Valve Cv Calculator
Turn a flow rate, pressure drop, and specific gravity into the flow coefficient a valve needs — in both US Cv and metric Kv.
Required flow coefficient
Cv = Q × √(SG / ΔP) for turbulent, non-choked liquid flow. Kv ≈ 0.865 × Cv. Gases, flashing, or cavitating service need a different method.
What the Cv number tells you
Every control or isolation valve has a flow capacity, and Cv is the shorthand for it. Because the flow coefficient rolls the valve’s geometry into one figure, you can size a valve to a duty, check whether an existing valve will pass the flow you need, or work out the pressure it will cost you — all from the same square-root relationship between flow, pressure drop, and fluid density.
The number you calculate here is the Cv requiredby your duty. Compare it against the rated Cv on the valve’s data sheet at the opening you expect to run. If the required Cv is close to the valve’s fully-open rating, the valve will spend its life nearly wide open and control poorly; if it is a tiny fraction of the rating, the valve is oversized. Aim to land somewhere in the middle of the travel.
Frequently Asked Questions
What is a valve Cv?
Cv, the flow coefficient, is the flow of 60 °F water in US gallons per minute that passes through a fully open valve at a 1 psi pressure drop. A valve with a Cv of 25 flows 25 gpm of water at 1 psi. It is a single number that captures how much a valve restricts flow, letting you compare valves and predict flow or pressure drop.
What formula does this calculator use?
For a liquid in turbulent, non-choked flow it uses Cv = Q × √(SG / ΔP), where Q is the flow in US gpm, ΔP is the pressure drop across the valve in psi, and SG is the specific gravity relative to water. Rearranged, that same relationship gives flow or pressure drop when the other quantities are known.
How do I convert Cv to Kv?
Kv is the metric flow factor — cubic metres per hour of water at a 1 bar drop. Kv ≈ 0.865 × Cv, so a Cv of 100 is a Kv of about 86.5. The calculator reports both. Going the other way, Cv ≈ 1.156 × Kv.
Does specific gravity change the Cv much?
Cv scales with the square root of specific gravity, so it changes slowly. A fluid 20% denser than water (SG 1.2) needs a Cv about 10% larger for the same flow and pressure drop, because √1.2 ≈ 1.095. Use the fluid's actual specific gravity at operating temperature for a better estimate.
When does this liquid Cv equation stop being valid?
The simple Cv = Q√(SG/ΔP) form assumes an incompressible liquid in turbulent, non-choked flow. It does not cover gas or steam (which use a compressible-flow sizing equation), flashing or cavitating service, very viscous or laminar flow, or two-phase mixtures. Those cases need the correction factors in a full sizing standard such as ISA/IEC 60534.
Engineering estimates for preliminary sizing — not professional engineering advice; verify with the valve manufacturer’s data and a qualified engineer before finalizing a selection.