French Drain Calculator

How it works

The stone fills the trench from the bottom up to the soil cover, so its depth is the trench depth less the cover. Length × width × that depth is the stone envelope; the perforated pipe inside it takes up π × (diameter ÷ 2)² of every foot, so its volume is subtracted. A 4 in pipe takes about 0.087 cu ft per foot. The net cubic feet are converted to cubic yards at 27 per yard and to tons at the stone's loose density.

Filter fabric lines the bottom and both sides of the stone and folds over its top, so the fabric width across the trench is the width + twice the stone depth + the fold-over; times the trench length that is the fabric area. The pipe runs the full trench length. The fall is the drop the pipe needs over that length at the slope you enter: length × slope.

Formula

stone depth      = trench depth − soil cover
pipe (cu ft)     = π × (pipe diameter ÷ 2)² × length
gravel (cu ft)   = length × width × stone depth − pipe
gravel (cu yd)   = gravel (cu ft) ÷ 27;   tons = cu yd × density
fabric width     = width + 2 × stone depth + fold-over
fabric (sq ft)   = length × fabric width
fall             = length × slope ÷ 100

Example

A 50 ft trench 12 in wide and 24 in deep with 6 in of soil cover holds 18 in of stone: 50 × 1 × 1.5 = 75 cu ft. A 4 in pipe takes π × (2⁄12)² × 50 = 4.36 cu ft, leaving 70.64 cu ft, or 2.62 cu yd; at 1.4 tons per cu yd that is 3.66 tons.

The fabric is 12 + 2 × 18 + 12 = 60 in (5 ft) across, so 50 × 5 = 250 sq ft (27.78 sq yd). At a 1% slope the 50 ft of pipe falls 6 in.

Assumptions and limitations

• The trench is rectangular with vertical sides and a uniform depth; the pipe is solid for volume purposes and sits wholly in the stone. A sloped trench bottom, sloping sides or a bedding layer under the pipe changes the volume.
• Stone density is a typical, editable figure; FM 5-434, Earthmoving Operations (US Army, 2000), Table 1-2, lists dry gravel at 2,700–3,000 lb per loose cubic yard. The supplier's figure for the product governs. Stone settles when placed, so a little more than the computed volume may be used.
• Fabric laps at roll ends, fabric over the trench ends and wastage are not included. Fittings, outlets, cleanouts and the soil cover are not included.
• The slope is an input; 1% is a typical minimum, not a design. Whether a drain is suitable, its outlet and its depth depend on the site; this is a quantity estimate, not engineering and not a statement of any code or permit requirement.

Frequently asked questions