How the gravel volume is worked out
A French drain is a gravel-filled trench with a perforated pipe at the bottom, wrapped in filter fabric so silt cannot clog the stone. The gravel volume is trench length × width × depth, converted to cubic yards by dividing by 27 — then we subtract the space the pipe itself occupies, which is π × radius² × length. On a 50 ft run of 4 inch pipe that displacement is only about a sixth of a cubic yard, but on a long 6 inch run it is worth accounting for. Weight is estimated at roughly 1.4 tons per cubic yard, since drainage stone is almost always sold and delivered by the ton rather than by volume.
Use washed, angular stone in the 3/4 inch to 1-1/2 inch range: rounded pea gravel packs tight and drains slower, and unwashed stone brings its own fines. Lay the fabric first, wide enough to line the trench and fold over the top with an overlap — the calculator tells you the minimum roll width for your trench, which is the width plus twice the depth plus about a foot of overlap. Set the perforated pipe holes facing down so water entering the stone bed can rise into the pipe, keep a fall of at least 1 inch per 8 feet toward the outlet, and make sure the outlet actually daylights somewhere lower than the trench. A drain that ends nowhere is just a buried reservoir.
Stone weight and price vary by quarry and stone type, and site conditions change how wide or deep the trench really needs to be. Check local code for outlet and discharge rules, and have buried utilities located before you dig.