French Drain Calculator

French drain material formulas

The calculator models one straight trench as a rectangular prism and one pipe as a cylinder. All dimensions are converted internally before the volumes are calculated. Let L be trench length, W trench width, D average excavation depth, A aggregate depth, d pipe outside diameter, s slope percent, g aggregate allowance and p the pipe-and-fabric purchase allowance.

The pipe calculation uses its outside diameter because the full pipe body displaces stone. Do not reduce an aggregate order by the stone’s void percentage: bulk aggregate is purchased with the spaces between its particles already present. If the pipe diameter is left blank, the calculator treats displacement as zero and does not produce a pipe-purchase length.

Worked material takeoff

Consider a 50 ft trench that is 12 in wide and averages 18 in deep. The aggregate envelope is 14 in deep around a 4 in outside-diameter pipe. At a 1% slope, with 10% aggregate allowance, 5% pipe-and-fabric allowance and a supplier density of 100 lb/ft³:

• Excavation is 50 × 1 × 1.5 = 75 ft³, or about 2.78 yd³.
• The gross aggregate envelope is 58.33 ft³.
• The pipe displaces about 4.36 ft³, leaving 53.97 ft³ of net aggregate.
• With 10% allowance, the aggregate order is about 59.37 ft³ or 2.20 yd³.
• At the entered density, the estimated order mass is about 5,937 lb, or 2.97 US short tons.
• The remaining cap space is 16.67 ft³. This is geometry only; it is not automatically the amount of reusable excavated soil.
• A 1% grade over 50 ft gives 0.5 ft (6 in) of fall.
• The pipe purchase length is 52.5 ft, and the full-wrap fabric estimate including the ends is about 230 ft².

Supplier rounding, stock pipe lengths, roll widths, fittings and field cuts can change the actual purchase. The entered density must come from the chosen aggregate supplier or specification; wetness, gradation and compaction make a generic tonnage unreliable.

Site, outlet and excavation checks

A French drain collects subsurface water through a permeable aggregate envelope and normally a perforated pipe, but it only works as intended when water has a suitable destination. Confirm a lower, stable and legally permitted outlet before excavation. A daylight outlet may need erosion protection, and discharge must not be directed onto neighbouring land, an unstable slope, a sanitary sewer, a storm system, a ditch or a waterway without the required permission. Solid outlet pipe, fittings, couplings, cleanouts, catch basins, headwalls and erosion protection are not included in this takeoff.

Use clean, washed drainage aggregate and a drainage geotextile selected for the native soil. Fabric opening size and permeability matter: an unsuitable landscape fabric can clog, while an unsuitable aggregate or filter can allow fine soil to migrate into the drain. Follow the pipe manufacturer and the project specification for perforation orientation, bedding, cover and load rating.

Before digging, locate and mark underground gas, electric, communications, water and sewer services through the applicable local utility-location service. Trenches can collapse even when they look stable. Keep people out of an unprotected excavation and follow local excavation rules for soil assessment, spoil setback, access, water accumulation, sloping, shoring or shielding. Work near foundations, retaining walls, traffic, steep ground, high groundwater, contaminated soil or recurrent flooding needs qualified site-specific advice.

This calculator does not size the drain for rainfall, runoff, soil infiltration, groundwater drawdown or pipe flow. It does not determine frost depth, structural pipe loading, geotextile specification, permits, property rights or code compliance. Have the site and outfall reviewed before relying on a material order.