Use our free valve Cv calculator to instantly work out the flow coefficient (Cv) your valve needs for any liquid duty. Enter your flow rate, pressure drop across the valve, and fluid specific gravity — the tool returns the required Cv in US units and the equivalent Kv value in metric units simultaneously.
Cv is the industry-standard measure of valve capacity: a Cv of 1 passes one US gallon per minute of water at a pressure drop of 1 psi. The higher the Cv, the greater the flow a valve can handle at a given pressure loss. Getting the sizing right is critical — an undersized valve creates excess pressure drop and reduced flow, while an oversized valve operates near-closed and suffers from poor control and premature wear.
Supports US units (gpm & psi) and metric units (m³/h & bar) , with a Water (SG 1.0) shortcut for common water applications. Switch between unit systems freely — all values recalculate instantly.
Liquid Water (SG 1.0)
US units (gpm · psi) Metric (m³/h · bar)
? Select a valve whose published Cv at full open comfortably exceeds this value, then verify the opening percentage at duty.
? How Cv is calculated US units: Cv = Q ÷ √(ΔP / SG) where Q is in US gpm, ΔP in psi
Metric: Kv = Q ÷ √(ΔP / SG) where Q is in m³/h, ΔP in bar
Conversion: Kv = Cv × 0.8646
Disclaimer: This calculator is provided for guidance only. Always verify valve sizing with the manufacturer’s data sheets and consult a qualified engineer for critical applications.
What Is Valve Cv and Why Does It Matter? The valve flow coefficient (Cv) is the universally accepted measure of a valve’s capacity to pass fluid. Defined by ISA standard ISA-75.01 and IEC 60534, a Cv of 1 means the valve will pass 1 US gallon per minute of water (SG = 1.0) at a pressure differential of 1 psi. Every valve manufacturer publishes a Cv figure for each valve size and style at full open — your calculated required Cv must sit comfortably below that figure.
The metric equivalent is Kv , defined under IEC 60534 as the flow in m³/h of water at a pressure drop of 1 bar. The relationship is fixed: Kv = Cv × 0.8646 . European valve datasheets typically quote Kv; North American and global oil & gas datasheets use Cv. This calculator gives you both at once so you can cross-reference any datasheet without manual conversion.
How to Use This Valve Cv Calculator Sizing a valve with this tool takes three inputs:
Flow rate (Q) — the volumetric flow the valve must handle at your duty point. Enter in US gpm, m³/h, or L/min.Pressure drop (ΔP) — the allowable pressure loss across the valve at that flow rate. Enter in psi, bar, or kPa. This is the difference between upstream and downstream pressure at the valve at full flow.Specific gravity (SG) — the density of your fluid relative to water at 15 °C. Water = 1.0. Light hydrocarbons are typically 0.65–0.85; glycol solutions above 1.0. Use the “Water” tab to skip this field for standard water applications.Once you have your required Cv, look it up against the manufacturer’s published Cv for the valve you’re considering. Choose a valve whose full-open Cv exceeds your calculated value — typically by a margin of 20–30% — and then verify what percentage open the valve will operate at your duty point. Actuated ball valves and butterfly valves ideally operate between 30% and 70% open for stable control.
Cv Formula Explained The standard liquid Cv formula (ISA/IEC non-choked, turbulent flow) is:
Cv = Q ÷ √(ΔP / SG) [US units: Q in gpm, ΔP in psi]Kv = Q ÷ √(ΔP / SG) [Metric units: Q in m³/h, ΔP in bar]
The formula assumes incompressible (liquid) flow, fully turbulent conditions, and that the valve is not approaching choked flow. For gas, steam, or two-phase flow, different equations apply — contact our technical team for help with those applications.
Frequently Asked Questions What is Cv in a valve? ? Cv is the valve flow coefficient — a dimensionless number that describes how much fluid a valve can pass at a given pressure drop. It is defined as the flow rate in US gallons per minute of water (specific gravity 1.0) that produces a 1 psi pressure drop across the valve. Every valve has a published Cv at full open; the higher the number, the larger the capacity. Cv is used in valve sizing to ensure the selected valve can handle your required flow without excessive pressure loss.
What is the difference between Cv and Kv? ? Cv is the US/imperial flow coefficient (gpm, psi); Kv is the European metric equivalent (m³/h, bar). They measure the same thing — valve capacity — in different unit systems. The conversion is: Kv = Cv × 0.8646 , or conversely Cv = Kv × 1.156. Most European valve datasheets quote Kv; North American and global oil & gas sheets use Cv. Our calculator outputs both simultaneously so you never need to convert manually.
How do I calculate Cv for a ball valve? ? The required Cv for any liquid duty is calculated using: Cv = Q ÷ √(ΔP / SG) , where Q is the flow rate in US gpm, ΔP is the allowable pressure drop in psi, and SG is the specific gravity of the fluid. Use the calculator above — simply enter your flow rate, pressure drop, and specific gravity and it calculates the required Cv instantly.
Once you have your Cv, compare it to the manufacturer’s published Cv for the ball valve you are considering. The valve’s full-open Cv must exceed your calculated value. For a brass ball valve or stainless steel ball valve, full-bore designs have a Cv very close to that of the equivalent pipe bore, making them ideal for low-pressure-drop applications.
What pressure drop should I use for valve sizing? ? The pressure drop to use in the Cv formula is the allowable pressure loss across the valve at your design flow rate — not the total system pressure. For on/off isolation ball valves, the full-bore pressure drop is typically very low (under 0.1 bar at normal velocities) and is rarely the design constraint. For control valve applications, a pressure drop of 10–30% of the available system differential is a common starting point, balancing good control authority against energy waste.
What does specific gravity mean in valve sizing? ? Specific gravity (SG) is the density of your fluid relative to water at 15 °C. Water itself has SG = 1.0. Heavier fluids (SG > 1.0), such as glycol solutions or some acids, require a higher Cv to achieve the same flow at the same pressure drop. Lighter fluids (SG < 1.0), such as diesel (SG ≈ 0.84) or many solvents, require a lower Cv. If you are sizing for standard water duties, use the “Water (SG 1.0)” tab to skip this input.
Can I use this calculator for gas or steam? ? No — this calculator uses the incompressible liquid Cv formula and is only valid for liquids. Gas and steam sizing requires different equations that account for compressibility, absolute inlet pressure, and (for steam) superheat or dryness fraction. For gas valve sizing, the ISA/IEC formula uses a gas-specific flow coefficient equation incorporating inlet pressure and specific gravity relative to air. If you need help sizing a valve for gas, steam, or mixed-phase flow, please contact our engineering team .
How much Cv margin should I allow when selecting a valve? ? For on/off isolation duties (ball valves by type , gate valves), choose a valve whose full-open Cv is at least equal to your calculated required Cv — a margin is not strictly necessary because the valve operates fully open or fully closed. For throttling and control duties , a common guideline is to select a valve with a full-open Cv approximately 1.3× to 1.5× your calculated value, so the valve operates at roughly 60–75% open at the design point. This leaves headroom for higher-than-expected flow while keeping the valve out of the near-closed, high-velocity zone where erosion and noise occur.
Where can I find the Cv for Ball Valve UK products? ? Every product in our range of ball valves includes a full technical datasheet with the published Cv (and Kv) value for each port size. You can find this in the “Downloads” or “Technical Data” tab on any product page. If you need help matching your calculated Cv to the right valve size and material, our team is happy to help — call us on 0151 547 1221 or use our contact form .
