Fire Hydrant Flow Test Calculator
Analyse a fire hydrant flow test: discharge from the pitot pressure and outlet size (Q = 29.83 c d² √P), the available flow at 20 psi residual using the NFPA 291 Hazen–Williams exponent 0.54, flow at any other residual pressure, the supply curve, and hydrant colour class.
How it works
Q_test = 29.83 × c × d² × √P_pitot; Q_R = Q_test × [(P_static − P_R) ÷ (P_static − P_residual)]^0.54; class from Q at 20 psi
A hydrant flow test measures how much water a distribution main can give up before its pressure falls to the 20 psi that protects it from collapse and back-siphonage. One hydrant is flowed while another nearby reads static and residual pressure; the discharge is computed from a pitot gauge reading in the stream (a velocity-pressure orifice equation), and because friction loss in the main follows the Hazen–Williams relationship, flow scales with pressure drop to the power 0.54. NFPA 291 asks for a drop of at least 25% so the curve is anchored by a real, substantial flow; the result sets the hydrant's colour band and is the starting point for every sprinkler and fire-pump design that connects to that main.
Worked example
A test showing 68 psi static, 52 psi residual and 28 psi on the pitot from one 2½″ outlet flows 29.83 × 0.9 × 6.25 × √28 = 888 gpm. Extrapolated to 20 psi: 888 × (48 ÷ 16)^0.54 = 1,610 gpm — a class AA (light blue) hydrant; at 35 psi residual about 1,310 gpm is available.
Frequently asked questions
Why 20 psi?
Below about 20 psi in the main there is risk of pipe collapse under vacuum, back-siphonage of contaminants and starved pumps; codes and NFPA 291 treat 20 psi residual as the floor for design flow.
Why does the pitot gauge need the outlet coefficient?
The stream contracts differently depending on the outlet shape: a smooth rounded outlet passes nearly its full area (0.9), a square-edged one less (0.8), and an outlet that projects into the barrel least (0.7). Use the wrong one and the flow is off by 10–20%.
Related calculators
Uses the standard fire-service field formulas (IFSTA / NFPA 1002 hydraulics, NFA fire-flow formula, NFPA 13 and 291 methods) in simplified form for training and planning. Department SOPs, the authority having jurisdiction, listed equipment data and a fire-protection engineer govern real operations and designs.