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Calculating water flow at a fire hydrant
- Pitot formula


The formula below calculates the water flow at a fire hydrant when flow tested with a Pitot tube. To get the water flow, use the formula below to plug in the the Pitot tube reading, and the diameter and shape of the hydrant opening.

Pitot formula: Q [flow in GPM] = 29.83 x √P2 [square root of Pitot reading in PSI] x D2 [square of orifice diameter in inches] x Cd [orifice coefficient - see table at bottom].
Pitot formula
Pitot
reading
P [in PSI]		 Orifice
Diameter
D [in Inches]		 Orifice
Coefficient
see table below		 Water flow
US gallons
per minute
Key in your
value below		 Key in your
value below		 Key in your
value below		 Result of
calculation below
25 2.25 0.8 604.06


Orifice Coefficients
Type of
orifice		 Coefficient
Rounded 0.9
Square 0.8
Projected 0.7




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