Valve Cv vs Kv: Converting Flow Coefficients Without Guesswork

Every control valve datasheet quotes a flow coefficient, but it may come as Cv or Kv depending on where the valve was made. The two numbers describe the same thing — how much flow a valve passes under defined conditions — but they use different units. Converting between them is a routine sizing step, and getting the factor wrong changes the calculated valve size.

The Short Answer

Cv is the flow coefficient in US customary units: the number of US gallons per minute of water that flows through the valve at 60 °F with a 1 psi pressure drop. Kv is the metric equivalent: the number of cubic meters per hour of water at 20 °C with a 1 bar pressure drop. The standard conversion is Cv = 1.156 × Kv, and Kv = Cv ÷ 1.156. Use the factor consistently and always note which coefficient the datasheet uses.

How the Coefficients Are Defined

Cv (US)

Cv is defined with water as the test fluid: 1 Cv means 1 US gallon per minute at a pressure drop of 1 psi. It is the default in North American valve sizing and is the value used in most US-based sizing equations and software.

Kv (metric)

Kv uses the same concept in metric units: 1 Kv means 1 cubic meter per hour at a pressure drop of 1 bar. It is common on European and Asian valve datasheets and in ISO-based sizing practice. Because the two definitions are the same physical relationship in different units, a fixed conversion factor connects them.

Key Conversion Values

From	To	Multiply by
Kv	Cv	1.156
Cv	Kv	0.865

Example: a valve with Kv = 40 has Cv ≈ 46.2. A valve with Cv = 100 has Kv ≈ 86.5.

Where Conversion Errors Happen

• Mixing the two coefficients in one sizing calculation — for example, entering a Kv value into a Cv-based equation without converting.
• Using an approximate factor; the commonly quoted 1.156 is the correct factor based on the unit definitions, and other rounded values create errors in larger valves.
• Forgetting that the conversion assumes water and the same pressure-drop definition; for gases, the expanded flow equations use the same coefficient but must be applied with the correct units.
• Copying a coefficient from a datasheet without noting whether it is Cv or Kv; always label the unit in the sizing document.

Practical Recommendation

Standardize every sizing calculation in one coefficient — convert all datasheet values to Cv (or all to Kv) before comparing. Write the coefficient and its unit next to every valve in the line list. When the fluid is not water, keep the coefficient in the native unit of the sizing standard you follow and convert only at the comparison step.

Conclusion

Cv and Kv are the same flow coefficient in different unit systems, linked by the factor 1.156. Label the unit on every datasheet, convert before comparing, and the sizing math stays clean. The conversion itself is simple; the errors come from mixing unlabeled units.

This article provides general educational knowledge only and is not professional engineering advice.

Tags: Valve Calculations