French Drain Calculator — Gravel, Fabric and Pipe
Estimate the primary materials for a straight French drain trench. Enter finished trench dimensions and a gravel displacement allowance for the pipe and open space.
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View all saved project estimatesWhat does the french drain calculator calculate?
Multiply trench length, width, and gravel depth to find gross volume, then reduce it for pipe displacement and add waste. The same dimensions estimate pipe length and filter fabric. This calculator plans materials only; slope, outlet capacity, soil, utilities, and local drainage rules require site-specific review.
French drain material formula
Width, depth and overlap convert from inches to feet before calculating trench volume and the fabric wrap. Pipe displacement removes the estimated non-gravel portion. The allowance covers uneven excavation, settlement, fabric cuts, handling loss and supplier ordering increments.
How to measure for an accurate estimate
- Mark the proposed route and measure its centerline length, including bends that will contain pipe.
- Measure the planned finished trench width and gravel depth rather than the pipe diameter alone, then confirm how much fabric overlap closes the wrap.
- Identify the high point, low point, and outlet elevation to verify that a continuous fall is physically possible.
- Call the local utility-locating service and confirm setbacks before treating the marked route as buildable.
Fifty-foot yard drainage trench
A 50 ft trench is 12 in wide and 18 in deep, with 12 in of fabric overlap, 10% displacement and a 10% material allowance.
The estimate is 2.75 yd³ of gravel, about 330 sq ft of filter fabric, and 55 linear ft of pipe.Before you purchase material
- Choose clean, washed, angular drainage aggregate with minimal fines rather than compactable base material.
- Use filter fabric and perforated pipe specified for the soil and application, not simply the least expensive roll.
- Ask whether gravel is sold by weight or volume and obtain the supplier’s product-specific conversion.
- Buy compatible fittings, cleanouts, outlet protection, and solid pipe for sections that should not collect water.
Common questions
What gravel should I use in a French drain?
Clean, washed, angular stone is commonly selected because open voids let water move toward the pipe. Product names and acceptable sizes vary locally. Avoid unwashed aggregate with fines unless the drainage design specifically calls for it, and confirm compatibility with the chosen fabric.
How much slope does a French drain need?
A drain needs continuous positive fall to a safe outlet, but the appropriate slope depends on site elevations, soil, expected flow, pipe size, and local requirements. Survey the route instead of assuming a universal percentage, and obtain professional drainage advice when buildings could be affected.
Does the French drain calculator include the pipe volume?
Yes, the displacement field reduces gross trench volume for pipe and intentional void space. Ten percent is only a planning default. A large pipe in a narrow trench displaces more gravel, while irregular excavation may offset that reduction and require a larger allowance.
Can a French drain discharge onto the street?
Do not assume street discharge is allowed. Municipal stormwater rules, easements, neighboring property rights, erosion control, and freezing conditions can restrict outlets. Confirm a lawful discharge point with the relevant authority before buying materials or excavating, especially where runoff could affect another parcel.
Sources and reference standards
- USDA NRCS — Engineering Field Handbook, Chapter 14 (2nd ed.)Current second-edition federal engineering guidance covering drainage investigation, design factors, construction, and safe outlets.
Use this result in a complete project plan
Yard drainage planner
A wet yard is a water-path problem before it is a gravel, pipe or pavement problem. Roof discharge, neighboring grades, compacted soil, irrigation leaks and high groundwater can create similar symptoms but require different responses. This guide organizes observations, elevations, contributing areas, outlet review and material estimates without presenting a generic trench as a universal drainage design.
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