Manning Pipe Capacity (Full-Bore Gravity Flow)
The discharge a circular gravity pipe carries flowing full at a given slope.
Example
You enter
- Pipe diameter (in) 8
- Pipe slope S (ft/ft) 0.01
- Pipe material concrete
You get
- Full-flow velocity 3.46 ft/s
- Full-flow capacity 1.208
- Q (GPM) 542.4
Details, formula, and sources
V = (1.486/n) R^(2/3) sqrt(S) with R = D/4, Q = V (pi/4) D^2. An 8 in concrete pipe (n 0.013) at 1% slope carries 1.21 cfs (542 gpm) at 3.46 ft/s; because Q scales with sqrt(S), doubling the slope raises capacity only 1.41x. Circular pipe flowing full, normal depth; the partial-flow depth is separate. A design aid; the engineer of record and the sewer code govern.
English Manning V = (1.486 / n) R^(2/3) S^(1/2); full-bore circular pipe R = D/4, A = (pi/4) D^2, Q = V A; Q_gpm = 448.831 Q_cfs.
Manning equation; n-values from USGS WSP-2339 (public domain).
USGS WSP-2339 free at pubs.usgs.gov.
Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
Field names used by the API: d_in, slope, material, v_fps, q_cfs, q_gpm
- Full-bore hydraulic radius R = D/4, A = (pi/4) D^2 for a circular pipe flowing fullcircular cross-section geometry
- Roughness n from the standard tables (PVC 0.009, concrete 0.013, corrugated metal 0.024), entered by materialUSGS WSP-2339
- Uniform full flow steady normal-depth full flow; the partial-flow depth is separatescope of this tile