Cv vs Kv Valve Flow Coefficient: Same Capacity, Different Units

The Short Answer

Cv and Kv are both flow coefficients that describe how much fluid a valve passes at a given pressure drop, and the only real difference is the unit system. Cv is the US convention: the flow of water in US gallons per minute at 60 °F with a 1 psi pressure drop. Kv is the metric convention: the flow of water in cubic meters per hour with a 1 bar pressure drop. The relationship is roughly Kv = 0.86 × Cv, so a valve with Cv = 100 has Kv of about 86. The conversion is approximate and standardized in ISA-75.01.01 / IEC 60534, and it is useful whenever a datasheet from one region is compared with a calculation from another. Both coefficients are meaningless without the associated pressure drop, and neither one alone tells you whether a valve is correctly sized for your line.

What Cv Means

The valve flow coefficient Cv was defined to give engineers a single number for valve capacity. For liquids, it is the number of US gallons per minute of 60 °F water that will flow through a fully open valve with a 1 psi pressure differential. A Cv of 50 means the valve passes 50 GPM of water at 1 psi drop when fully open — and, for the same valve, roughly 100 GPM at 4 psi drop, because liquid flow through a valve follows an approximately square-root relationship with pressure drop under non-cavitating conditions. Cv is published by manufacturers for every valve size and travel position and is the basis for sizing calculations, most commonly with the ISA-75 / IEC 60534 sizing equations.

What Kv Means

Kv is the same physical idea expressed in metric units: the number of cubic meters per hour of water (at a reference temperature, typically 5-40 °C range with water properties) that flows through the fully open valve with a pressure drop of 1 bar. It is the coefficient used in European sizing practice and appears on datasheets from European and Asian manufacturers. Because 1 bar equals 14.5 psi and 1 m³/h equals about 4.4 US GPM, the two coefficients are numerically different for the same valve — which is exactly why the conversion factor exists and why mixing the two without conversion is a common and costly error.

Cv and Kv Conversion

Cv	Kv (approximate)	Notes
10	8.6	Multiply Cv by 0.86
25	21.5	Approximate, per ISA-75 / IEC 60534
50	43	Approximate
100	86	Approximate
200	172	Approximate
400	344	Approximate

The exact relationship is Kv = Cv × 0.865 (and Cv = Kv × 1.156), following the unit conversion between US GPM and m³/h and psi and bar; treat the numbers above as working values, and use the standard’s stated factor for formal calculations (data needs further verification for your exact standard edition).

Application Example: Matching a Datasheet to a Calculation

An engineer in Houston sized a control valve with a required Cv of 90 and asked a European supplier for a quotation. The supplier returned a valve with Kv = 78 and wrote “Kv 78 ≈ Cv 90” in the covering email. Using the conversion, Kv 78 corresponds to Cv ≈ 90 (78 / 0.865 ≈ 90), so the valve was indeed correctly matched — but only because the conversion was applied. Without it, a purchasing clerk comparing numbers directly would have rejected a correct valve or accepted a wrong one. The same mixing happens in reverse when a European design in Kv is sent to a US fabricator. Keeping the coefficient and its unit together on every document prevents the error — the same discipline as keeping units straight in sizing calculations like valve Cv calculation example.

Common Misconceptions

The first misconception is that Cv and Kv are different capacities — they are the same capacity in different units. The second is that a higher Cv always means a better valve — oversizing is as bad as undersizing: a control valve sized far above the required Cv operates near its minimum travel, loses resolution, and can cause hunting and instability (see control valve hunting causes for the downstream symptoms). The third is using the coefficient without the pressure drop — Cv and Kv are defined at a specific reference drop and are only meaningful in a sizing equation. The fourth is applying the liquid definition to gas — gas flow uses the same coefficient in compressible-flow equations with expansion factors, not the plain liquid formula. The fifth is assuming the coefficient is constant at all travels — published Cv curves show how capacity changes with travel, and only the fully-open value is the single number.

Practical Recommendation

State the flow coefficient together with its unit and reference conditions in every specification and datasheet — “Cv 100 (ISA-75, water at 60 °F, 1 psi)” removes all ambiguity. When comparing valves from different regions, convert with Kv = 0.865 × Cv and verify against the sizing equation, not by direct number comparison. Size control valves to a Cv that keeps the valve between roughly 20% and 80% travel at design conditions, and record the calculated Cv alongside the selected valve’s Cv so the margin is visible for review.

Conclusion

Cv and Kv describe the same valve capacity in two unit systems, related by a simple conversion factor. Use the coefficient with its unit and reference conditions, apply the conversion when crossing regions, and size to a sensible travel range — and the flow coefficient will never be the silent source of an undersized or oversized valve again.

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

Sources & Further Reading

ISA, IEC

Tags: Valve Calculations