About this calculator
This calculator turns a building's perimeter into a French (perimeter) drain materials list — pipe length and pieces, the gravel envelope around the pipe, geotextile fabric to wrap it, inspection wells at regular intervals, and the trench's own excavated volume. Enter the outline as a rectangle or as a list of straight runs.
Formula
total length = 2 x (length + width) [rectangle]
= sum of entered run lengths [list]
pipe pieces = ceil(total length / stock pipe length)
excavated volume = total length x trench width x trench depth
pipe volume = total length x (pi/4) x pipe diameter^2
gravel volume = total length x trench width x (bed thickness + cover thickness) - pipe volume
geotextile wrap width = trench width + 2 x (bed thickness + cover thickness)
geotextile area = total length x wrap width x (1 + overlap%)
inspection wells = floor(total length / well spacing) + 1
Slope is entered and shown in the breakdown but never used in a calculation — this calculator does not size the pipe or slope for actual water discharge.
Worked example
A closed 10 x 8 m rectangular perimeter, 400 mm wide x 600 mm deep trench, a 110 mm pipe in 6 m stock lengths, a 100 mm gravel bed plus a 200 mm gravel cover, 15% geotextile overlap, inspection wells every 15 m.
total length = 2 x (10 + 8) = 36 m
pipe pieces = ceil(36 / 6) = 6
excavated volume = 36 x 0.4 x 0.6 = 8.64 m³
gravel volume = 36 x 0.4 x 0.3 - pipe volume ≈ 4.32 - 0.34 ≈ 3.98 m³
geotextile area = 36 x (0.4 + 2x0.3) x 1.15 = 36 x 1.0 x 1.15 = 41.4 m²
inspection wells = floor(36/15) + 1 = 3
FAQ
Why doesn't this calculator size the pipe or check the slope? Sizing a drain for actual water flow needs a hydraulic calculation — catchment area, rainfall intensity, pipe roughness and the site's real fall — which is well outside a quantity take-off. The slope figure you enter is only carried through to the results for your own reference; verify it against a proper drainage design before building.
Why does the gravel volume subtract the pipe's own volume? The gravel fills the trench around the pipe, not through it — the pipe itself takes up space inside that same envelope, so counting the full trench cross-section as gravel would overstate the gravel needed by the pipe's own volume.
How is the geotextile area worked out? As a single strip wrapped around the gravel envelope's cross-section — along the trench bottom, up both sides, with an overlap allowance for the fold at the top. It is a simplified model of how the fabric is typically installed, not a real cutting/fold layout.
Which standard does this follow? None is cited. This is plain geometry (pipe/gravel/geotextile/excavation volumes) plus a simple evenly-spaced well count — no standard governs perimeter-drain quantities generally, and hydraulic sizing (which this calculator doesn't attempt) would be the place any such standard would apply.
Assumptions and limits
Gravel bed/cover thickness, geotextile overlap, stock pipe length and inspection well spacing are all user inputs with example defaults, not data for a specific drainage system or soil condition. The trench is modelled with vertical sides and no side-slope allowance (unlike the general excavation calculator, which offers one) — add extra excavated volume yourself if the trench needs battered sides. No drawing is produced — the result is a materials/excavation list only.