Valve Cv and Kv Calculator (Liquids)
Valve flow coefficient Cv and Kv for liquids from flow rate, pressure drop and specific gravity — plus the valve size to select and the flow through it at other pressure drops.
How it works
US: Cv = Q(gpm) × √(SG ÷ Δp(psi)) · metric: Kv = Q(m³/h) × √(SG ÷ Δp(bar)) · Cv = 1.156 × Kv, Kv = 0.865 × Cv · flow at another drop: Q₂ = Q₁ × √(Δp₂ ÷ Δp₁)
The flow coefficient describes how much a valve lets through: Cv is the US gallons per minute of 60 °F water that pass with a 1 psi drop, and Kv is the cubic metres per hour of water with a 1 bar drop. Flow rises with the square root of the pressure drop and falls with the square root of the liquid’s specific gravity. Control valves are usually chosen so the required Cv falls at about 60–80% of the valve’s rated Cv, leaving room to control. The simple formula applies to turbulent, non-flashing liquids; gases, steam, cavitating and viscous services need the full ISA-75.01 / IEC 60534 equations.
Worked example
Passing 10 m³/h of water (SG 1.00) with a 1 bar drop needs Kv = 10 × √(1 ÷ 1) = 10.00, which is Cv = 11.56. To keep that at about 80% of the valve’s capacity, pick a valve rated Cv 14.4 (Kv 12.5) or more. Doubling the flow through the same valve would need a 4 bar drop.
Sources: Engineering ToolBox — Flow coefficients Cv and Kv · Wikipedia — Flow coefficient
Frequently asked questions
What does Cv mean on a valve?
Cv is the flow of 60 °F water in US gallons per minute through the fully open valve with a 1 psi pressure drop. A valve with Cv 10 passes 10 gpm of water at 1 psi, 20 gpm at 4 psi.
How do I convert Kv to Cv?
Cv = 1.156 × Kv and Kv = 0.865 × Cv. Kv is the metric equivalent: m³/h of water at a 1 bar drop.
How do I size a control valve?
Compute the required Cv at maximum flow and the available pressure drop, then choose a valve whose rated Cv puts that point at roughly 60–80% open (or 80% of rated Cv). An oversized valve controls poorly near its seat.
Can I use this for gas or steam?
Not directly. Compressible flow needs the gas Cv equations with the expansion factor and choked-flow limits from ISA-75.01 or IEC 60534. This calculator is for liquids that do not flash or cavitate.
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This calculator is for educational purposes. Double-check important results before acting on them.