Quick answer: A sprinkler hydraulic calculation starts from the design density (LPM/m²) over the design area, finds the flow per head from Q = K√P, then adds Hazen-Williams friction losses back to the source to find the required pump pressure and total demand.
The method
Flow per head: Q = K × √P (K-factor, P = pressure)
Friction (Hazen-Williams): p = 6.05×10^5 × Q^1.85 / (C^1.85 × d^4.87)
Design density × area = base demand
Balance node by node back to the riserSteps
- Pick hazard class → design density and design area (NFPA 13 / IS 15105).
- Compute flow at the most remote head from Q = K√P.
- Add friction and elevation losses pipe by pipe.
- Sum to the total demand and required pump duty.
Worked example
Ordinary Hazard: density ~5 mm/min over 144 m². With K=80 heads at ~0.5 bar, each flows ~57 LPM; balancing the remote area and friction gives the pump duty point — typically a few hundred LPM at 5–7 bar.
Design density, flow and Hazen-Williams pipe balance to NFPA 13. Free tool.
Open the Sprinkler Hydraulic Calculator →Frequently Asked Questions
What is the K-factor in sprinkler calculations?
The K-factor relates a sprinkler head flow to pressure by Q = K times the square root of P. A standard head has K around 80 (metric), so higher pressure gives proportionally more flow.
What is design density in NFPA 13?
Design density is the minimum water application rate over the design area, set by hazard class — for example about 5 mm/min for Ordinary Hazard. It defines the base water demand for the system.