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Weir Flow Rate Calculator

Work out weir flow rate instantly with clear inputs, formula shown and shareable results.

About Weir Flow Rate Calculator

A sharp-crested rectangular weir passes Q = 1.84 L H^1.5, so 2.5 m of crest under 0.3 m of head gives 0.756 m3/s. A 90 degree V-notch follows H^2.5, which makes it far more sensitive at low flows and the standard choice for gauging small streams. Head must be measured upstream of the drawdown curve.

Formulas

Rectangular weir

Q = 1.84 x L x H^1.5

90 degree V-notch

Q = 1.38 x H^2.5

Worked example

With the default values below, Weir Flow Rate Calculator returns Discharge Over The Weir of 0.7559. Change any input above and every figure here is recalculated in your browser.

Inputs used

Weir crest lengthminimum 0 m
2.5 m
Head over the crestminimum 0 m
0.3 m
Weir typeone of Rectangular sharp-crested, V-notch, 90 degrees, Broad-crested
Rectangular sharp-crested

Results

Discharge Over The Weir
0.7559 m3/s
Same Discharge In Litres Per Second
755.9
Volume Per Day
65,306 m3
Head Exponent Used
1.5

Step by step

  1. 1Values used
  2. 2Rectangular weir
  3. 390 degree V-notch
  4. 4Discharge Over The Weir= 0.7559 m3/s
  5. 5Same Discharge In Litres Per Second= 755.9
  6. 6Volume Per Day= 65,306 m3
  7. 7Head Exponent Used= 1.5

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does a V-notch use a higher exponent?

Because the flow width grows with head as well as the depth, so discharge rises faster - which gives better resolution at low flows.

Where should the head be measured?

At least four times the head upstream of the crest, beyond the surface drawdown, or the reading will be too low.

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