Home › Guides › French Drain Calculator
French Drain Calculator
Quick answer: Gravel = (length × width × depth) − pipe volume. Use washed angular 3/4″–1-1/2″ stone, wrap the fabric right around the gravel, and give it at least 0.5% fall — 6″ per 100 ft.
Stone volume and tonnage, pipe length, and the fabric width you actually need to wrap the trench — plus the slope and stone specification that decide whether the drain still works in ten years.
Results update automatically as you type.
What the numbers mean
A French drain is a gravel-filled trench with a perforated pipe at the bottom, wrapped in filter fabric. The gravel volume is the trench volume minus the space the pipe occupies — which is what the calculator does rather than ignoring the pipe, as most estimates do.
Fabric goes around the gravel, not just under it
The fabric’s job is to keep soil fines out of the gravel. Fines are what clog a French drain, and they arrive from all sides — so the fabric must line the trench and fold over the top of the gravel before backfill. Lining only the bottom is the single most common reason a French drain stops working within a few years.
The fabric width you need is the trench width plus twice the depth plus overlap for the fold — which is what the calculator returns.
Slope
| Slope | Drop per 100 ft | Verdict |
|---|---|---|
| 1% | 12″ | Ideal where you have the fall available |
| 0.5% | 6″ | The practical minimum |
| Under 0.5% | Under 6″ | Water sits; needs a different solution |
A French drain is gravity-driven. Without continuous fall to a legitimate outlet — daylight on a slope, a dry well, or a storm connection where permitted — it becomes a trench that stores water beside your foundation.
Pipe: perforated, and which way up
Use rigid perforated PVC rather than corrugated flexible pipe where you can. Corrugated is cheaper and easier to lay but its ridges trap sediment and it is far harder to snake out later.
Stone
Use washed, angular stone, 3/4″ to 1-1/2″. Washed matters because unwashed stone brings its own fines. Angular matters because it locks together and leaves consistent voids; rounded pea gravel compacts and reduces flow. Do not use crusher run or anything with fines in the mix — it will set up almost like concrete.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply length by width by depth in feet, then subtract the volume the pipe occupies. A 60 ft trench 12 in wide and 24 in deep holds 120 cubic feet; a 4 in pipe displaces about 5 cubic feet, leaving roughly 4.3 cubic yards of stone.
Should French drain pipe holes face up or down?
Down. Water rises into the pipe from the surrounding gravel as the water table climbs, so downward-facing holes let the pipe begin draining at the lowest possible level. Holes facing up means water has to rise nearly to the top of the pipe before it starts moving.
What slope does a French drain need?
At least 0.5%, which is 6 inches of fall per 100 feet. One percent — 12 inches per 100 feet — is better where you have the fall available. Without continuous fall to a real outlet the trench simply stores water.
What size gravel for a French drain?
Washed, angular stone between 3/4 in and 1-1/2 in. Washed matters because unwashed stone carries fines that clog the drain, and angular matters because it locks together and holds consistent voids. Avoid crusher run and anything containing fines.