Calculators / Rainwater and drainage
How much gravel for a French drain?
Put in the trench length, width and depth, the pipe and the slope. You get the gravel, pipe and fabric to buy and the fall end to end.
Updated · How we check our numbers
Your French drain
3.5 tons of gravel
A 50 ft trench 12 in wide and 18 in deep with 4 in perforated pipe: about 2.6 cu yd of gravel, 300 sq ft of fabric, and 3 in of fall end to end at 0.5% (⅝ in per 10 ft).
- Gravel
- 2.6 cu yd3.5 tons
- Pipe
- 50 ft4 in perforated
- Fabric
- 300 sq ft
- Fall
- 3 in0.5% (⅝ in per 10 ft)
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Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
Where to send the waterA dry well or rain garden
trench = 50 ft × 12/12 × 18/12 = 75 cu ft
pipe = 50 × π × (4/24)² = 4.4 cu ft
gravel = (75 − 4.4) ÷ 27 = 2.6 cu yd × 1.35 = 3.5 tons
fabric = sides, bottom and a top overlap ≈ 300 sq ft
Fall by slope
50 ft run- 0.2 in per 10 ftADS homeowner guide (minimum)1 in fall
- 4 in per 100 ftUniversity of Missouri Extension EQ 4012 in fall
- 0.5% (⅝ in per 10 ft)Minnesota PCA underdrains3 in fall
- 1% (1¼ in per 10 ft)Steeper drains faster6 in fall
Daylight the pipe downhill, into a dry well or rain garden, or to a storm connection your town allows, never onto a neighbor. NRCS uses a solid (unperforated) pipe for the last 10 ft, ending at least a foot above the ditch bottom.
Line the trench with landscape fabric, lay a few inches of gravel, set the pipe, fill with gravel and fold the fabric over the top (Fairfax County).
Call 811 before you dig. Some cities also want a permit for drainage work.
How we work it out
gravel = trench volume − pipe volume
Fairfax County, Virginia describes a standard French drain as about 1½ ft deep and 10 to 12 in wide with a 4 to 6 in perforated pipe, lined with landscape fabric; Delaware's DNREC calls 8 to 24 in deep yard drains a DIY job; ADS's detail drawing sets 8 in by 8 in as the smallest trench.
Published minimum slopes run from ADS's 0.2 in per 10 ft to University of Missouri Extension's 4 in per 100 ft and Minnesota's 0.5% for underdrains; more fall drains faster. NRCS's subsurface drain standard ends a drain in at least 10 ft of solid pipe, a foot above the ditch bottom.
Where the numbers come from
- Fairfax County, VA: drainage problems and runoff
- Delaware DNREC: French drain for a yard
- ADS: French drain detail STD-106A
- ADS: solving home drainage problems
- University of Missouri Extension EQ 401: curtain drains
- NRCS Conservation Practice Standard 606: Subsurface Drain (2022)
- Minnesota PCA: permeable pavement (underdrain slope)
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