving.
Frequently asked questions
How much gravel do I need for a French drain?
About 1.4 tons per cubic yard of trench volume — for a typical 12in-wide, 18in-deep trench that’s roughly 0.08 tons per linear foot.
What size perforated pipe is used for a French drain?
Most residential drains use 4-inch perforated pipe, wrapped in fabric sock and surrounded by gravel to filter out sediment.