MachineCalcs

Trench Drain Sizing Calculator

Size a trench or channel drain from known flow or runoff, grate open area, rectangular channel capacity and outlet pipe capacity.

Hydraulics 17 inputs 11 results
Used in Piping Trades 28

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.

Trench drain sizing starts from runoff Q = C*i*A or a known design flow, then checks grate open area, rectangular channel capacity and outlet pipe capacity. This calculator reports required grate open area, installed inlet velocity, channel utilization, required channel depth, outlet utilization and limiting drainage area. It is not a manufacturer trench-drain selector, grate load-rating table, ADA/heelproof check or municipal stormwater approval worksheet.

Continue workflow

All Hydraulics
Workflow placement Piping Trades Stage 2: Lay out the install
Next 1 Size catch basin Use catch-basin sizing when the trench drain outlet discharges to a basin or when point inlets are a better collection method. Next 2 Size gutters Start with roof runoff when downspouts discharge toward the trench or channel drain. Next 3 Lay out pipe slope Lay out trench channel slope, outlet fall and downstream invert elevations after the hydraulic screen. Chart Use reference data Pipe fitting takeout chart

How to use this calculator

  1. Choose flow mode. Use runoff mode for rainfall and drainage area, or known-flow mode when the design flow is already specified.
  2. Set grate assumptions. Enter trench length, grate width, open-area percent and target inlet velocity.
  3. Set channel geometry. Enter channel width, usable flow depth, slope and Manning roughness.
  4. Check outlet pipe. Enter outlet pipe ID, outlet slope and outlet Manning n.
  5. Review limiting side. Compare grate, channel and outlet utilization plus the required channel depth and limiting drainage area.

How it works

Runoff mode starts with Q = C i A where C is runoff coefficient, i is rainfall intensity and A is the contributing area. Known-flow mode starts from the entered flow. Both modes apply the safety or debris allowance before the drain checks.

The grate screen uses A_open = Q / v. The trench channel is treated as a rectangular open channel using Q = A R^(2/3) S^(1/2) / n with A = W y and R = A / (W + 2y). The outlet pipe uses full circular Manning capacity.

Use adjacent drainage tools for the rest of the path: catch basin sizing, gutter and downspout sizing, swale capacity and pipe slope layout.

Worked example

Verified against the live calculator

With the defaults, a 1,500 ft^2 paved area at 3 in/h rainfall, C = 0.9 and a 20% allowance produces about 50 gpm of design flow. A 20 ft long by 6 in grate with 55% open area has ample open-area capacity at 2 ft/s, while the 4 in by 4 in channel and 4 in outlet pipe screen the downstream hydraulic margin.

Frequently asked questions

How do you size a trench drain by flow rate?

Enter a known design flow or estimate runoff from area, rainfall intensity and runoff coefficient, then check grate open area, trench channel capacity and outlet pipe capacity.

Does grate open area equal real capture?

No. Open area is only a first screen. Actual capture depends on grate geometry, pavement slope, ponding depth, bypass, debris and manufacturer hydraulic data.

What Manning n should I use for the channel?

Use a roughness value for the actual channel material and condition. Smooth polymer concrete is lower than rough, damaged or sedimented channels.

Is this a code or manufacturer trench-drain selector?

No. Final selection still needs local stormwater criteria, manufacturer grate and channel data, load class, ADA or heelproof requirements, outlet/tailwater checks and civil review.

Method & assumptions

  • Rainfall intensity and runoff coefficient must come from local project assumptions.
  • Grate open area is only a velocity screen; real inlet capture needs manufacturer and site data.
  • The trench channel is treated as a simple rectangular open channel at the entered usable depth.
  • The outlet pipe is checked as a full circular Manning pipe at the entered ID and slope.
  • Final trench-drain work still needs local stormwater criteria, pavement slope, bypass and ponding checks, grate load class, ADA/heelproof requirements, debris and sediment maintenance, outlet/tailwater checks, manufacturer data and civil review.
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