Formulas
Smooth-Bore Flow Formula
The smooth-bore flow formula estimates a solid-stream nozzle's flow from its tip diameter and nozzle pressure. Bigger tips and higher pressures both raise the flow — and the reaction force the crew must hold.
A smooth bore's flow is predictable because nothing inside it moves: the discharge follows from the opening size and the pressure behind it. That predictability makes smooth bores easy to plan for at the panel.
The same formula reveals the trade-off. Going up a tip size or up in pressure to gain flow also raises the nozzle reaction, so flow and handling are decided together, not separately.
Formula
GPM = 29.7 × D² × √NP
D = tip diameter in inches; NP = nozzle pressure; √NP = its square root. Bigger tips and higher pressures both raise the flow — and the reaction force the crew must hold.
Common questions
- What is the smooth-bore flow formula?
- The smooth-bore flow formula is GPM = 29.7 × D² × √NP: a coefficient times the tip diameter squared times the square root of the nozzle pressure. It estimates a solid-stream nozzle's flow from its tip size and the pressure behind it.
- Why is a smooth-bore nozzle's flow easy to predict?
- Because nothing inside it moves. The discharge follows directly from the fixed opening size and the nozzle pressure, so once you know the tip diameter and the pressure you can plan the flow at the panel with confidence.
Where this shows up
Calculator
Nozzle Flow & Reaction