EngineerHub.orgDrawing No. EH–FM–004 // Fluid Mechanics & Piping
Control Valve Cv / Kv Calculator
Reviewed August 2026
Size Cv/Kv, inspect liquid choking risk and compare metric and US flow coefficients with transparent assumptions.
01 // Service
02 // Sizing result
Liquid mode uses the conventional turbulent-flow coefficient definition and checks the IEC/ISA liquid choking limit using FL and the liquid critical-pressure factor.
Gas mode is an ideal-gas effective-area screening model only. It does not convert area into Cv or Kv because that conversion is not universal for compressible valve sizing. Final selection should use IEC 60534-2-1 / ISA sizing factors for the actual valve geometry and manufacturer data.
Background & method
IEC 60534-2-1 contains the harmonized IEC/ISA sizing framework for incompressible and compressible control-valve flow. This page implements a strong liquid screening calculation and clearly separates the less complete gas screening model from final standard sizing.
FAQ
Cv is the US customary flow coefficient and Kv is the metric flow coefficient. For the same valve, Cv is approximately 1.156 × Kv.
At sufficiently large pressure drop the local pressure at the vena contracta can reach the vapour-pressure region, limiting flow and creating cavitation or flashing risk.
No. Gas mode is intentionally labelled as a screening model. Final compressible sizing needs the actual valve xT, Fp, Fγ, Reynolds corrections and manufacturer data required by IEC/ISA procedures.