p = 4.52 × Q^1.85 / (C^1.85 × d^4.87)

NFPA 13 form: p in psi/ft, Q in gpm, C = roughness coefficient, d = actual internal diameter (in). Straight-pipe friction only — fittings and elevation not included. C defaults are for wet systems; use C=100 for dry/preaction steel.