How it works
The calculator fills the trench with gravel, subtracts the volume of the pipe, counts pipe pieces, and sizes the filter fabric from the trench perimeter plus the top overlap.
Gravel depth = Trench depth − Soil cover
Gravel = (Length × Width × Gravel depth − π × (Pipe OD ÷ 2)² × Length) × (1 + Extra ÷ 100)
Gravel weight = Gravel × Weight per volume
Pipe pieces = ⌈ Length ÷ Length per piece ⌉
Couplings = Pipe pieces − 1
Fabric width = 2 × Gravel depth + Width + min(Overlap, Width)
Fabric area = Fabric width × Length
⌈ ⌉ means rounding up to whole pieces. Gravel is not rounded; suppliers sell it in their own increments.
Gravel and pipe
The pipe sits in the gravel, so the space it takes is not filled with stone. The calculator subtracts a full cylinder of the pipe’s outside diameter – ADS gives 4.6 in (117 mm) for its 4 in single wall pipe – along the whole trench. Perforated pipe comes in rigid lengths and in coils: ADS lists its TripleWall pipe in 10 ft (3 m) lengths and its single wall pipe in coils from 100 ft (30 m), joined with snap or split couplers.
Filter fabric
The Mirafi installation guideline for French and trench drains says to cut the geotextile wide enough to conform to the trench perimeter with at least a 12 in (30 cm) top overlap, or the trench width if that is less, place 3–6 in of drainage aggregate before the pipe, fill the trench and fold the leftover fabric over the stone. If you cover the gravel with soil, the fabric wraps only the gravel layer, so the calculator uses the gravel depth for the sides.
What it does not decide
How deep the drain goes, how much it falls and where it ends are site questions. The calculator takes your trench size as given and returns the materials for it.
Worked example
Inputs
- Trench length
- 50 ft
- Trench width
- 12 in
- Trench depth
- 18 in
- Pipe outside diameter
- 4.6 in
- Pipe length per piece
- 10 ft
- Gravel weight per volume
- 1.4 tons/cu yd
- Fabric overlap on top
- 12 in
- Extra gravel
- 0 %
Result
- Gravel
- 2.56 cu yd (69.23 cu ft)
- Gravel weight
- 3.59 short tons (7,179 lb)
- Perforated pipe pieces
- 5
- Couplings
- 4
- Filter fabric width
- 60 in (5 ft)
- Filter fabric area
- 250 sq ft
Common mistakes
- Forgetting the fall. A French drain needs a continuous slope to a place where the water can leave; the trench depth grows along the run.
- Ordering gravel by the trench volume when part of the trench is topsoil. Enter the soil cover so only the gravel layer is counted.
- Cutting filter fabric to the trench width. It lines both sides and the bottom and folds over the top, so it is several times wider than the trench.
- Using a pipe length that leaves the last piece short. Pieces are rounded up, and each joint between pieces needs a coupling.
- Laying pipe on the trench bottom. The Mirafi guideline places 3–6 in of drainage aggregate under the pipe before it goes in.
Assumptions and limits
- The trench is a straight rectangle in section with the same width and depth along its length.
- Gravel = trench volume of the gravel layer minus the pipe’s outside volume, plus the extra you enter. Without a pipe diameter the whole layer is gravel.
- Weight = volume × the density you enter. No density is assumed.
- Pipe pieces = trench length ÷ length per piece, rounded up; couplings = pieces − 1. Fittings, end caps and outlet pipe are not included.
- Fabric width = 2 × gravel depth + trench width + top overlap (no wider than the trench), following the Mirafi guideline; fabric area = width × trench length. Roll-end overlaps are not added.
- Quantities only. Drain layout, slope, outlet and depth are site decisions for you, local rules or a professional.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply the trench length by its width and the depth of the gravel layer, then subtract the space the pipe takes. A 50 ft trench 12 in wide and 18 in deep holds 75 cu ft; a 4 in pipe with a 4.6 in outside diameter takes about 5.8 cu ft, leaving about 69 cu ft, or 2.56 cu yd, of gravel.
How wide should the filter fabric be for a French drain?
Wide enough to line the trench and lap over the top. The Mirafi guideline says to cut the fabric to the trench perimeter plus at least a 12 in top overlap or the trench width, whichever is less. For a 12 × 18 in trench that is 18 + 12 + 18 + 12 = 60 in.
Should I use rigid or flexible perforated pipe?
Both are sold for drainage. ADS sells its single wall corrugated pipe in coils from 100 ft and its TripleWall perforated pipe in 10 ft lengths. Coils need fewer couplings; rigid lengths hold their grade more easily. Choose for your site and enter the length of the product you buy.
How many tons is a cubic yard of drain rock?
It depends on the stone. A Wisconsin DOT study found compacted aggregate at about 1.5–2.2 tons per cubic yard depending on gradation, and loose material weighs less. Ask your supplier for the figure for the product.
How deep should a French drain be?
That depends on what it drains and where the water goes, so this calculator does not set it. Enter the depth you or your contractor have planned, and call before you dig to have buried utilities marked.
Sources
- Single Wall Pipe — Advanced Drainage Systems (ADS). Annular corrugated single wall pipe 3–15 in, "100'-5,500' (30-1676 m) coils, depending on pipe diameter. 10'-20' (3-6 m) sticks"; internal, external snap or split band couplers.
- S3.01 Single Wall Pipe Specification — Advanced Drainage Systems (ADS). Nominal 4 in (100 mm) I.D. pipe: average O.D. 4.6 in (117 mm) (reference value; 3 in 3.6, 6 in 7.0); all diameters with or without perforations; joints with split or snap couplings.
- HDPE Drainage Products catalog — Advanced Drainage Systems (ADS). ADS 3000 TripleWall pipe "in 3” (75 mm) and 4” (100 mm) diameters in 10’ (3 m) lengths, either solid or with standard 5/8” perforations"; single wall pipe in coils, perforated or non-perforated.
- Installation Guideline – Geotextiles used in filtration and drainage applications (2023) — Solmax (Mirafi). French and trench drains: trench minimum 8 in (20 cm) wide; geotextile width "enough to conform to the trench perimeter with at least a 12 in (30 cm) top overlap or the trench width (whichever is less)"; 3–6 in of aggregate under the pipe; fold leftover geotextile over the aggregate.
- Aggregate unit weights – WisDOT research report 0092-22-05 — Wisconsin Department of Transportation. Compacted aggregate unit weights of about 1.5–2.2 tons per cubic yard depending on gradation; loose material weighs less.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 6, 2026