FLOW COEFFICIENT · ISA / IEC 60534
Calculate the pressure drop across a control or isolation valve from its flow coefficient (Cv or Kv), or work backwards to find the flow a valve will pass at a given differential pressure — for liquids, gases and steam.
Solve for
Pressure Drop ΔP
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Flow Velocity
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Equation used
Engineering note: Results use the standard ISA / IEC 60534 sizing relationships and are for guidance only. They assume turbulent, non-choked flow and a fully-open valve unless a Cv for the actual opening is entered. Always confirm against the manufacturer’s published Cv/Kv, and check for choked flow, cavitation and flashing at high pressure drops.
Cv vs Kv: Cv is in US gpm at 1 psi drop (60 °F water); Kv is in m³/h at 1 bar drop (water). Kv = 0.865 × Cv. Pick the coefficient that matches your valve data sheet.
Frequently Asked Questions
Common questions about valve pressure drop calculations, Cv & Kv flow coefficients, and the ISA / IEC 60534 sizing standard.
Valve pressure drop (ΔP) is the difference in fluid pressure measured immediately upstream and downstream of a valve. It represents the energy lost as the fluid passes through the valve’s internal flow path. Higher pressure drop means the valve is creating more resistance to flow — this affects pump sizing, system energy consumption, and the ability to achieve the required flow rate. For control valves, a correctly sized pressure drop across the valve is essential for stable, controllable flow regulation.
Cv (flow coefficient) is a measure of a valve’s capacity to pass flow. It is defined as the number of US gallons per minute of water (at 60 °F) that will flow through a fully open valve with a pressure drop of 1 psi. The higher the Cv, the larger the valve’s flow capacity. Cv is the standard used in North America and is published on every valve data sheet. To size a valve, you calculate the required Cv for your process conditions (flow rate, pressure drop, fluid properties) and select a valve whose rated Cv meets or slightly exceeds that value.
Cv and Kv are both flow coefficients but use different unit systems. Cv is defined in US customary units (US gpm at 1 psi drop with water at 60 °F) and is commonly used in North America. Kv is the metric equivalent (m³/h at 1 bar drop with water at 15 °C) and is standard in Europe and most international applications. The two are related by the conversion: Kv = 0.865 × Cv, or Cv = 1.156 × Kv. Both values are published on manufacturer data sheets — always use the coefficient that matches your unit system and process conditions.
For liquid service, the ISA / IEC 60534 formula is: ΔP = SG × (Q / (0.865 × Cv))², where ΔP is in bar, Q is in m³/h, SG is the specific gravity of the liquid relative to water, and Cv is the valve flow coefficient. For Kv, the formula simplifies to: ΔP = SG × (Q / Kv)². For gas and steam service, compressibility and pressure ratio effects must also be considered, as described in the full IEC 60534-2 standard. Use the calculator above to apply these equations instantly without manual calculation.
For isolation valves (gate, ball, butterfly) that are designed to be fully open during normal operation, the pressure drop should be very low — typically less than 0.1 bar to minimise energy loss. For control valves, the design pressure drop is usually selected as a proportion of the total system pressure drop, often 10–30%, to provide adequate controllability. A commonly used rule of thumb is to design for at least 0.3–0.5 bar across the control valve at maximum flow. Too little pressure drop results in poor control authority; too much wastes energy and may cause cavitation or noise.
Yes. The calculator includes dedicated tabs for Gas and Steam service. For gases, it uses the IEC 60534 approach accounting for specific gravity relative to air, inlet pressure, and temperature to handle compressibility effects. For steam, it uses the steam-specific sizing equations with inlet pressure and temperature as inputs. These calculations assume non-choked (subsonic) flow conditions; if the pressure ratio across the valve exceeds the critical pressure ratio, the actual flow will be choked and this calculator’s results will overestimate flow capacity — consult your valve manufacturer for choked flow sizing.
Cavitation occurs in liquid service when the local static pressure inside the valve drops below the vapour pressure of the liquid, causing vapour bubbles to form. When these bubbles are carried into a higher-pressure region downstream, they collapse violently, producing noise, vibration, and severe erosion damage to the valve trim and body. Cavitation risk increases with high pressure drop, high flow velocities, and liquids with elevated vapour pressures (such as hot water or light hydrocarbons). To avoid cavitation, select a valve with anti-cavitation trim, reduce the pressure drop per stage by using multiple valves in series, or choose a valve designed for high-recovery applications.
Specific gravity (SG) directly scales the pressure drop for a given flow rate and Cv. Since pressure drop is proportional to SG, a liquid that is denser than water (SG > 1, such as brine or heavy oils) will produce a higher pressure drop than water at the same flow and Cv. Conversely, a lighter liquid (SG < 1, such as diesel or ethanol) will produce a lower pressure drop. For water at standard conditions, SG = 1.0 exactly. Always use the specific gravity at your actual operating temperature, as SG changes significantly with temperature for most liquids.