Hydrant Flow

GPM from Pitot pressure and outlet diameter.

Run the calculator

Example

You enter

Pitot pressure (psi) 10
Outlet diameter (in) 2.5
Coefficient of discharge 0.9

You get

Flow 530 gpm

Checked against published worked examples from: NFPA.

Details, formula, and sources

Formula or tableFlow GPM = 29.83 × c × d² × sqrt(P_pitot). c = coefficient of discharge (0.90 typical for rounded outlets, 0.80 for square outlets, 0.70 for cocked-back outlets).

Source and editionAWWA M17 (Installation, Field Testing, and Maintenance of Fire Hydrants) by name; ISO Public Protection Classification fire-flow methodology.

Free accessAWWA M17 licensed; principles free at most state-DOT and water-utility training pages.

What governsEstimate. AHJ and licensed fire protection engineer govern.

API field namespitot_psi, outlet_diameter_in, c, flow_gpm

Default coefficient 0.90 (rounded outlet) unless user suppliesAWWA M17 typical