Parshall Flume Flow Calculator
Read the upstream head in a Parshall flume and get the flow — free-flow rating for standard throat widths from 1 inch to 8 feet.
Flume Discharge (Free Flow)
| Item | Value |
|---|
How the calculation works
Free-flow formula (Q in cfs, Ha in feet)
Q = C × Han
C and n come from the throat-width table below.
Ha (ft) = Ha (in) ÷ 12
1 cfs = 448.831 gpm = 0.646317 MGD
Q = C × Han
C and n come from the throat-width table below.
Ha (ft) = Ha (in) ÷ 12
1 cfs = 448.831 gpm = 0.646317 MGD
| Throat width | C | n |
|---|---|---|
| 1 in | 0.338 | 1.55 |
| 2 in | 0.676 | 1.55 |
| 3 in | 0.992 | 1.55 |
| 6 in | 2.06 | 1.58 |
| 9 in | 3.07 | 1.53 |
| 1 ft | 4.00 | 1.522 |
| 1.5 ft | 6.00 | 1.538 |
| 2 ft | 8.00 | 1.550 |
| 3 ft | 12.00 | 1.566 |
| 4 ft | 16.00 | 1.578 |
| 5 ft | 20.00 | 1.587 |
| 6 ft | 24.00 | 1.595 |
| 7 ft | 28.00 | 1.601 |
| 8 ft | 32.00 | 1.607 |
Notes & limitations
Free-flow C and n values are the standard Parshall flume constants published in the USBR Water Measurement Manual (free-flow discharge table). Each throat size has its own calibrated constants — do not mix them.
- Free flow only: this rating assumes the flume is not submerged. If the downstream head Hb is high relative to Ha (submergence ratio above roughly 0.6–0.8 depending on throat size), apply the submerged-flow correction instead.
- Measure Ha at the prescribed gauge location in the converging section, not at the throat.
- Keep the approach channel straight, clean, and free of debris; skewed or turbulent approach flow biases the reading.
- Verify the flume dimensions against the standard — a non-standard or settled flume needs its own calibration.
← All water treatment calculators · Related: Manning Calculator