Manning Open Channel Flow Calculator
Calculate discharge (Q) and velocity (V) for gravity flow using the Manning equation — SI units.
Manning equation (SI)
Q = (1/n) · A · R2/3 · S1/2 · V = Q / A
Rectangular: A = b·y, P = b + 2y, R = A/P
Circular (full): A = πd²/4, P = πd, R = d/4
Q = (1/n) · A · R2/3 · S1/2 · V = Q / A
Rectangular: A = b·y, P = b + 2y, R = A/P
Circular (full): A = πd²/4, P = πd, R = d/4
Typical Manning’s n values
| Surface | n |
|---|---|
| PVC / HDPE pipe | 0.009 – 0.011 |
| Concrete pipe / channel | 0.012 – 0.014 |
| Finished metal | 0.012 |
| Brick | 0.015 |
| Earth channel, clean | 0.022 |
| Earth channel, weedy | 0.030 |