What This Solves
Calculates the time required for runoff to travel from the most distant point in a watershed to the outlet using various empirical methods.
Best Used When
- You need to determine storm duration for peak flow estimation in the Rational Method
- You are analyzing composite flow paths with sheet flow, shallow concentrated flow, and channel segments
- You want to compare multiple Tc methods (Kirpich, NRCS, FAA, etc.) to select a representative value
Do NOT Use When
- You are calculating runoff volume and the method already includes time of concentration — Use SCS Curve Number Calculator
- You need peak flow directly and prefer a single integrated tool — Use Rational Method Calculator
Key Assumptions
- Flow path is a single continuous route from the most distant hydraulic point to the outlet
- Surface roughness and slope are uniform within each flow segment type
- Time of concentration assumes the entire watershed is contributing at equilibrium
- Empirical equations are valid only within their original calibration ranges (area, slope, length)
- No significant detention or storage features interrupt the flow path
Input Quality Notes
Different methods can produce widely varying results. Use methods appropriate for your land use and watershed size, and consider averaging or selecting the method that best matches local conditions and regulatory guidance.
Try a Common Scenario
Click to pre-fill the calculator with realistic values.
Estimate the time of concentration (Tc) — the time for runoff to travel from the hydraulically most distant point in a watershed to its outlet — using five established methods (Kirpich, FAA, NRCS Lag, Kerby-Hathaway and the TR-55 segmental method) in US customary or SI units.
Calculation Method
Best for: Small agricultural watersheds with overland and channel flow. Developed from data on Tennessee watersheds.
Which Method Should I Use?
Kirpich
- - Small agricultural watersheds
- - Natural channels
- - Simple preliminary estimates
FAA
- - Airport drainage
- - Parking lots
- - Small impervious areas
NRCS Lag
- - SCS hydrograph methods
- - Rural watersheds
- - When CN is known
Kerby-Hathaway
- - Overland flow only
- - Natural surfaces
- - Flow length under 1000 ft
TR-55 Composite
- - Complex flow paths with multiple segments
- - Urban and rural watersheds
- - Most accurate for varied terrain
- - Required for detailed drainage design
How it works: the governing equations
Each method is an empirical or physically based relationship between flow-path length, slope and surface roughness. The calculator works internally in US customary units (length in feet, slope as a ft/ft decimal unless noted) and returns Tc in minutes.
Kirpich
Tc = 0.0078 · L0.77 · S−0.385
L = longest flow-path length (ft); S = average slope = H / L (ft/ft), where H is the elevation drop along the path. Developed from small agricultural watersheds in Tennessee. Source: FHWA HEC-22, Eq. 3-3.
FAA
Tc = 1.8 · (1.1 − C) · L0.50 / S0.333
L = overland flow length (ft); C = Rational-method runoff coefficient; S = slope expressed as a percent. Best for airport drainage and small impervious areas. Source: FHWA HEC-22, Eq. 3-5.
NRCS (SCS) Lag
Lag = L0.8 · (S + 1)0.7 / (1900 · Y0.5) · Tc = Lag / 0.6
L = hydraulic length (ft); Y = average watershed slope (percent); S = maximum potential retention = 1000 / CN − 10 (inches), from the SCS curve number CN. Lag is in hours and Tc ≈ Lag / 0.6. Source: NRCS TR-55 (1986) and NEH Part 630.
Kerby-Hathaway
Tc = 0.83 · (L · N)0.467 · S−0.235
L = overland flow length (ft, max ≈ 1000 ft); N = Kerby retardance coefficient; S = slope (ft/ft). For pure overland (sheet) flow on natural surfaces. Source: NRCS NEH Part 630, Ch. 15.
TR-55 segmental (sheet + shallow + channel)
Tc is the sum of travel times along the longest flow path. Sheet flow (max ≈ 300 ft):
Tt = 0.007 · (n · L)0.8 / (P20.5 · S0.4)
where n = sheet-flow Manning's roughness, P2 = 2-year 24-hour rainfall depth (in), S = slope (ft/ft). Shallow concentrated flow velocity V = 20.328√S (paved) or 16.1345√S (unpaved) ft/s; channel flow velocity from Manning's equation V = (1.486 / n) · R2/3 · S1/2. For each segment Tt = L / (60 · V) minutes. Source: NRCS TR-55 (1986).
Equations are implemented exactly as shown above in the calculator's library
(src/lib/calculations/time-of-concentration.ts). SI inputs are converted to feet/inches
internally. Always validate Tc against your local drainage manual and apply any required minimum.
Sheet-flow Manning's roughness (n) reference
Manning's roughness coefficients for shallow sheet flow, used in the TR-55 sheet-flow travel-time equation. Values are from TR-55, Table 3-1 (USDA NRCS, 1986) and match the coefficients built into this calculator.
| Surface | Manning's n |
|---|---|
| Smooth asphalt | 0.011 |
| Smooth concrete | 0.012 |
| Ordinary (rough) concrete / tar & gravel | 0.013 – 0.014 |
| Fallow soil (no residue) | 0.05 |
| Cultivated soil (with residue) | 0.06 |
| Range (natural) | 0.13 |
| Short-grass prairie | 0.15 |
| Dense grass | 0.24 |
| Bermuda grass | 0.41 |
| Woods, light underbrush | 0.40 |
| Woods, dense underbrush | 0.80 |
Note: these sheet-flow n values are intentionally higher than open-channel Manning's n for the same material because they account for raindrop impact and very shallow depths. Use channel-flow n values (typically 0.011–0.05 for pipes and lined channels) only for the channel-flow segment.
Worked example (Kirpich)
A small watershed with a 4,000 ft longest flow path dropping 100 ft in elevation:
- Average slope S = H / L = 100 / 4,000 = 0.025 ft/ft
- Apply Kirpich: Tc = 0.0078 × 4,0000.77 × 0.025−0.385
- Tc ≈ 19.2 minutes
This matches the verification case in the calculator's library (HEC-22, Eq. 3-3).
Frequently asked questions
What is time of concentration (Tc)?
Time of concentration is the time it takes for runoff to travel from the hydraulically most distant point in a watershed to the outlet. At that point the entire drainage area is contributing flow simultaneously, which produces the peak discharge for a storm of uniform intensity. Tc is a key input to the Rational Method (where rainfall intensity is read at a duration equal to Tc) and to NRCS/SCS unit-hydrograph methods. It is usually reported in minutes.
Which time of concentration method should I use?
It depends on the watershed. The Kirpich equation suits small agricultural watersheds (under about 200 acres) with mixed overland and channel flow. The FAA method was developed for airports, parking lots and small impervious areas and uses a runoff coefficient. The NRCS (SCS) Lag method suits rural watersheds where a curve number is known and pairs with SCS hydrograph methods. Kerby-Hathaway covers pure overland (sheet) flow up to about 1,000 ft. The TR-55 segmental method is the most rigorous for varied terrain: it sums sheet flow, shallow concentrated flow and channel flow travel times along the longest flow path.
What is the maximum sheet flow length in TR-55?
TR-55 limits sheet flow to a maximum of about 300 ft (roughly 91 m). Beyond that distance, shallow flow concentrates into rills and small channels, so the remaining path should be modeled as shallow concentrated flow and then channel flow. Some agencies use a more conservative limit of 100 ft for sheet flow; always check your local drainage manual.
Is there a minimum time of concentration?
Many agencies enforce a minimum Tc — commonly 5 to 10 minutes — even when a calculation returns a smaller value. Very short times of concentration produce unrealistically high rainfall intensities in the Rational Method, so a floor is applied to keep designs reasonable. Confirm the minimum required by your reviewing jurisdiction.
How does time of concentration affect peak flow?
A shorter Tc means the watershed concentrates flow quickly, so a shorter-duration, higher-intensity rainfall drives the peak — generally increasing peak discharge. Development that adds impervious cover, smooth surfaces, gutters and storm sewers shortens Tc and raises peak flows, which is why post-development drainage often requires detention to offset the change.
Standards & related tools
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Last verified: February 2026