Time of Concentration Calculator

Pick a method

• Kirpich was calibrated on small (1–112 acre), steep, well-drained agricultural watersheds in Tennessee. Underpredicts for flat or urban watersheds; do not use for impervious or very flat ground.
• NRCS lag (TR-55) is the workhorse for US stormwater hydrology. Works for basins under 2000 acres. Accounts for surface roughness via the curve number — higher CN (more impervious) gives shorter t_c.
• SCS sheet/shallow/channel segmental method (not in this calculator) splits the longest flow path into three regimes: sheet flow (~100 ft max), shallow concentrated flow, and channel flow. Sum the travel time of each segment. Most rigorous; required by many regulators for designs over 1 acre.

Why t_c matters

In the Rational Method (Q = CIA), t_c selects the rainfall intensity I to use. Shorter t_c → higher I → higher peak flow Q. Underestimating t_c oversizes pipes, culverts, and BMPs (conservative but expensive). Overestimating misses the peak and undersizes hydraulic infrastructure.

In NRCS hydrograph methods, t_c determines the time-to-peak and the unit hydrograph shape. Both peak flow and the volume of the rising limb are sensitive to t_c.

Practical minimums

Most stormwater regulatory agencies impose a 5- or 10-minute minimum t_c regardless of computed value. Even on a tiny lot, depression storage and surface roughness add at least a few minutes of lag, so a computed 2-minute t_c is not physically realistic.

Slope sensitivity

Both formulas have t_c ∝ S^-0.4 roughly. Doubling slope shortens t_c by ~25%. Halving slope adds ~33%. For very flat basins (S < 0.005 ft/ft), neither formula is reliable; use the segmental method.

Reference: USDA NRCS (1986). Urban Hydrology for Small Watersheds (TR-55). Original: Kirpich, Z.P. (1940). "Time of concentration of small agricultural watersheds." Civil Engineering, 10(6), 362.