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
This 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