Drainage
Manning's Equation / Pipe Flow
Manning's equation calculator for full-pipe capacity and velocity using Manning's n — solve for discharge or back-solve for slope.
Drainage · Hydraulics
Manning's Equation / Pipe Flow
Manning's equation relates flow rate to channel geometry, slope, and roughness. For circular pipes flowing full, hydraulic radius equals D/4 and cross-sectional area equals πD²/4, producing a direct formula for full-pipe capacity and velocity.
The Manning's roughness coefficient (n) depends on pipe material and condition. Common values: PVC 0.009–0.011, concrete 0.011–0.013, corrugated HDPE 0.011–0.013, corrugated metal 0.021–0.025. This calculator also back-solves for the slope required to convey a target design flow.
Manning's Equation (Imperial)
Q = (1.486/n) · A · R^(2/3) · S^(1/2)
Full circular pipe: A = πD²/4, R = D/4
Velocity: V = Q / A
Reference: ASCE Manual of Engineering Practice No. 36; FHWA HDS-5; state DOT drainage design manuals.
Manning's Equation Calculator
Circular pipe, full-flow — imperial units
Example: 0.005 = 0.5%
PVC 0.010 Concrete 0.013 HDPE 0.012
Pipe Flow Results
Full-Pipe Capacity (Q)—
Velocity (V)—
Flow in GPM—
Cross-Section Area—
Hydraulic Radius (R)—
Full-pipe flow only. For partial flow apply Q/Q_full ratios from hydraulic tables. Verify velocity 2–10 fps for storm drains. Requires PE review.