Hydraulic Calculations
Start from the density/area curve
For each hazard you select a density (gpm/ft²) from the curve and a design area. Enter the curve with either value to read the other, but the design area cannot be below the minimum for the hazard - commonly 1500 ft² for light and ordinary hazard and 2500 ft² for extra hazard. Multiply density by area to get sprinkler flow, then add the hose allowance (100 gpm light, 250 gpm ordinary, 500 gpm extra hazard) at the base of the riser.
Then do the hydraulics node by node
Discharge from a sprinkler is Q = K√P, so a 5.6 K sprinkler at 7 psi flows about 14.8 gpm. Move through the system accounting for friction loss (Hazen-Williams), fitting equivalent lengths, elevation at 0.433 psi per foot, and velocity pressure where required. Use the actual K-factor from the listing, never a nominal pipe-size assumption.
Traps: the 30% increases for dry-pipe and double-interlock preaction systems, and the reductions permitted for quick-response sprinklers, apply only under the standard's stated conditions and are not freely stackable; adding hose allowance at each remote node rather than once at the supply; forgetting the minimum operating pressure at the most remote standard spray sprinkler; checking supply against a hydrant test without adjusting it to the design basis.
Rule of thumb: 0.20 gpm/ft² × 1500 ft² = 300 gpm of sprinkler demand; add 250 gpm hose allowance for a 550 gpm system demand.