Time of Concentration Calculator (Kirpich)
Tc sets which rainfall intensity your IDF curve gives you. Enter the longest flow path and its average slope to get the Kirpich time of concentration.
Time of Concentration (Kirpich)
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How the calculation works
Kirpich equation (1940)
Tc (min) = 0.0078 × L0.77 × S−0.385
L = flow path length (ft), S = average slope (ft/ft, decimal)
Average velocity along the path = L ÷ (Tc × 60) (ft/s), shown for a sanity check.
Tc (min) = 0.0078 × L0.77 × S−0.385
L = flow path length (ft), S = average slope (ft/ft, decimal)
Average velocity along the path = L ÷ (Tc × 60) (ft/s), shown for a sanity check.
Notes & limitations
The Kirpich equation (Z.P. Kirpich, 1940) was developed for small agricultural watersheds in Tennessee and is widely adopted for urban drainage in FHWA HEC-22 and state DOT manuals. It tends to underpredict Tc on very flat or heavily vegetated overland flow — many designers pair it with a sheet-flow equation (e.g., the SCS/TR-55 kinematic-wave form) for the overland segment.
- Most agencies impose a minimum Tc of 5 minutes for storm drain design; some use 10 minutes. Use the agency minimum when the computed value is smaller.
- The flow path should follow the hydraulically longest route, combining overland, shallow concentrated, and channel/pipe segments. For a segmented path, compute each segment’s travel time and sum them instead of using one average slope.
- Kirpich was calibrated on watersheds up to a few hundred acres; for large or complex watersheds use TR-55 or a hydrograph method.
- Use the resulting Tc as the storm duration when reading intensity from your IDF curve for the Rational Method.
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