French Drain Calculator
Plan the materials for a French drain. Enter the trench length, width and depth and the pipe size to get the drain gravel in cubic yards and tons, the perforated pipe to buy, the landscape fabric to line the trench and how much the run should fall.
Last updated:
Usually 9–12 in, or pipe diameter + 6–8 in
Average depth; 18–24 in is common
0 if gravel goes to the surface
At least 1% (1/8 in per ft); 1 in per 8 ft is also common
US tons per cubic yard
For settling, uneven trench walls and spillage
How to use the French Drain Calculator
- Enter the trench length, width and depth, and any soil cap on top.
- Pick the pipe size and how it is sold, or choose no pipe.
- Choose the drain rock and how the fabric will line the trench.
- Set the slope and a gravel allowance.
- Read the gravel, pipe, fabric and digging volume.
French drain formulas
- Gravel (ft³) = length (ft) × width (ft) × gravel depth (ft) − pipe volume
- Pipe volume (ft³) = π × (outside diameter in ÷ 24)² × length
- Cubic yards = ft³ ÷ 27; tons = yd³ × density (≈ 1.4 t/yd³ for #57 stone)
- Fabric strip width = 2 × width + 2 × depth + overlap (for a wrapped “burrito” drain)
- Fall = length × slope (1% = 1/8 in per ft)
Example: 50 ft × 12 in × 18 in with a 4 in pipe. Trench volume 50 × 1 × 1.5 = 75 ft³; the pipe (about 4.5 in outside) takes 5.5 ft³; with 10% extra the gravel is 76.4 ft³ = 2.83 yd³, about 4 tons. Pipe: 53 ft, so six 10 ft sections. Fabric: a 72 in strip, 52.5 ft long.
Gravel per 10 ft of trench (with 4 in pipe, no extra)
| Width × depth | ft³ per 10 ft | yd³ per 10 ft | Tons per 10 ft (1.4 t/yd³) |
|---|---|---|---|
| 9 × 12 in | 6.4 | 0.24 | 0.33 |
| 12 × 18 in | 13.9 | 0.51 | 0.72 |
| 12 × 24 in | 18.9 | 0.70 | 0.98 |
| 18 × 24 in | 28.9 | 1.07 | 1.50 |
| 24 × 36 in | 58.9 | 2.18 | 3.05 |
Building tips
- Use clean, washed, angular stone; fines and sand clog the drain over time.
- Put 2–3 in of gravel under the pipe, set the pipe on a steady slope, then fill around and over it.
- The outlet needs somewhere to go: daylight on lower ground, a dry well, or an approved storm connection.
- Check the slope with a string line and line level, or a laser level, as you dig.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply length × width × gravel depth in feet, subtract the pipe, and divide by 27 for cubic yards. A 50 ft trench 12 in wide and 18 in deep holds 75 ft³; minus about 5.5 ft³ for a 4 in pipe and plus 10% extra, that is about 2.8 yd³, or roughly 4 tons of #57 stone.
How many tons is a cubic yard of drain gravel?
Washed #57 stone and pea gravel weigh roughly 1.35–1.5 US tons per cubic yard; the calculator uses 1.4. Wet or finer material weighs more, so ask your supplier for the weight of their product.
How deep and wide should a French drain be?
Many yard drains are about 12 in wide and 18–24 in deep. Foundation drains are set at or near the footing. The trench should be at least 6 in wider than the pipe so gravel surrounds it.
What slope does a French drain need?
A minimum of about 1% (1/8 in per foot, or 1 in every 8 ft) so water flows to the outlet. Over 50 ft that is a fall of about 6 in. Steeper is fine as long as the outlet can daylight or reach a dry well.
How much landscape fabric do I need for a French drain?
To line the trench and fold the fabric over the top, you need a strip about 2 × width + 2 × depth + 12 in of overlap. For a 12 × 18 in trench that is 72 in, so a 6 ft wide roll, or two runs of 3 or 4 ft fabric, the length of the trench.
Should French drain pipe holes face up or down?
Most guidance says to install perforated drain pipe with the holes facing down, so water rises into the pipe from the bottom and sediment is less likely to settle in it. Follow the pipe maker's instructions.
Do I need a permit or to call before I dig?
Always contact your local utility locator service before digging, and check local rules about where you may discharge water; you should not direct runoff onto a neighbor's property.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.