Guide25 May 2026

Cv vs Kv: Valve Flow Coefficients & How to Convert

Cv and Kv are the two flow coefficients used to describe how freely a valve passes flow. Both answer the same question — how much flow gets through for a given pressure drop? — and a higher number means a less restrictive valve. The only real difference is the unit system: Cv is US/imperial, Kv is metric. Mixing them up is one of the most common valve-sizing mistakes, so it's worth getting straight.

What is Cv?

The flow coefficient Cv is the flow of water, in US gallons per minute (gpm) at 60 °F, that passes through a fully open valve with a pressure drop of 1 psi. A valve with Cv = 25 passes 25 gpm of water at a 1 psi drop. The bigger the Cv, the less the valve restricts flow.

What is Kv?

Kv is the metric equivalent: the flow of water in cubic metres per hour (m³/h) at 5–30 °C with a pressure drop of 1 bar. A valve with Kv = 25 passes 25 m³/h at a 1 bar drop. Same idea, metric units.

Converting between Cv and Kv

Because the two are defined in different units (gpm vs m³/h, psi vs bar), they relate by a fixed factor:

K_v = 0.865 \, C_v \qquad C_v = 1.156 \, K_v

So a valve rated Cv = 10 has Kv ≈ 8.65, and a valve rated Kv = 10 has Cv ≈ 11.56.

US Cv vs UK Cv

Watch out for a second trap: UK (imperial) Cv uses imperial gallons, which are about 20% larger than US gallons, so its conversion factor is different:

K_v = 1.039 \, C_{v(\text{UK})} \qquad C_{v(\text{UK})} = 0.962 \, K_v

For the same valve, the imperial Cv comes out roughly 17% lower than the US Cv — not interchangeable. Most modern valve datasheets and IEC 60534 use US Cv or Kv, but always confirm which one a number refers to.

For a full lookup table across common values, see the Cv / Kv conversion table.

Sizing a valve with Cv / Kv

For turbulent, non-choked liquid flow, the required coefficient is the flow divided by the square root of the pressure drop scaled by specific gravity:

C_v = Q \sqrt{\frac{SG}{\Delta P}}

$Q$ — flow rate (US gpm for Cv; m³/h for Kv)
$\Delta P$ — pressure drop across the valve (psi for Cv; bar for Kv)
$SG$ — specific gravity of the fluid (water = 1)

Compute the required Cv/Kv for your duty, then choose a valve whose rated coefficient comfortably exceeds it. Gas, vapour, and choked flow need the fuller IEC 60534 method, which adds an expansion factor (Y) and a pressure-drop ratio (xT).

Common mistakes to avoid

Mixing US Cv and Kv. A "Cv 10" valve is not a "Kv 10" valve — they differ by about 15%. Always convert before comparing two valves or a duty point.
Confusing US and UK Cv. Imperial Cv runs ~17% below US Cv for the same valve. Check the standard behind any published Cv.
Sizing on rated (full-open) Cv. Rated Cv is at 100% travel. A control valve should sit mid-travel for good controllability — size so the duty point isn't near fully open or fully shut.
Ignoring choked flow. For gas or vapour, once the pressure ratio is high enough the flow chokes and the simple liquid formula no longer holds. Use the IEC 60534 gas method.

Quick reference

Kv = 0.865 × Cv (US)
Cv (US) = 1.156 × Kv
Cv (UK) = 0.962 × Kv

SimuPipe's valve sizing calculator converts Cv ↔ Kv and sizes valves for liquid and gas using IEC 60534, and the Cv / Kv reference table lists the full conversion plus representative valve values. For the complete control-valve method — choked flow, the expansion factor Y, and xT — see Understanding Control Valve Sizing (IEC 60534).