Trench dimensions
Estimated material cost:
Cross-section diagram
Illustrative trench view: top cover, gravel envelope and pipe. The diagram updates to show the gravel bed you specify.
Illustrative diagram, not a hydraulic drainage design.
What this estimate includes
Excavation: length × width × full trench depth. Gravel envelope: length × width × (depth minus top cover). If a pipe is included, its outside cylindrical displacement is subtracted from the gravel envelope, not the empty volume inside the pipe.
Full-wrap geotextile: to cover the bottom, both sides and top of the gravel, the required fabric width is 2 × (trench width + gravel height) + overlap. Multiply this by the trench length. This assumes a continuous rectangular wrap; available roll widths, joins between fabric lengths and cutting losses can require more material.
Before ordering materials
Confirm the aggregate bulk density, actual bulk-bag weight and pipe outside diameter with your supplier. Subtract the soil cover from the gravel-filled depth if the trench will be finished with turf or topsoil. Excavation volume is not the same as gravel order volume.
Does this size the drainage system?
No. This tool estimates materials only. Gradient, hydraulic capacity, discharge destination, ground conditions and geotextile selection require site-specific assessment. Check for underground utilities and excavation safety before digging.
How we calculate: formulas and sources
Geometric formulas use the values you enter. Material defaults are editable estimates: no one density, seam overlap or layout is appropriate for every job.
View formulas, default sources and limitations
Equations and geometric model
Excavation (m³) = L × W × full depth. Gravel envelope (m³) = L × W × (depth − top cover), all in metres.
Pipe displacement (m³) = π × (outside diameter / 2)² × L. Net gravel = envelope − displacement. Pipe length equals trench length, with no fittings allowance.
Ordered gravel = net gravel × (1 + allowance/100). Weight = ordered gravel × entered bulk density. Bag counts round up using the entered bag weight.
Developed geotextile width (m) = 2 × (trench width + gravel fill height) + closure overlap. Order area (m²) = developed width × trench length × (1 + allowance/100). This assumes a single continuous rectangular wrap, without lengthwise joins or extra roll-cutting waste.
Default assumptions and their origin
Example density: 1,600 kg/m³. Áridos Internacionales lists this approximate value for its Mediterranean gravel, but ~1,000 kg/m³ for volcanic gravel. Check the supplier's actual bulk-density specification instead of assuming a universal figure.
Example overlap: 30 cm. DRAINTEX installation guidance describes 300 mm overlap and full-wrap drainage applications. Texel illustrates a 300 mm top wrap for French drains. Follow your project's design and selected manufacturer's requirements.
Example bedding depth: 5 cm. This is a configurable geometry assumption, not a universal design minimum. 10% order allowance and 1,000 kg/bag are planning examples, not guaranteed supply characteristics.
References
- U.S. DOE / PNNL — Footing Drain Pipe — clean gravel, pipe and fabric; U.S. footing-drain context, not a general standard for all sites.
- DRAINTEX — Installation Guidelines (PDF) — complete wrapping and reference overlaps.
- Texel — Geotextile Installation Guide (PDF) — illustrated French-drain detailing.
- Áridos Internacionales — Grava Mediterránea — commercial example of ~1,600 kg/m³ bulk density.
- Áridos Internacionales — Grava Volcánica Marrón — commercial example of ~1,000 kg/m³.
Sources reviewed 21 Oct 2026 · Internal formula review, not an external technical certification.
Limitations: Uniform rectangular trench and straight pipe fully inside the gravel envelope only. No gradients, water flow, hydraulic capacity, sloped trench walls, joined fabric panels, fittings, compaction, taxes or delivery. Excavation safety and drainage design require site-specific assessment.
How to calculate drainage trench materials
Excavation volume and the gravel-filled part of the trench are calculated separately. The outside volume of the pipe is subtracted from the gravel envelope where a pipe is included. Filter fabric is estimated for a full wrap around the gravel section: bottom, both sides, top and seam overlap. The purchase allowance is applied to gravel and fabric, not to the pipe length. Enter your supplier's actual aggregate density and prices when available. This is a material estimate, not a hydraulic, outlet or slope design. You can specify a lower gravel bed, check geometric pipe clearances and compare required fabric width with the available roll. View the method and cited references. Defaults are examples, not universal design parameters.
Results are estimates and depend on the supplied values.