Drainage
Culvert Sizing
Culvert sizing calculator — minimum culvert diameter from Manning's equation given design discharge and slope, with standard pipe size selection and velocity check.
Culvert Sizing
Reviewed & updated August 28, 2026This calculator determines the minimum circular culvert diameter needed to convey a design peak flow at a given slope without exceeding full-pipe capacity. It applies Manning's equation for circular pipes flowing full — a conservative preliminary sizing step before headwater and inlet/outlet control analysis.
The computed minimum diameter is rounded up to the next standard pipe size. Full-pipe velocity is then checked against the specified minimum (typically 2.5 fps for storm culverts, 3.0 fps for sanitary or debris-laden flows). For final design, headwater depth must be evaluated under both inlet control (FHWA HDS-5 nomographs) and outlet control (energy balance with entrance, friction, and exit losses).
Q = (1.486/n) · A · R^(2/3) · S^(1/2) [A = πD²/4, R = D/4]
D_min = [Q · n / (0.4632 · S^0.5)]^(3/8) [ft]
Constant 0.4632 = 1.486 · (π/4) · (¼)^(2/3)
A 25 cfs design flow on a 0.5% slope through reinforced concrete pipe (n = 0.013) requires a minimum diameter of 28.4 inches, rounding up to the next standard size of 30 inches. At that size, full-pipe capacity is 29.0 cfs — more than the required 25 cfs — with a full-flow velocity of 5.91 fps, which clears the 2.5 fps self-cleansing minimum and confirms the pipe won't silt in over time.
Culvert Sizing Calculator
Circular pipe — Manning's full-flow · imperial units