How this calculator works
The rectangular trench volume is reduced by the outside cylinder of the pipe, because gravel cannot occupy its bore or wall. A cited 4 in Schedule 40 product has a 4.500 in (114.3 mm) outside diameter. The SDR35 starting value of 107.06 mm follows the cited IPEX sewer manual; verify actual dimensions for the product being installed. Pipe length follows the entered slope, and each stock piece is rounded up. Wrapped trench fabric covers bottom, two sides and top plus one overlap; end caps are excluded.
Formula
Fall = horizontal length x slope; pipe length = sqrt(length^2 + fall^2); gravel = trench length x width x depth - pi x OD^2 x pipe length / 4. Trench-wrap width = 2(width + depth) + overlap; pipe-wrap width = pi x OD + overlap.
Worked example
A 10 m by 0.3 m by 0.5 m trench without pipe contains 1.5 m3 gravel. A full fabric wrap with 0.2 m overlap is 1.8 m wide and has 18 m2 area before end allowances.
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Frequently asked questions
Does this select the pipe slope?
No. Enter the slope specified for the site and drainage design. The 1% default is an editable example, not a universal minimum.
Why use the outside pipe diameter?
Both the wall and empty bore displace gravel. Nominal pipe size or inside diameter would understate displacement.
Does the fabric estimate include every seam?
It includes one continuous longitudinal overlap, without trench ends, cross seams or roll cutting losses. Add these from the installation plan.