How this calculator works
The ideal exit speed follows from converting pressure head to kinetic energy. A discharge coefficient accounts for jet contraction and losses. The default 0.61 is an approximate fully contracted sharp-edged value from the USBR manual, not a calibration for every opening. Use the upstream-to-downstream head difference for a submerged opening. A large opening spanning a substantial depth range requires integration rather than one centreline head.
Formula
Q = Cd A sqrt(2gH); A = pi d^2/4. H = [Q/(Cd A)]^2/(2g). Equivalent diameter assumes a circular opening.
Worked example
For A = 0.01 m2, Cd = 0.6, H = 2 m and g = 10 m/s2, Q = 0.03794733192 m3/s. With H = 0.5 m the discharge is 0.01897366596 m3/s.
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Frequently asked questions
Where is head measured?
Measure the vertical energy-head difference to the opening centre, using the downstream surface for a submerged outlet.
Does Cd always equal 0.61?
No. It depends on edge shape, contraction, Reynolds number and the surrounding boundaries.
Can this size a gas nozzle?
No. Compressibility and possible choking require a gas-flow model.