Flow Formulas & Cv Factors

General Information About Valve Sizing:

Valves with a "Full Port" have an internal seat diameter that is the same as the nominal pipe size, i.e. a 1 inch pipe size valve with a full port has a 1 inch diameter seat. Valves with a "Reduced Port" have an internal seat diameter that is smaller than the nominal pipe size.

The valve's flow coefficient, Cv, is a value that is determined by flow testing for each valve size. Full port valves will have a higher Cv than reduced port valves. The Cv rating for each valve is listed in the tables found in the valve catalog.

The definition of Cv is the # of gallons of water that will flow through the valve with a 1 PSI pressure differential when the valve is open.

The equations below can be used to determine:

  • Flow Rate, given the Cv and ΔP
  • Cv, given the Flow Rate and ΔP
  • ΔP, given the Flow Rate and Cv

Cv = Valve's flow coefficient (dimensionless value)
S = Specific Gravity (1.0 for air or water)
T = Absolute Temperature in °R (°R = °F + 460)
P1 = Inlet Pressure in PSIG
ΔP = Pressure Differential in PSI across valve in the open position
V = Specific Volume in Cubic Feet per Pound

Flow Formulas

Cv Factors

The CV factor is the number of U.S. gallons per minute that will pass through a valve with a pressure drop of (1) PSI.

Port Size in Inches 3/32" 1/8" 5/32" 3/16" 1/4" 5/16" 3/8" 1/2" 3/4" 1" 1-1/4" 1-1/2" 2" 2-21/2" 3"
Cv Factor 0.2 0.3 0.36 0.53 0.7 1.7 2.0 3.5 7.5 13.0 17.0 25.0 48.0 60.0 100.0