Rational Method Runoff Calculator (Q = CiA)
Peak stormwater runoff from catchment area, runoff coefficient and rain intensity.
Valid for small catchments (< ~80 ha) where storm duration ≥ time of concentration. Choose i from local IDF curves at that duration.
Formula
Rational Method Runoff Calculator (Q = CiA) is a free rational method for process, mechanical and water engineers — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Rational Method Runoff Calculator (Q = CiA)
Peak stormwater runoff from catchment area, runoff coefficient and rain intensity. The calculation implements Q = C·i·A (consistent units; common form Q = CiA/360 with mm/h & ha) (ASCE / WEF — urban drainage design; CPHEEO manual (India)). Valid for small catchments (< ~80 ha) where storm duration ≥ time of concentration. Choose i from local IDF curves at that duration.
How to use Rational Method Runoff Calculator (Q = CiA)
- 1Enter Runoff coefficient C (Pavement 0.9 · roofs 0.85 · lawns 0.2 · mixed urban 0.7).
- 2Enter Rainfall intensity in mm/h.
- 3Enter Catchment area in ha.
- 4Read Peak runoff, Peak runoff instantly — no submit button needed.
- 5Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Rational Method Runoff Calculator (Q = CiA)?
- ✓Implements the standard formula — Q = C·i·A (consistent units; common form Q = CiA/360 with mm/h & ha)
- ✓Reference cited on-page: ASCE / WEF — urban drainage design; CPHEEO manual (India)
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Live worked example: the substitution recomputes from your numbers
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Rational Method Runoff Calculator (Q = CiA) use?+
It computes Q = C·i·A (consistent units; common form Q = CiA/360 with mm/h & ha), per ASCE / WEF — urban drainage design; CPHEEO manual (India). The formula is displayed under the result along with a worked example substituted with your own inputs.
What should I keep in mind when using this calculator?+
Valid for small catchments (< ~80 ha) where storm duration ≥ time of concentration. Choose i from local IDF curves at that duration.
Does this work for any fluid?+
Yes — density and viscosity are inputs (with common fluids suggested in the field hints), so the same physics applies to water, oils, gases and process fluids. Compute always runs in SI internally, so unit mix-ups can't corrupt the result.
Is the Rational Method Runoff Calculator (Q = CiA) free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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