Free calculators
The full app’s engine, running in your browser with no account. Each opens on a published worked example, with the source its formula follows.
- Curve number and runoffComposite CN from cover type and hydrologic soil group, then TR-55 runoff depth for a 24-hr rainfall.TR-55 (1986) eq. 2-3 and 2-4, p. 2-1
- Time of concentrationTR-55 Tc from a sheet flow, a shallow concentrated flow and a channel flow segment, in hours and minutes.TR-55 (1986) eq. 3-1 to 3-4, pp. 3-1 to 3-4, and Appendix F p. F-1
- Rational method peak flowPeak runoff Q = Cf·C·i·A from the runoff coefficient, rainfall intensity and drainage area, with the frequency factor for the storm.FHWA HEC-22 4th ed. (2024) eq. 4.1, p. 23; Cf from HEC-22 3rd ed. (2009) Table 3-1, p. 3-8
- Detention volume estimateStorage needed to hold post-development runoff to an allowable release rate, by the modified rational critical-duration method.NYC DEP Criteria for Detention Facility Design (2012) pp. 1 to 4; UDFCD Drainage Criteria Manual Vol. 2 (2007) eq. SO-6 to SO-10. A preliminary estimate, not a NYSDEC method.
- NY water quality volumeWQv = P·Rv·A/12 from the 90th percentile rainfall, impervious cover and contributing area, per the NYSDEC design manual.NYS Stormwater Management Design Manual (July 31, 2024) § 4.2, p. 4-2, and Table 4.1