How Much Gravel Do I Need for a French Drain?

The short answer

The standard residential French drain — a 10×12 inch trench with 4-inch pipe — takes about 0.75 cubic feet of gravel per linear foot. Multiply by your run length and divide by 27: a 100-foot drain needs about 2.8 cubic yards (order 3). The formula: trench cross-section in square feet × length in feet, minus the pipe's volume, divided by 27.

Gravel per linear foot, by trench size

Cubic feet per foot is the number to remember, because every drain length scales from it. The pipe's own volume is subtracted (a 4-inch pipe displaces about 0.09 cu ft per foot; a 6-inch pipe about 0.20):

Suppliers sell whole and half yards, so round up — and add about 10% overage, because hand-dug trenches are never perfectly uniform. If your run lengths don't match the columns, our gravel calculator handles any trench size.

Worked example: an 80-foot drain

An 80-foot run with a 12×18 inch trench and 4-inch pipe: 1.41 cu ft per foot × 80 = 113 cubic feet — that's 4.19 cubic yards. Order 4.5 to 5 yards (suppliers round to half yards, and the overage covers trench irregularities). In tons — how bulk stone is often priced — that's 4.5 × 1.4 = 6.3 tons.

Choose the right stone: #57, not pea gravel

The standard is #57 stone: angular, washed crushed rock roughly ½ to 1 inch. The angular faces lock together while leaving big voids for water to move through — that's the whole point of the drain. The common mistake is rounded pea gravel: the round stones migrate, pack tighter over time, and shed fines that clog the system. Washed matters too — unwashed “crusher run” carries dust that can seal the voids. If your supplier asks what you need, “clean #57” is the magic phrase.

Yards to tons for ordering

Bulk stone is often priced per ton: gravel weighs about 2,800 lb per cubic yard, so multiply yards by 1.4. Quick conversions for the standard 10×12 inch trench with 4-inch pipe:

Install: fabric wrap and 1% slope

Quantity is only half the job — a drain installed wrong stops working. Line the trench with non-woven geotextile fabric before any stone goes in, and wrap it over the top burrito-style after backfilling. The fabric keeps silt from infiltrating the voids; skip it and the drain silts up in a few years. (We cover fabric selection in detail in our landscape fabric under rock guide.) Keep a 1% slope — about 1 inch of drop per 8 feet — toward the outlet the entire run, lay the perforated pipe holes-down, bed it on 2 inches of stone, cover it with stone, then wrap the fabric closed and top with soil or sod.

Where does the water go?

A French drain is only as good as its outlet. The three standard endings: daylight — the pipe exits downhill above grade (cheapest and most reliable, if the slope allows); a dry well — a buried pit of stone that the pipe drains into when there's nowhere downhill to go; or a sump pump basin for flat sites. The outlet does not change your gravel quantity much — budget an extra half yard of #57 for a dry well's surround — but decide it before you dig, because a drain with no exit just moves the puddle.

Call 811 before you dig

This costs nothing and avoids cutting a gas or fiber line. In most US states you call 811 a few business days before digging and utilities mark their buried lines for free. Some municipalities require a permit for drainage work that alters runoff, especially near property lines — a five-minute call to your city's permits desk settles it.

How much fabric do you need?

Size the fabric from the trench's perimeter. A 12×18 inch trench has about 3.5 feet of perimeter (12 + 18 + 18 inches, plus a few inches of overlap at the top) — so a 100-foot run needs roughly 350 square feet of fabric, or one 4×100-foot roll with overlap to spare. Buy non-woven geotextile rated for drainage (cheap woven landscape weed barrier can clog); 4-ounce non-woven is the common residential choice. Overlap sections by 6–12 inches rather than cutting separate pieces for each side — the burrito wrap needs to be continuous to keep silt out.

Common mistakes

Rounded stone. Pea gravel migrates and compacts — #57 angular is the standard. Skipping fabric. Silt kills drains. Pipe holes up. Holes go down so water rises into them and exits. Flat runs. Water doesn't move without slope — check it with a level as you go. Ordering exact yards. Hand-dug trenches wander; 10% overage is cheaper than a second delivery fee.