French Drain Sizing Calculator

Inputs

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Roof, yard, patio or retaining-wall area draining toward this French drain.
ft²
Use the local design rainfall intensity for the duration and return period you are screening.
in/h
Weighted runoff coefficient for the contributing surface. Hardscape is higher; lawn and planted areas are lower.
Extra factor applied to runoff for clogging, sediment, construction tolerance and uncertainty.
%
Installed length of the stone-filled French drain trench.
ft
Inside trench width after excavation.
in
Stone-filled depth used for storage. Do not include cover soil above the drain envelope.
in
Percent of stone envelope volume available to store water.
%
Actual inside diameter of the perforated or slotted pipe.
in
Outside diameter used to subtract pipe displacement from the gravel void storage.
in
Perforated pipe slope as fall divided by run times 100.
%
Full-pipe roughness coefficient for the conveyance screen. Corrugated pipe may need a higher value.
How many minutes of design runoff you want the trench storage to buffer.
min

Results

Default result
Edit inputs
Design runoff flow(Qd)
14.96gpm
Pass

Raw runoff before allowance: 47.19 L/min.

Also computed

Pipe capacity(Qp)Pass85.41gpm

Full-pipe Manning capacity at the entered ID, slope and roughness.

Pipe utilization(Up)Pass17.52%

Available storage(V)Pass657.5gal

2291 L in gravel voids plus 197.7 L inside the pipe.

Required storage(Vreq)Pass224.4gal

Design runoff multiplied by 15.00 min.

Storage utilization(Us)Pass34.13%

Storage duration(ts)Pass43.94min

Available storage divided by design runoff.

French drain preview: pipe 18 percent used, storage 34 percent usedFrench drain previewRunoff, gravel void storage and perforated pipe capacitydesign runoff57 L/minstorage2489 LPipe18%StorageScreen only. Soil infiltration, geotextile, sediment, tailwater, cover, cleanouts and local stormwater rules can control final design.
Method notes 4 notes
  • Runoff screen uses Q = C*i*A with user-entered rainfall intensity, runoff coefficient and safety/clog factor.
  • Storage subtracts pipe outside-diameter displacement from the stone envelope, applies the entered gravel void ratio, then adds pipe internal volume.
  • Pipe capacity uses full-flow Manning capacity. Real perforated-pipe inlet capacity, slots, sock, aggregate envelope, sediment and outlet tailwater can control performance.
  • Final drainage design still needs soil infiltration or outlet capacity, cleanouts, geotextile, frost/cover, setbacks, daylight or dry-well discharge, local stormwater criteria and civil/geotech review.

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