Stormwater Runoff Calculator
Estimate the peak stormwater runoff from your site with the Rational Method (Q = C · i · A) — the standard civil-engineering approach for small watersheds. Enter your zip code and we'll pull NOAA rainfall data for a nearby project-area point. Each calculation is fully displayed for transparency, replicability, and ease of use.
Rainfall isn't evenly distributed; it varies enormously from one location to another. Whether you're in the desert or along the Gulf Coast, we'll pull NOAA rainfall data for your ZIP-code centroid (Perica et al., 2013), giving you a defensible planning estimate for nearby project sites.
How do I choose a return period?
| Typical use | Return period |
|---|---|
| Lawn / landscape / minor nuisance drainage | 2–5-year |
| Residential yards, driveways, patios | 10-year |
| Commercial sites, around foundations/structures | 25-year |
| Critical areas, basements, where flooding is costly | 50–100-year |
Add each surface that sheds water towards your drainage. Choosing a surface type loads a typical runoff coefficient (C), the fraction of rain that runs off instead of soaking in, from the Texas Department of Transportation (TxDOT) Hydraulic Design Manual, Table 4-10 (TxDOT, n.d.). The tool blends them into one area-weighted C.
The Rational Method calculates rainfall at a duration equal to the time of concentration (tc) — how long water takes to travel from the farthest point of your area to the drain. At the time of concentration, the entire area is contributing and flow peaks (TxDOT, n.d.). Let the tool estimate tc from your flow path, or enter a duration yourself.
Overland (sheet) flow uses the Kerby equation (Kerby, 1959); if your runoff collects into a defined channel/swale, that leg should be added separately with the Kirpich equation (Kirpich, 1940). This tool computes the overland (Kerby) time only. A 10-minute minimum is applied in automatic mode to align with the TxDOT Rational Method guidance.
Methodology & sources (show the engineering basis)
The model. Peak runoff is computed with the Rational Method, Q = C · i · A (Kuichling, 1889), the standard method for small, fairly uniform drainage areas (≤ ~200 ac; TxDOT, n.d.). In U.S. customary units the exact relation is Q[ft³/s] = (43,560 / (12·3600)) · C · i[in/hr] · A[ac]; the leading constant equals 1.0083 (= 43,560 / [12 × 3,600]), conventionally taken as ~1, which the tool carries explicitly.
Rainfall (i). Retrieved live from NOAA Atlas 14 (Perica et al., 2013) for the ZIP-code centroid, then interpolated (log–log) to the selected duration = tc. The 90% confidence interval shown is NOAA's published lower/upper bound.
Runoff coefficient (C). Per-surface values are representative figures chosen within the ranges TxDOT HDM Table 4-10 gives for each surface, then area-weighted (TxDOT, n.d., Table 4-10). For storms ≥ 25-year a frequency factor Cf (1.1 / 1.2 / 1.25) is applied, with C·Cf capped at 1.0; those factors are reproduced in open DOT drainage guidance and trace back to the standard Rational Method frequency adjustment (Connecticut DOT, 2000; ASCE & WEF, 1992).
Time of concentration (tc). This tool computes the overland (sheet) flow time with the Kerby equation, as implemented in the TxDOT Hydraulic Design Manual §11, Eq. 4-14 (Kerby, 1959; TxDOT, n.d.). Where runoff later concentrates in a defined channel or swale, that leg should be added separately with the Kirpich equation, Eq. 4-15 (Kirpich, 1940); the combined Kerby–Kirpich approach follows Roussel et al. (2005). A low-slope adjustment (+0.0005 to slopes below 0.2%) follows Cleveland et al. (2012); automatic mode applies TxDOT's 10-minute minimum duration for Rational Method intensity selection.
References
American Society of Civil Engineers & Water Environment Federation. (1992). Design and construction of urban stormwater management systems (ASCE Manuals and Reports of Engineering Practice No. 77 / WEF Manual of Practice No. FD-20). American Society of Civil Engineers. ASCE product record
Cleveland, T. G., Thompson, D. B., Fang, X., & Li, M.-H. (2012). Establish effective lower bounds of watershed slope for traditional hydrologic methods (Report No. 0-6382-1). Texas Department of Transportation. https://rosap.ntl.bts.gov/view/dot/25168
Connecticut Department of Transportation. (2000). Drainage manual, Section 6.9.5, Infrequent Storm. https://portal.ct.gov/-/media/dot/drainage/69pdf.pdf
Federal Highway Administration. (2024). Urban drainage design manual (Hydraulic Engineering Circular No. 22, 4th ed.; Publication No. FHWA-HIF-24-006). U.S. Department of Transportation. https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif24006.pdf
Kerby, W. S. (1959). Time of concentration for overland flow. Civil Engineering, 29(3), 174.
Kirpich, Z. P. (1940). Time of concentration of small agricultural watersheds. Civil Engineering, 10(6), 362.
Kuichling, E. (1889). The relation between the rainfall and the discharge of sewers in populous districts. Transactions of the American Society of Civil Engineers, 20, 1–56. https://doi.org/10.1061/TACEAT.0000694
Perica, S., Martin, D., Pavlovic, S., Roy, I., St. Laurent, M., Trypaluk, C., Unruh, D., Yekta, M., & Bonnin, G. (2013). Precipitation-frequency atlas of the United States (NOAA Atlas 14). National Oceanic and Atmospheric Administration, National Weather Service. https://hdsc.nws.noaa.gov/pfds/ [Volume, version, and year vary by region; the tool reports the volume and version it queries.]
Roussel, M. C., Thompson, D. B., Fang, X., Cleveland, T. G., & Garcia, A. C. (2005). Time-parameter estimation for applicable Texas watersheds (Report No. 0-4696-2). Texas Department of Transportation. https://library.ctr.utexas.edu/digitized/texasarchive/phase2/4696-2-lamar.pdf
Texas Department of Transportation. (n.d.). Hydraulic design manual: Chapter 4. Hydrology. Retrieved June 1, 2026. Sections cited: Rational Method (§12); Runoff Coefficients (§12, Table 4-10); The Kerby Method (§11, Eq. 4-14); The Kirpich Method (§11, Eq. 4-15).
Informational use only — not engineered plans. Drainage Connect provides these tools and guides to help you understand drainage concepts and select products. Outputs are planning estimates based on simplified, standard methods and the values you enter together with published NOAA rainfall data; real sites vary. They are not a substitute for site-specific design. For any project involving vehicular loading, public right-of-way, structures, retaining walls, stormwater detention, regulated discharge, or risk to property or persons, have your design reviewed and stamped by a licensed Professional Engineer (PE) and confirm requirements with your local Authority Having Jurisdiction (AHJ). You are responsible for compliance with all local, state/provincial, and federal codes. Drainage Connect assumes no liability for decisions made using these resources and offers no warranty of fitness for a particular purpose. We sell products from many manufacturers and recommend the best-fitting solution at the lowest available cost — we are not the design engineer of record for your project.
