French Drain Gravel Calculator
Work out how much gravel your French drain trench needs. It starts with a 50 ft trench, 1 ft wide, filled to 14 inches deep.
1. Measure your area
2. Choose your gravel
Gravel density varies by material, moisture and supplier. Use your supplier's stated density when available. This calculator provides an estimate, not a supplier quote.
What a French drain is
A French drain is a trench filled with gravel, usually with a perforated pipe along the bottom, that collects water and carries it away from a wall, patio or low spot. Water seeps through the gravel into the pipe, and the pipe moves it downhill to a place where it can drain safely. The gravel is the bulk of the material you buy, so working out the quantity is the first step in budgeting the job.
How to measure for a French drain
Treat the trench as a long, narrow box. Measure the length of the run, the width of the trench and the depth of gravel you will place. In the calculator above, enter the length of the trench, the trench width in feet (for example 1 for 12 inches), and the gravel depth in inches. The gravel depth is the trench depth minus the layer of soil or turf you plan to put back on top, which is often a few inches.
Trenches are commonly around 6 to 12 inches wide and 12 to 24 inches deep, but the right size depends on your soil, how much water you need to move and local rules. Before you dig, call 811 (the US utility locator) so buried lines are marked, and check whether your area has rules about where drainage water may be sent.
Which gravel for a French drain?
Drains work best with clean, washed stone with little or no dust, commonly in the range of about 3/4 to 1-1/2 inches, because water passes through the gaps between the pieces. Fine material and sand can clog the drain. Many installers also line the trench with filter fabric before filling it, so the surrounding soil cannot wash into the gravel. Clean stone has more gaps than mixed stone and can weigh a little less, so the figures here lean slightly high, which errs on the side of having enough. If your supplier gives a density, use the Custom density option.
Worked example: a 50 ft drain, 1 ft wide
Trench: 50 ft long × 1 ft wide, gravel filled to 14 inches deep (an 18-inch trench with about 4 inches of soil on top).
Volume: 50 × 1 × (14 ÷ 12) = 58.33 cubic feet
Add 10% for waste and settling: = 64.17 cubic feet = 2.38 cubic yards
Weight at 110 lb per cubic foot: 7,058 lb = 3.53 tons
Should I subtract the pipe?
The pipe takes up space that would otherwise be gravel. A 4-inch pipe along this 50 ft trench occupies about 4.36 cubic feet, roughly 7% of the trench volume. The calculator does not subtract it, and the 10% waste allowance more than covers the difference. It is cheap insurance, because you will lose some gravel to uneven trench walls and settling.
Gravel per 100 feet of French drain
Quantities for common trench sizes at 110 lb per cubic foot, with no waste allowance. Add 10% for a real job.
| Trench width | Gravel depth | Cu ft per foot | Cu yd per 100 ft | Tons per 100 ft |
|---|---|---|---|---|
| 6 in | 12 in | 0.50 | 1.85 | 2.75 |
| 8 in | 12 in | 0.67 | 2.47 | 3.67 |
| 12 in | 12 in | 1.00 | 3.70 | 5.50 |
| 12 in | 18 in | 1.50 | 5.56 | 8.25 |
| 18 in | 18 in | 2.25 | 8.33 | 12.38 |
Slope and outlet
A drain only works if water can flow out of it. A gentle, steady slope is usual, commonly about 1 inch of drop for every 8 to 10 feet of pipe, and the end of the pipe needs somewhere suitable to discharge. This calculator covers the gravel only. For the slope, pipe and outlet, follow your supplier's or a contractor's guidance, especially if the drain is protecting a building.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply the trench length by the width and the gravel depth, then add about 10% for waste. A 50 ft drain that is 1 ft wide and filled to 14 inches deep needs roughly 2.4 cubic yards, or about 3.5 tons. Enter your own measurements in the calculator above.
How much gravel per foot of French drain?
A trench 12 inches wide with 12 inches of gravel needs 1 cubic foot of gravel per foot of drain, roughly 110 lb. The table above lists other trench sizes.
What size gravel is best for a French drain?
Clean, washed stone in roughly the 3/4 to 1-1/2 inch range is common, because water flows freely through it. Avoid stone with a lot of dust, which can clog the drain. Ask your supplier what they recommend for drainage.
Do I need filter fabric?
Many installers wrap the gravel in filter fabric so soil cannot work into the stone and clog it over time. It depends on your soil, so ask a local supplier or contractor.
How deep should a French drain be?
Commonly 12 to 24 inches, but it depends on where the water is coming from and local conditions. Enter the gravel depth, not the full trench depth, in the calculator if you are putting soil or turf back on top.
Do I subtract the pipe volume?
You can, but it is a small amount, usually less than the 10% waste allowance. For most jobs it is simpler to keep the allowance and order the full quantity.
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Last updated 2026-10-06. Quantities are estimates; densities and coverage vary by material, moisture and supplier.