Register to start using the calculator!
One account unlocks every calculator on the site — try it free for 5 days, no card required.
Rainwater Flow Rate Calculations
Rain intensity
[l/s*ha]
Runoff coefficient
ψ [-]
Catchment area [m2]
Flow rate [dm³/s]
Designing a sewer network? Build the longitudinal profile and a CAD-ready DXF.
Paste your coordinates — Altivo computes slopes and elevations, and the DXF export drops straight into AutoCAD. No install, no dongle, in the browser.
Rainwater flow rate calculator — stormwater drainage design
This tool calculates the design stormwater flow rate q [dm³/s] from a single catchment or a set of catchments. It is used when sizing gutters, downpipes, roof outlets, lateral connections and stormwater sewer networks in line with PN-EN 12056-3 and standard engineering practice.
The calculator handles multiple catchments at once — it sums the flows and feeds them into the gravity sewer diameter selection tool, so you can size the entire stormwater section in one place.
How to use the calculator
Pick the design rain intensity I [dm³/s·ha] — from the list of authors (Bogdanowicz-Stachý, Blaszczyk, PN-EN 12056-3) or enter your own value, e.g. from a local IDF atlas.
Choose the runoff coefficient ψ matching the catchment surface. If none of the presets fits, select "Custom ψ value" and enter a number between 0 and 1.
Enter the catchment area A [m²]. For each additional catchment, add a new row with the "Add row" button.
Export the result to PDF or Excel — ready to attach to the project documentation or send to the client.
Formula and basis of calculations
q = ψ · A · I / 10 000
q — design stormwater flow rate [dm³/s]
ψ — runoff coefficient (dimensionless, 0–1)
A — catchment area [m²]
I — design rain intensity [dm³/(s·ha)]
The result is the instantaneous peak flow for a rain event of duration t and return period C. For stormwater drainage, t = 15 min and C = 1, 2 or 5 years are typical, depending on how critical the protected facility is.
How to choose the rain intensity I
The design intensity depends on the location, rain duration and return period (C = 1, 2, 5, or even 10 years for critical infrastructure). The calculator includes the most common values from Bogdanowicz-Stachý and Blaszczyk formulas, plus the 300 dm³/s·ha value recommended by PN-EN 12056-3 for sizing roof drainage.
The most accurate Polish source is the PANDa Atlas (Polish IDF Atlas), which provides I values on a kilometer grid. If you work with such data, use the "Custom value" option and enter the number read from the atlas.
Runoff coefficient ψ — what determines it
The runoff coefficient expresses how much of the rainwater reaches the sewer and how much is lost to infiltration, evaporation and retention. The more impermeable the surface, the closer ψ is to 1.
- Pitched roofs, asphalt, sealed concrete slabs: ψ ≈ 0.9–1.0
- Stone pavement with joints, paved squares: ψ ≈ 0.7–0.85
- Gravel roofs, extensive green roofs: ψ ≈ 0.3–0.5
- Lawns, parks, gardens, sports fields: ψ ≈ 0.05–0.25
Design tips
• For a mixed catchment (e.g. roof + driveway + lawn), enter each surface as a separate row — the calculator will sum the flows.
• Check local water utility guidelines — some require C = 5 years for stormwater systems serving urban squares.
• Remember about retention: when designing a detention tank, the design flow q is the starting point for the hydrological balance.
• After computing the flow, use the "Diameter selection" tool that appears below — it automatically checks pipe velocity and fill ratio.
PDF and Excel export
Once data is entered, export buttons appear. The PDF contains a table with every catchment, its I, ψ, A values and the flow q per catchment plus the grand total — ready to attach to the design documentation. The Excel file lets you continue calculations in your own spreadsheet, e.g. for retention balance.