Compute a control valve's flow coefficient (Cv) from liquid flow rate, pressure drop, and specific gravity using the standard sizing formula Cv = Q × √(SG ÷ ΔP).
Results
Calculated
Flow Coefficient (Cv)
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Cv = Q × √(SG ÷ ΔP), US gpm/psi⁰·⁵
Metric Coefficient (Kv)
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Kv = 0.865 × Cv, m³/h per √bar
Flow Rate (metric)
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Q converted to m³/h
Pressure Drop (metric)
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ΔP converted to bar
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How to use this calculator
Enter the liquid flow rate in US gallons per minute, the pressure drop across the valve in psi, and the fluid's specific gravity (water = 1.0), then click Calculate. Click Clear to reset all fields to the example values.
Understanding the flow coefficient (Cv)
The valve flow coefficient, Cv, is the standard industry measure of how much liquid a valve will pass for a given pressure drop. It is defined by the sizing equation Cv = Q × √(SG ÷ ΔP), where Q is the volumetric flow rate in US gallons per minute, ΔP is the pressure drop across the valve in psi, and SG is the fluid's specific gravity relative to water. In plain terms, Cv is "how many gallons per minute of 60°F water will pass through the valve with a 1 psi drop across it" — a higher Cv means a less restrictive valve at the same pressure drop. This calculation assumes turbulent, non-cavitating, non-flashing, single-phase liquid flow; it does not apply the additional piping-geometry (Fp) or choked-flow (FL) correction factors used in full ISA/IEC valve sizing.
Interpreting the results
The Flow Coefficient (Cv) card is the primary result, in the US customary units (gpm, psi) used on most valve nameplates and manufacturer Cv tables. The Metric Coefficient (Kv) card converts it to the metric equivalent using the standard factor Kv = 0.865 × Cv, for comparing against valves specified in m³/h and bar. The remaining two cards restate your flow rate and pressure drop in metric units (m³/h and bar) so the whole calculation can be cross-checked in either unit system.
Frequently Asked Questions
What is a valve flow coefficient (Cv)?
Cv is a standard measure of a valve's flow capacity: the number of US gallons per minute of 60°F water that will pass through the valve with a pressure drop of 1 psi across it. It is calculated from Cv = Q × √(SG ÷ ΔP), where Q is the flow rate, ΔP is the pressure drop, and SG is the fluid's specific gravity relative to water.
How is Cv different from Kv?
Cv and Kv are both valve flow coefficients that differ only in units and reference fluid. Cv uses US gallons per minute and psi; Kv uses cubic meters per hour and bar. They convert with the standard factor Kv = 0.865 × Cv, equivalently Cv = 1.156 × Kv.
What does this simplified formula assume?
This calculator uses the basic incompressible-liquid sizing equation, which assumes turbulent, non-cavitating, non-flashing, single-phase flow with no unusual piping near the valve. Full valve sizing under ISA/IEC standards also applies correction factors for piping geometry, choked flow, and viscosity for more precise selection — consult the valve manufacturer's sizing software for final selection.
How do I pick a valve once I know Cv?
Compare the calculated Cv to a manufacturer's Cv table for each valve size and type at your desired opening, commonly 60 to 80 percent open at design flow to leave margin for control. An undersized valve cannot pass enough flow even fully open; an oversized valve throttles near the closed position and loses control precision.