text: In a design problem, one may select pipe for a particular hydraulic slope S based on the candidate pipes diameter D' and its roughness ε. 
  With these quantities as inputs, the friction factor f can be expressed in closed form in the Colebrook–White equation or other fitting function, and the flow volume Q and flow velocity V can be calculated therefrom.
  In the case of water (ρ = 1 g/cc, μ = 1 g/m/s) flowing through a 12-inch (300 mm) Schedule-40 PVC pipe (ε = 0.0015 mm, D = 11.938 in.), a hydraulic slope S = 0.01 (1%)  is reached at a flow rate Q = 157 lps (liters per second), or at a velocity V = 2.17 m/s (meters per second). 
  The following table gives Reynolds number Re, Darcy friction factor f, flow rate Q, and velocity V such that hydraulic slope S = h / L = 0.01, for a variety of nominal pipe (NPS) sizes. 
  Note that the cited sources recommend that flow velocity be kept below 5 feet / second (~1.5 m/s).
  Also note that the given f in this table is actually a quantity adopted by the NFPA and the industry, known as C, which has the imperial units psi/(100 gpmft) and can be calculated using the following relation:
  where  is the pressure in psi,  is the flow in 100gpm and  is the length of the pipe in 100ft'