Trench Drain Sizing Calculator

Inputs

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Use runoff mode for local rainfall assumptions, or known-flow mode when a design flow is already specified.
Paved, doorway, driveway, garage, patio or slab area draining to this trench drain.
ft²
Use the local design-storm intensity for the selected duration and return period.
in/h
Dimensionless runoff coefficient. Concrete, asphalt and roof areas are usually high; pervious surfaces are lower.
Extra allowance applied to runoff or known design flow for debris, partial blockage and uncertainty.
%
Installed grate length collecting flow along the trench.
in
Gross grate or slot width used with open-area percent.
in
Effective open fraction of the grate or slot. Use manufacturer data when available.
%
Velocity used to convert design flow into required grate open area.
ft/s
Inside trench channel width below the grate.
in
Usable rectangular flow depth inside the trench channel.
in
Longitudinal channel slope as fall divided by run times 100.
%
Roughness coefficient for the trench channel. Smooth polymer concrete is lower than rough or sedimented channels.
Actual inside diameter of the trench drain outlet pipe.
in
Outlet pipe slope as fall divided by run times 100.
%
Roughness coefficient for the full circular outlet pipe.

Results

Default result
Edit inputs
Design flow(Qd)
50.49gpm
Pass

Input flow before allowance: 159.3 L/min.

Also computed

Required grate open area(Ao req)Pass8.1in²

Open area needed at the target inlet velocity.

Installed grate open area(Ao)792in²

Capacity at target velocity: 1.869e+4 L/min.

Grate utilization(Ug)Pass1.023%

Installed inlet velocity(v)Pass0.02045ft/s

Channel capacity(Qc)Pass93.16gpm

Rectangular Manning channel capacity. Full-depth velocity: 569.4 mm/s.

Channel utilization(Uc)Pass54.2%

Trench drain preview: design flow 191 L per minute, channel capacity 353 L per minuteTrench drain previewGrate open area, rectangular channel and outlet pipe capacitydesign flow191 L/minchannel capacity353 L/minGrateChannelOutletScreen only. Final trench-drain selection needs local stormwater criteria, manufacturer grate/channel data, load rating, bypass, debris, sediment, outlet and tailwater checks.
Method notes 3 notes
  • Runoff mode uses Q = C * i * A, then applies the safety/debris allowance. Known-flow mode applies the same allowance to the entered flow.
  • Grate sizing uses required open area = Q / target inlet velocity. Actual capture can be lower because of grate geometry, bypass, slope toward the drain, ponding head, debris and manufacturer hydraulics.
  • Channel and outlet capacity use Manning screens. Final trench-drain selection still needs local stormwater criteria, manufacturer grate/channel capacity, load class, ADA/heelproof requirements, debris and sediment maintenance, outlet capacity, tailwater and civil review.

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