MachineCalcs

French Drain Sizing Calculator

Estimate runoff flow, perforated pipe capacity, gravel void storage, storage duration, required trench length and limiting drainage area for a French drain screen.

Hydraulics 13 inputs 9 results
Used in Piping Trades 28

Calculator

Inputs

View results
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.

French drain sizing starts with runoff, Q = C*i*A, then asks whether the trench can buffer that flow and whether the perforated pipe can convey it. This calculator subtracts pipe OD displacement from the stone envelope, applies the entered gravel void ratio, adds pipe internal storage, checks full-pipe Manning capacity and reports storage duration, required trench length and the limiting drainage area.

Continue workflow

All Hydraulics
Workflow placement Piping Trades Stage 2: Lay out the install
Next 1 Size catch basin Use a catch-basin or yard-drain inlet screen when surface runoff needs collection before entering the French drain. Next 2 Size gutters Start with roof runoff, gutter capacity and downspout count when the trench receives roof-edge discharge. Next 3 Lay out pipe slope Lay out trench fall, pipe pitch and invert context after the storage and pipe-capacity screen. Chart Use reference data Pipe fitting takeout chart

How to use this calculator

  1. Enter drainage area. Use the area that actually drains toward this trench, such as roof, patio, yard or retaining-wall backfill area.
  2. Set rainfall and runoff. Use local design rainfall intensity and a weighted runoff coefficient for the surface mix.
  3. Enter trench geometry. Use installed trench length, width, stone depth and a gravel void ratio for the drainage aggregate.
  4. Enter pipe data. Use actual pipe ID, OD, slope and Manning n so storage displacement and conveyance are separate.
  5. Check the bottleneck. Compare pipe utilization, storage utilization, storage duration, required trench length and limiting drainage area.

How it works

Runoff starts with the Rational Method screen: Q = C i A where C is runoff coefficient, i is design rainfall intensity and A is the contributing area. The calculator then applies the entered safety or clogging allowance.

Storage is the stone-envelope void volume plus the pipe's internal volume. The pipe outside diameter is subtracted from the trench section before the gravel void percentage is applied, so the pipe is not counted as both stone voids and internal pipe storage.

Pipe capacity is screened with full-flow Manning capacity: Q = (1/n) A Rh^(2/3) S^(1/2) using actual pipe ID, slope and roughness. Use the catch basin sizing calculator, gutter and downspout calculator and pipe slope calculator for adjacent drainage checks.

Worked example

Verified against the live calculator

With the defaults, a 1,000 ft^2 mixed drainage area at 2 in/h rainfall and C = 0.60 produces about 15 gpm of design runoff after the 20% allowance. An 80 ft by 18 in by 24 in trench with 35% gravel voids and a 4 in pipe stores roughly 665 gal, enough for about 44 minutes at the default design flow.

Frequently asked questions

How do you size a French drain from drainage area?

This page converts drainage area and rainfall intensity into runoff flow, then compares that flow with trench storage and perforated-pipe capacity.

Does this include soil infiltration?

No. Soil infiltration, drawdown and percolation must come from site testing or a drainage report. This calculator only screens runoff, storage and pipe conveyance.

Why enter both pipe ID and pipe OD?

Pipe ID controls internal pipe storage and Manning capacity. Pipe OD controls how much stone-envelope volume the pipe displaces.

Can this replace local stormwater or plumbing code?

No. Use local rainfall criteria, discharge rules, setbacks, outlet/tailwater checks, cleanouts, frost cover and civil or geotechnical review for final design.

Method & assumptions

  • Rainfall intensity must come from local design-storm criteria or project requirements.
  • Runoff coefficient is user-entered and should represent the weighted surface mix.
  • Gravel storage uses rectangular trench geometry, pipe OD displacement and the entered void ratio.
  • Pipe capacity is a full-flow Manning screen, not a perforation, sock, geotextile or sediment rating.
  • Final work still needs soil infiltration or outlet checks, cleanouts, frost/cover, setbacks, dry-well/daylight discharge and local stormwater review.
Embed this calculator on your site free

Paste this where you want the calculator to appear. It stays in sync — same formulas, metric & imperial, light/dark — and a small credit link helps people find more tools.

Open widget

Live preview