Weir Flow Calculator
Read the head over a weir crest and get the flow — for rectangular weirs and 90° V-notch weirs in channels, weir boxes, and stilling basins.
Weir Discharge
| Item | Value |
|---|
How the calculation works
Formulas (Q in cfs, L and H in feet)
Rectangular (suppressed): Q = 3.33 × L × H1.5
90° V-notch: Q = 2.5 × H2.5
Head conversion: H (ft) = H (in) ÷ 12
1 cfs = 448.831 gpm = 0.646317 MGD
Rectangular (suppressed): Q = 3.33 × L × H1.5
90° V-notch: Q = 2.5 × H2.5
Head conversion: H (ft) = H (in) ÷ 12
1 cfs = 448.831 gpm = 0.646317 MGD
| Weir type | Formula (ft, cfs) | Best for |
|---|---|---|
| Rectangular, suppressed | Q = 3.33·L·H1.5 | Medium to large flows |
| 90° V-notch | Q = 2.5·H2.5 | Small flows, precise at low heads |
Notes & limitations
The 3.33 and 2.5 coefficients are the standard Francis / V-notch weir constants for free-flowing (unsubmerged) weirs, consistent with the USBR Water Measurement Manual and standard hydraulics references. They assume a sharp crest, ventilated nappe, and negligible velocity of approach.
- Free flow only: if the downstream water level rises near the crest (submerged weir), these formulas over-read — use a submerged-weir correction or a flume instead.
- Measure head far enough upstream (about 4× the maximum head) where the drawdown curve has not started, and zero the gauge to the crest.
- For contracted rectangular weirs (crest shorter than the channel), the Francis correction Q = 3.33·(L − 0.2H)·H1.5 applies; this tool assumes a suppressed weir spanning the full channel width.
- Debris, algae, or a damaged crest changes the effective coefficient — inspect the weir before trusting the reading.
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