Convert tributary roof area and user-entered rainfall intensity into runoff flow before code-based drain sizing.
Why it matters: Rainfall data, projected area, and runoff basis must be explicit before selecting drains or conductors.
ΣCALCULATE · VERIFY · INTERPRET
Calculated result
Engineering evaluation
Calculation complete—verify project criteria
Roof runoff flow (Qʳ)415.584 gpm
Hourly runoff volume (Vʰ)
24,935.07 gal/h
Engineering interpretation
The entered rainfall basis produces 415.5844 gpm ideal roof runoff.
Important limitations
Rainfall and storm-drainage screen only. Do not size drains or establish compliance from this result. Primary/secondary drainage, roof geometry, ponding, blockage, detention, structural stability, adopted code, and manufacturer ratings require review.
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2026-08-07 · Free Library
Governing relationships
Qʳ,ᴵᴾ=0.0103896111·Aʳ·i·Cʳ
Vʰ,ᴵᴾ=60Qʳ
Qʳ,ˢᴵ=Aʳ·i·Cʳ/3600
Vʰ,ˢᴵ=Aʳ·i·Cʳ/1000
Current substitution
Qʳ=415.5844 gpm; Vʰ=24935.07 gal/h for Aʳ=10000, i=4, Cʳ=1.
Calculation sequence
Step-by-step method
Confirm location-, duration-, and recurrence-specific rainfall source.
Multiply horizontal tributary area by rainfall intensity and the visible runoff factor.
Apply the exact unit conversion for flow and hourly volume.
Equation legend
Inputs and calculated quantities
Aʳ
Tributary horizontal roof area — ft² or m²
i
Design rainfall intensity — in/h or mm/h
Cʳ
Runoff factor — 0–1
Qʳ
Roof runoff flow — gpm
Vʰ
Hourly runoff volume — gal/h
Applications
How this calculation is used
Building design
Screen roof runoff using documented rainfall data.
Construction
Check tributary area and drainage-path assumptions.
Commissioning
Document the hydraulic input basis without implying drain capacity or compliance.
Quality controls
Assumptions and limitations
Assumptions
Entered values describe one documented system and use a consistent unit, energy, pressure, elevation, and measurement basis.
Measurements are representative and synchronized where the calculation combines field readings.
Limitations
Uniform rainfall and constant runoff are assumed; ponding, detention, blockage, time of concentration, roof geometry, and drain hydraulics are omitted.
Primary and secondary drainage, rainfall selection, ponding stability, and drain ratings require adopted-code and professional review.
Field use
Verification procedure
Confirm the calculation boundary, source documents, formula symbols, and units.
Record representative field values or approved project inputs and their sources.
Review the result against the applicable code, criteria, manufacturer data, and measurement uncertainty.
Common engineering mistakes
Combining values from different system boundaries, operating periods, temperature bases, or unit conventions.
Treating a calculated screening value as a hidden pass/fail threshold or final design decision.
International Plumbing Code, 2024The legally adopted edition and local amendments govern water, sanitary, storm, sump, and pumping requirements; protected code tables are not reproduced.
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
