Drainage Pipe Calculator
A drainage pipe calculator uses Manning's equation to determine the flow capacity of a circular drainage pipe based on its diameter, slope, and interior roughness. Manning's equation is the standard method used by civil engineers and hydrologists for calculating open-channel and gravity pipe flow. It is used to design storm drains, culverts, sanitary sewers, and agricultural drainage systems. By calculating the flow capacity at full-pipe conditions, you can determine whether a proposed pipe size is adequate for a design storm or check an existing pipe for hydraulic sufficiency. Enter the pipe diameter, slope, and Manning's roughness coefficient to get the full-pipe flow capacity and flow velocity.
Manning's equation for circular pipe
A = pi x (D/2)^2 [flow area for full pipe]
R = D / 4 [hydraulic radius for full circular pipe]
V = (1.486 / n) x R^(2/3) x S^(1/2) [velocity in ft/s]
Q = V x A [flow in cfs]
D is diameter in feet, n is Manning's roughness coefficient, S is slope in ft/ft. This formula applies to full pipe flow; capacity at 93% full is slightly higher due to the non-linear relationship.
Frequently asked questions
What is Manning's equation for pipe flow?
Manning's equation calculates the flow velocity in an open channel or partially full pipe: V = (1.486/n) x R^(2/3) x S^(1/2), where V is velocity (ft/s), n is Manning's roughness coefficient, R is hydraulic radius (ft), and S is the slope (ft/ft). Flow Q = V x A, where A is the cross-sectional area of flow.
What Manning's n value should I use for drainage pipe?
Common Manning's n values: smooth concrete pipe = 0.013, corrugated metal pipe (CMP) = 0.024, HDPE corrugated = 0.012-0.024, PVC smooth = 0.010-0.011, clay pipe = 0.013-0.015. Use 0.013 for most concrete storm drains. Always verify with your pipe manufacturer's data.
What minimum slope should drainage pipe have?
For self-cleaning velocity (typically 2 ft/s minimum), concrete pipe needs at least 0.5% slope (0.005 ft/ft). Steeper slopes improve flow and reduce sediment deposition. Minimum cover over storm drains is typically 1 foot; deeper installations require hydraulic design review.
What pipe size do I need to handle a 10-year storm?
Pipe sizing for a 10-year storm requires a hydrologic analysis: calculate the design flow using the Rational Method (Q = CiA) with local IDF curves, then size the pipe so it flows full or at a designated fraction of full capacity at or above the design flow.
What is the hydraulic radius for a full circular pipe?
For a circular pipe flowing full, the hydraulic radius R = D/4, where D is the pipe diameter. For a half-full pipe, R is the same (D/4). Maximum flow in a circular pipe actually occurs at about 93% full, not 100% full, due to the Manning's equation relationship.
Official sources
- Federal Highway Administration: FHWA HEC-22 Urban Drainage Design Manual.
- U.S. Army Corps of Engineers: USACE EM 1110-2-1601 Hydraulic Design of Flood Control Channels.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.