Valve Flow Coefficient Calculator
The valve flow coefficient (Cv or Kv) is the fundamental parameter for sizing control valves and isolation valves. This calculator uses the ISA-75.01 / IEC 60534-2-1 liquid flow equation to calculate the required Cv and Kv from your flow rate, pressure drop, and fluid specific gravity. Enter your process conditions and the calculator returns both Cv (US) and Kv (metric) values with a recommended valve Cv including a 30% safety margin.
Valve flow coefficient formula (liquid service)
Kv = Q / sqrt(dP / SG)
Cv = Kv / 0.865 = QGPM / sqrt(dPpsi / SG)
Kvrecommended = 1.3 × Kvrequired
Where: Q = volumetric flow rate (m^3/h for Kv; US GPM for Cv), dP = pressure drop across the valve (bar for Kv; psi for Cv), SG = specific gravity of the fluid relative to water at 15 degrees C (60 degrees F). The factor 1.3 is the ISA-recommended sizing margin.
Valve sizing guidance
- At maximum design flow, the selected valve should be approximately 70 to 80% open to ensure controllability.
- At minimum flow, the valve should be at least 10 to 15% open to maintain stable control.
- For gas or steam service, additional compressibility and critical flow corrections per IEC 60534-2-1 are required.
- Always check with the valve manufacturer's published Cv tables for the specific valve style and trim.
Valve Cv calculator: frequently asked questions
What is the valve flow coefficient Cv?
The valve flow coefficient Cv is the flow rate of water (US gallons per minute, GPM) that will pass through a fully open valve with a pressure drop of 1 psi (6.895 kPa) at 60 degrees F (15.6 degrees C). It is used in the US system for valve sizing. The European equivalent is Kv (m^3/h of water at 1 bar pressure drop). Kv = 0.865 * Cv.
What is the ISA/IEC formula for liquid Cv?
For incompressible (liquid) flow, Cv = Q * sqrt(SG / dP), where Q is flow in US GPM, SG is specific gravity relative to water, and dP is pressure drop in psi. In SI units with Kv: Q (m^3/h) = Kv * sqrt(dP_bar / SG).
How do I calculate Kv in SI units?
Kv = Q / sqrt(dP / (rho / rho_water)), where Q is volumetric flow in m^3/h, dP is pressure drop in bar, and rho/rho_water is the specific gravity. Kv has units of m^3/h per bar^0.5. For water at standard conditions, SG = 1.00, so Kv = Q / sqrt(dP_bar).
What is the difference between Cv and Kv?
Cv is the US customary flow coefficient (GPM/psi^0.5). Kv is the metric flow coefficient (m^3/h per bar^0.5). The conversion is: Kv = 0.865 * Cv, or Cv = 1.156 * Kv. Valve manufacturers publish Cv values for US-market products and Kv values for European-market products.
What safety factor should I apply to the calculated Cv?
ISA-75.01 and IEC 60534-2-3 recommend selecting a valve with a Cv at least 1.3 times the calculated required Cv at maximum flow (30% over-sizing). This provides rangeability margin and allows for process variations. For control valves, the maximum Cv should correspond to approximately 70 to 80% of the valve's rated open position.
Official sources
- ISA-75.01.01 Flow Equations for Sizing Control Valves: ISA-75.01.
- IEC 60534-2-1: Industrial-process control valves: iec.ch.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.