{"id":"french-drain-calculator","name":"French Drain Calculator — Gravel, Fabric and Pipe","shortName":"French Drain Calculator","canonicalUrl":"https://calcurhome.com/calculators/french-drain-calculator","catalogUrl":"https://calcurhome.com/api/calculators","category":{"id":"landscaping","name":"Landscaping","url":"https://calcurhome.com/calculators/category/landscaping"},"language":"en-US","audience":"US homeowners, DIYers and home-project planners","primaryKeyword":"french drain calculator","secondaryKeywords":["french drain gravel calculator","how much gravel for a french drain","drainage trench calculator","french drain fabric calculator","french drain pipe length"],"description":"Estimate French drain gravel, filter fabric and perforated pipe from trench dimensions and waste. Plan materials before confirming drainage design.","directAnswer":"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.","inputs":[{"key":"length","label":"Trench length","unit":"ft","defaultValue":50,"min":1,"max":1000,"step":1},{"key":"width","label":"Trench width","unit":"in","defaultValue":12,"min":4,"max":72,"step":1},{"key":"depth","label":"Gravel depth","unit":"in","defaultValue":18,"min":4,"max":96,"step":1},{"key":"fabricOverlap","label":"Fabric overlap","unit":"in","defaultValue":12,"min":0,"max":48,"step":1,"help":"Additional width for closing the wrap over the gravel; confirm the required overlap for the selected fabric."},{"key":"displacement","label":"Pipe and void displacement","unit":"%","defaultValue":10,"min":0,"max":60,"step":1,"help":"Approximate share of the trench volume not occupied by gravel."},{"key":"waste","label":"Material allowance","unit":"%","defaultValue":10,"min":0,"max":50,"step":1}],"formula":{"title":"French drain material formula","expression":"Gravel yd³ = length × width × depth × (1 − displacement %) ÷ 27 × allowance; fabric ft² = length × (two sides + bottom + top + overlap) × allowance.","explanation":"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."},"measurementGuide":["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."],"buyingTips":["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."],"workedExample":{"title":"Fifty-foot yard drainage trench","scenario":"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.","result":"The estimate is 2.75 yd³ of gravel, about 330 sq ft of filter fabric, and 55 linear ft of pipe."},"commonQuestions":[{"question":"What gravel should I use in a French drain?","answer":"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."},{"question":"How much slope does a French drain need?","answer":"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."},{"question":"Does the French drain calculator include the pipe volume?","answer":"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."},{"question":"Can a French drain discharge onto the street?","answer":"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":[{"name":"USDA NRCS — Engineering Field Handbook, Chapter 14 (2nd ed.)","url":"https://directives.nrcs.usda.gov/sites/default/files2/1712930918/31801.pdf","note":"Current second-edition federal engineering guidance covering drainage investigation, design factors, construction, and safe outlets."}],"defaultCalculation":{"inputs":{"length":50,"width":12,"depth":18,"fabricOverlap":12,"displacement":10,"waste":10},"result":{"value":"2.75","unit":"cubic yards","label":"Estimated drainage gravel","lines":[{"label":"Gross trench volume","value":"75 ft³"},{"label":"Filter fabric","value":"330 sq ft"},{"label":"Perforated pipe","value":"55 linear ft"}],"summary":"Plan on about 2.75 cubic yards of gravel, 330 square feet of fabric, and 55 feet of pipe.","note":"Planning estimate only. Confirm trench slope, a lawful free-draining outlet, utility locations, soil suitability, fabric specification, and pipe sizing before excavation."}},"reviewedOn":"2026-07-24","disclaimer":"Planning estimate only. Verify product coverage, supplier rounding, local codes and structural requirements before purchase or construction."}