When to use: Size a storm drain by computing the discharge a circular pipe carries when flowing just full. Using Manning's equation Q = (1.49/n)·A·R^⅔·S^½, with the hydraulic radius of a full circular pipe equal to D/4, gives the full-flow velocity and capacity. Storm pipes should also reach a minimum self-cleaning velocity of about 3 ft/s to prevent sediment deposition; the calculator flags whether the current slope achieves this and reports the minimum slope required.
This calculator uses Manning's equation to compute the discharge capacity of a circular storm drain pipe flowing full, and checks whether the pipe slope achieves the minimum self-cleaning velocity — the two criteria that define an adequately sized storm sewer per ASCE and local drainage standards.
For a circular pipe flowing full, the wetted area is A = π·D²/4 and the hydraulic radius is R = D/4 (a convenient result of the full-circle geometry). Substituting into Manning's equation in US customary units: Q = (1.49/n) · (πD²/4) · (D/4)^(2/3) · S^(1/2), which simplifies to Q = (0.4632/n) · D^(8/3) · S^(1/2).
This full-flow capacity is the basis for storm drain sizing. Pipes are typically sized so that the design storm flow Q does not exceed the full-flow capacity Q_full, with the hydraulic grade line (HGL) checked to confirm pipes do not surcharge. Actual flow in a storm drain system is often at 50–90% of full-flow capacity during the design storm.
Storm drain design follows ASCE Manual of Engineering Practice No. 36, FHWA Urban Drainage Design Manual (HEC-22), and local municipal drainage design standards. Design storm frequencies are typically 10-yr for minor systems (streets, inlets, storm mains) and 100-yr for major conveyance and outfall structures per ASCE 7 stormwater provisions. Many cities also require 2-yr design for residential inlet spacing.
Pipe material specifications are governed by ASTM: D3034 (PVC), C76 (RCP), and A760/A762 (CMP). Manning's n values are: RCP 0.012–0.013, smooth HDPE 0.012, corrugated HDPE 0.016–0.020, CMP 0.022–0.026. AASHTO LRFD Bridge Design Specifications govern storm drain design for highway applications.
Minimum self-cleaning velocity is 2.5–3.0 ft/s in the full-pipe condition per ASCE HEC-22, to prevent sediment deposition that reduces capacity and causes odors. Maximum velocity is typically limited to 10–12 ft/s for RCP and 15 ft/s for HDPE to prevent erosion at joints and outlets. A velocity of 10 ft/s at full flow corresponds to significant erosion potential at outfall structures, requiring energy dissipators or riprap aprons.
Minimum pipe size for storm drains is 15 inches (12 inches for lateral drains in some jurisdictions) to facilitate maintenance access and cleaning. Minimum cover over the pipe crown is typically 2–3 ft for traffic loading, or the manufacturer's installation requirements for the given pipe material and load rating.
Select the pipe material to set Manning's n. Enter the inside pipe diameter (inches) and pipe slope (ft/ft). The calculator returns full-flow capacity Q (cfs and gpm), flow velocity V (ft/s), flow area, hydraulic radius, and the minimum slope required to achieve 3 ft/s self-cleaning velocity. If the current slope is below 3 ft/s, increase slope until the status turns green. Cross-check Q against the Rational Method or hydrograph-based peak flow to confirm the pipe is adequately sized for your design storm.
Design for full-flow capacity as the upper limit — the pipe should be able to carry the design storm flow when flowing just full (at atmospheric pressure). In practice, designing pipes so the HGL stays below the crown (partial flow) prevents pressurization and surcharging. Many drainage standards require the HGL to remain at least 1 ft below the inlet grate elevation for the design storm to prevent surface flooding.
Solving Q = (0.4632/0.013) · D^(8/3) · 0.005^(1/2) for D at Q = 50 cfs gives D ≈ 2.4 ft = 29 in. Round up to the next standard size (30 in). Always verify that the slope achieves 3 ft/s minimum velocity in the selected pipe. A smaller diameter at the same slope will increase velocity, which may be desirable if minimum velocity is marginal.
The minor storm system (typically 10-yr design) includes inlets, storm mains, and outfalls sized to handle frequent storms with minimal street flooding. The major storm system (100-yr design) is the overland flow path — streets, swales, and detention facilities — that handles the rare large storm when the minor system is overwhelmed. Both must be designed to function, and the major system flow path must not threaten life safety or structural damage.
CMP has Manning's n = 0.022–0.026 versus RCP n = 0.012–0.013. For the same diameter and slope, CMP carries approximately 50–55% of the capacity of RCP. This means a CMP pipe must be roughly one to two standard sizes larger than an RCP to pass the same design flow. HDPE smooth pipe (n = 0.012) is comparable to RCP in hydraulic efficiency.
The HGL represents the pressure head (water surface elevation) at each point in a pressurized pipe system. For a storm drain flowing at capacity, the HGL may rise above the pipe crown, creating pressure flow and potentially surcharging manholes and inlets. HGL analysis per HEC-22 energy method or SWMM hydraulic routing is required to confirm that surcharged pipes do not flood the surface at the design storm frequency.