French Drain Calculator
Work out the gravel, fabric, pipe and fall for a French drain. Enter the length, trench width and depth; the pipe volume comes out of the gravel.
How to Use
- Enter the length of the drain and the trench width and depth.
- Choose the perforated pipe size; its volume is taken out of the gravel.
- Choose the slope: 1% (⅛ in per foot) is the usual minimum, 2% clears faster.
- Adjust the fabric overlap at the top and the gravel weight per cubic yard if your supplier gives a different figure.
- Read the gravel by volume and weight, the fabric, the pipe and how much lower the outlet must be.
Worked Example
A 50 ft French drain, 12 in wide and 18 in deep, with a 4 in perforated pipe at 1%. Trench: 50 × 1 × 1.5 = 75 ft³. Pipe: π × (2 in)² = 12.57 in² = 0.0873 ft², × 50 ft = 4.36 ft³. Gravel: 75 − 4.36 = 70.6 ft³ = 70.6 ÷ 27 = 2.62 yd³, and at 1.4 tons per cubic yard, 3.66 tons.
Fabric: bottom 12 in + two sides of 18 in + top 12 in + 12 in overlap = 72 in = 6 ft wide, × 50 ft = 300 ft². Pipe: 5 lengths of 10 ft, or half a 100 ft coil. Fall: 1% of 50 ft = 0.5 ft = 6 in, so the outlet end of the trench bottom is 6 in lower than the start. The soil dug out, 75 ft³, is 2.8 yd³ before it bulks up.
The common mistake: ordering gravel by the trench length alone. “A yard every 20 feet” is right for a 12 × 18 in trench but doubles for a 24 in wide one. Always work from width × depth × length.
Show Work
Formulas
Henry French’s Drain
The French drain is not French. It is named after Henry Flagg French, a Massachusetts lawyer and farmer, who described gravel-filled trenches for draining wet fields in his 1859 book Farm Drainage. Farmers had long laid clay tiles or stones in trenches; French’s book set out how deep, how far apart and how steep to make them.
The modern version adds two inventions: perforated plastic pipe, which carries water away far faster than gravel alone, and non-woven geotextile fabric, which keeps fine soil out of the stone. With both, a properly sloped drain can work for decades.
About This Calculator
This calculator works out the materials for a French drain, curtain drain or footing drain: the gravel by volume and weight with the pipe’s volume removed, the filter fabric to wrap the trench, the pipe and the fall over its length, in feet and inches or metres. It draws the cross-section to scale.
Everything runs in your browser; nothing is sent anywhere. Pipe volume uses the nominal diameter; corrugated pipe is slightly larger outside, so the gravel figure has a small margin built in. Gravel weights vary by stone; ask your supplier for theirs.
Related calculators: Gravel Calculator, Excavation Calculator, and Slope & Grade Calculator.
Frequently Asked Questions
How much gravel do I need for a French drain?
The trench volume less the pipe. A 50 ft trench 12 in wide and 18 in deep holds 75 ft³; a 4 in pipe takes 4.4 ft³ of it, leaving 70.6 ft³, 2.62 yd³. At about 1.4 tons per cubic yard, that is 3.7 tons of washed stone.
How deep and wide should a French drain be?
Most yard drains are 6 to 12 in wide and 12 to 24 in deep, deep enough to intercept the wet layer and leave a few inches of gravel under the pipe. Footing drains sit beside the footing, with the pipe at or just below the bottom of the slab. Call your utility locator before digging.
How much slope does a French drain need?
At least 1%, which is ⅛ in per foot or 1 in in 8 ft. Over 50 ft the outlet must be 6 in lower than the start. 2% (¼ in per foot) carries silt away better; very flat drains slowly fill with sediment.
What gravel should I use?
Washed, clean stone of ¾ to 1½ in, often sold as #57 stone or drainage aggregate. Avoid pea gravel and crushed stone with fines, which pack tightly and slow the water. Washed stone weighs about 1.3 to 1.5 tons per cubic yard (1.5 to 1.8 t/m³).
Do I need landscape fabric?
Yes, in most soils. Non-woven geotextile lines the trench and folds over the top of the gravel, so soil cannot wash into the stone and clog it. Allow the width of the trench bottom, both sides and the top, plus about a foot of overlap.
How do I use the French Drain Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Yard drain
50 ft, 12 in wide, 18 in deep, 4 in pipe: 2.62 yd³ of gravel, 3.7 tons, 300 ft² of fabric.
Footing drain
120 ft around a house, 12 × 12 in: 4.06 yd³ of gravel and 600 ft² of fabric; 14.4 in of fall at 1%.
Narrow drain
30 ft, 6 in wide, 12 in deep, 3 in pipe: half a cubic yard of gravel, 0.70 tons.
Metric
15 m, 300 × 450 mm, 100 mm pipe: 1.91 m³ of gravel, 27 m² of fabric, 150 mm of fall.
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