Storm Drain Size Calculator
Size storm drains using the Rational Method from rainfall intensity and drainage area. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Q = C x i x A | D = [(Q x n) / (0.4631 x S^0.5)]^(3/8)
The Rational Method Q = CiA gives peak runoff (cfs) from runoff coefficient C, rainfall intensity i (in/hr), and drainage area A (acres). Pipe diameter D (ft) is found by solving Manning equation for a full circular pipe, then rounding up to the next standard size.
Worked Examples
Example 1: Suburban Subdivision Drainage
Problem:A 5-acre suburban subdivision (C=0.65) needs a storm drain designed for a 10-year storm with 4 in/hr rainfall intensity. Pipe slope is 1%. Size the pipe using Manning equation (n=0.013).
Solution:Q = C x i x A = 0.65 x 4.0 x 5.0 = 13.00 cfs\n\nRequired diameter: D = [(Q x n) / (0.4631 x S^0.5)]^(3/8)\nD = [(13.00 x 0.013) / (0.4631 x 0.1)]^(3/8)\nD = [0.169 / 0.04631]^(3/8) = [3.650]^(0.375) = 1.61 ft = 19.3 inches\n\nSelect standard 21-inch pipe\nCapacity = (0.4631/0.013) x (21/12)^(8/3) x (0.01)^0.5\n= 35.62 x 3.543 x 0.1 = 12.62... use 24-inch: capacity = 35.62 x 4.595 x 0.1 = 16.37 cfs\nVelocity = 16.37 / (pi x (1)^2/4) = 16.37/3.14 = 5.21 ft/s
Result:Design Flow: 13.00 cfs | Required: 19.3 in | Select: 24-inch pipe | Velocity: 5.21 ft/s
Example 2: Commercial Parking Lot Drainage
Problem:A 2-acre commercial parking lot (C=0.85) with 10-year, 5.5 in/hr rainfall intensity. Pipe slope 0.005 (0.5%). Size the storm drain.
Solution:Q = C x i x A = 0.85 x 5.5 x 2.0 = 9.35 cfs\n\nD = [(9.35 x 0.013) / (0.4631 x 0.0707)]^(3/8)\nD = [0.1216 / 0.03274]^(3/8) = [3.713]^(0.375) = 1.63 ft = 19.5 inches\n\nSelect 21-inch pipe\nCapacity = (0.4631/0.013) x (1.75)^(8/3) x (0.005)^0.5\n= 35.62 x 3.216 x 0.0707 = 8.10 cfs... need larger\nSelect 24-inch: = 35.62 x 4.595 x 0.0707 = 11.58 cfs > 9.35 OK\nVelocity = 11.58 / (pi x 1^2/4) = 3.69 ft/s
Result:Design Flow: 9.35 cfs | Select: 24-inch pipe | Capacity: 11.58 cfs | Velocity: 3.69 ft/s
Frequently Asked Questions
What is the Rational Method and when is it used for storm drain design?
The Rational Method is the most widely used technique for estimating peak stormwater runoff from small drainage areas, expressed as Q = C x i x A, where Q is the peak runoff rate (cfs), C is the runoff coefficient (dimensionless, 0-1), i is the rainfall intensity (inches/hour) for the design storm duration equal to the time of concentration, and A is the drainage area (acres). The method was developed in the 1880s and remains standard practice because of its simplicity and reasonable accuracy for small watersheds. It is appropriate for drainage areas up to about 200 acres (some agencies allow up to 640 acres). For larger areas, the SCS (NRCS) Curve Number method or unit hydrograph methods are more appropriate because they account for rainfall distribution over time and storage effects that the Rational Method ignores. The Rational Method assumes that rainfall intensity is uniform over the entire drainage area and that the peak runoff rate occurs when the entire area is contributing flow.
What is the time of concentration and how does it affect storm drain design?
Time of concentration (tc) is the time required for runoff to travel from the hydraulically most distant point in the drainage area to the point of interest (the storm drain inlet). It is the critical duration used to select the design rainfall intensity from intensity-duration-frequency (IDF) curves. A shorter tc means a higher rainfall intensity is used, resulting in a larger design flow. The tc consists of two components: overland flow time (sheet flow over ground surfaces) and channel/pipe flow time (concentrated flow in gutters, channels, and pipes). Common methods to estimate tc include: Kirpich formula for overland flow over natural surfaces, SCS sheet flow equation (limited to 300 feet), and pipe flow velocity calculations for downstream segments. Minimum tc is typically 5-10 minutes for small urban areas. As development replaces natural ground with impervious surfaces, tc decreases (faster runoff), which increases peak flow rates. This is a primary reason why urbanization increases flooding risk.
How are storm drain pipes sized using Manning equation?
Storm drain pipes are sized by first determining the design flow using the Rational Method (or other hydrologic method), then selecting a pipe diameter that can carry the design flow at the required slope using Manning equation. For a circular pipe flowing full, the capacity is Q = (0.4631/n) x D^(8/3) x S^(1/2), where n is Manning roughness coefficient, D is diameter in feet, and S is the pipe slope. The required diameter is D = [(Q x n) / (0.4631 x S^0.5)]^(3/8). The calculated diameter is rounded up to the next available standard pipe size (8, 10, 12, 15, 18, 21, 24, 30, 36 inches, etc.). The selected pipe is then checked for minimum velocity (typically 2-3 ft/s at design flow to prevent sediment deposition) and maximum velocity (typically 10-15 ft/s to prevent erosion). Most design standards require pipes to flow no more than full (not pressurized) for the design storm, with some agencies requiring a maximum depth-to-diameter ratio of 0.8.
What design storm frequency (return period) should be used for storm drains?
The design storm return period varies by facility type, land use, and local regulations. Common standards include: minor storm drains in residential areas use 10-year storms, commercial and industrial areas use 10-25 year storms, arterial road crossings use 25-50 year storms, and critical facilities (hospitals, emergency services) use 50-100 year storms. Many jurisdictions require a dual-design approach: the minor system (pipes and inlets) handles the 10-year storm, while the major system (overland flow paths, detention basins) safely conveys the 100-year storm without flooding buildings. The design rainfall intensity is obtained from local IDF curves, which relate rainfall intensity to storm duration and return period. For example, a 10-year, 15-minute storm in Atlanta has an intensity of about 6.5 in/hr, while the same storm in Phoenix has about 4.0 in/hr. These geographic variations make local IDF data essential for accurate storm drain design.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy