Stormwater Pipe Sizing
Size a stormwater pipe in minutes, with the AEP, runoff coefficient, and rainfall already handled for you.
The Quick Stormwater Calculator is the everyday tool for single-pipe stormwater sizing. Pick the catchment type, drop in the area, and let Clarus pull the right design AEP, the recommended runoff coefficient, and the rainfall intensity for your site. Get a sized pipe with the standards basis stamped on the result.
Sizing paths
Uses the Rational Method end-to-end. Choose a catchment type from the verified library of twelve options, enter the area, and Clarus applies the recommended design AEP and runoff coefficient for that surface. Box gutters default to 1% AEP, most roof and ground catchments to 5% AEP, and mixed catchments resolve from the receiving system.
For when you already have the design flow in L/s from a stormwater modelling tool or an upstream calculation. Skip the catchment and rainfall steps and go straight to pipe sizing.
Built in
When a catchment is part roof, part paving, part landscaping, the weighted components builder lets you split it by area. Clarus computes the area-weighted runoff coefficient automatically, no manual averaging.
Enter the design intensity manually, or use address lookup: type the site address and Clarus pulls the IFD rainfall intensity for the requested AEP and duration, stamping the source URL into the result. AU and NZ supported with country-aware providers.
Enter as a ratio (1:100), a percent (1%), or slope in m/m. Clarus normalises to the form the hydraulics need and shows the others for clarity.
Catalogues
Bread-and-butter sizing using standard PVC stormwater dimensions.
The manufacturer's catalogue dimensions and roughness for projects specifying that product.
Reinforced concrete pipe dimensions for RCP projects.
Results
Every Quick Stormwater result is complete and auditable.
With the runoff coefficient used and its source label, so reviewers can see how the flow was derived.
From the Colebrook-White solution at your slope. The unrounded number that tells you whether you're sized at the edge of the catalogue or with room to spare.
A real catalogue DN with internal diameter, velocity at design flow, hydraulic status against your slope and minimum-gradient requirement, and a standards-status flag.
Every size in the chosen catalogue with capacity, velocity, hydraulic status, and standards status. The recommended size is flagged; the rows around it show what happens going up or down.
For the chosen pipe DN, showing Australian and New Zealand requirements side by side. Country-aware.
The assumptions that fed the design flow: AEP, runoff coefficient basis, rainfall provider, intensity, and catchment breakdown.
Codes and messages for anything that needs attention before issue, from hydraulic failure to missing rainfall or an unresolved address.
Every snapshot shows exactly which catchment defaults, pipe catalogue version, and rainfall source the calculation used. The PDF carries the same stamps.
Why it's worth using
Box gutters are 1% AEP, eaves gutters are 5% AEP, mixed catchments resolve from the receiving system. Clarus encodes these so you don't have to remember which catchment type goes with which design event, and the reviewer sees the rule that was applied.
Twelve catchment types each carry a recommended coefficient with its basis labelled. Mixed catchments are area-weighted automatically, and the result stamps the coefficient and label so the report explains itself.
Type the site address, get the rainfall intensity for the design AEP and duration in seconds. The source URL is stamped on the result and goes into your PDF so reviewers can verify.
Whether you're sizing from a catchment area or from a flow figure handed to you by a stormwater modeller, the same calculator handles both. No tool switching.
Clarus shows you the catalogue around the answer with capacity, velocity, hydraulic status, and standards status per row. The "could we drop one size?" question is answered on screen.
Australia and New Zealand both supported with the right minimum-gradient table and the right rainfall provider for each. Switching between AU and NZ jobs is a single dropdown, not a separate tool.
Generic PVC, Vinidex StormPRO SN8, and RCP RRJ Class 2/3, each using the right internal diameter and roughness so velocity and capacity numbers actually match the product.
Save calculations as snapshots, attach them to a project, and export PDF reports stamped with the AEP, runoff coefficient, rainfall source, and pipe catalogue version. The issued document is auditable end to end.
When it grows
Single-pipe stormwater sizing for residential, commercial, and light infrastructure. When the design needs a full network, the same project record carries forward into Schematic Studio, where every branch is sized automatically as you sketch, with the same standards-aligned outputs behind every run.
Spin up a free trial and size your first pipe in minutes, with the AEP, runoff coefficient and rainfall handled for you, and a stamped, auditable PDF you can defend in review.
Start your first stormwater pipe sizing calculation