Size a pneumatic valve using standard gas flow, inlet and outlet absolute pressure, upstream temperature and gas specific gravity. The calculator distinguishes subcritical and choked flow using a published simplified gas model. A separate liquid mode retains the incompressible calculation.
What is Valve Flow Coefficient (Cv)?
Cv quantifies how much fluid gets through a valve with a given pressure drop. Higher Cv means less restriction—more flow with less pressure drop. It’s how you decide what size valve to use for the result you want.
Simple Explanation
Cv is like the setting on a hose nozzle. Open it up (high Cv), and water flows easily. Choke it down (low Cv), and flow slows. If you know the flow you need and your available pressures, Cv tells you the smallest valve that won’t throttle your system. Go too small, and equipment slows; go too big, and you’ll sacrifice fine control.
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Pneumatic Valve Flow System Diagram
Gas inputs: standard flow, absolute pressures, upstream temperature and gas specific gravity.
Gas behavior: the simplified model caps flow at P₂/P₁ ≤ 0.5. Actual valve behavior must be checked against manufacturer data.
Liquid mode: uses its own incompressible equation; air flow is never converted into an equivalent liquid flow.
Pneumatic Valve Cv Calculator
Enter standard gas flow at 60°F and 14.7 psia. Standard L/min uses the same reference conditions; it is not normal litres at 0°C. For other local atmospheric pressures, enter absolute pressures.
This calculator is intended for education, concept evaluation, and preliminary design. Results are based on the equations and assumptions described on this page, but cannot account for every real-world load case, tolerance, material property, environmental condition, installation detail, safety factor, code, or regulatory requirement. Verify all inputs, assumptions, units, and results independently before selecting components or using the result in a real application. Safety-critical, structural, medical, lifting, transportation, or regulated applications must be reviewed by a qualified engineer.
Questions about this calculator or found an error? Message our engineering team.
How to Use This Calculator
- Select gas or liquid, then units.
- For gas, use standard flow at 60°F and 14.7 psia; enter upstream temperature and gas specific gravity.
- Select gauge or absolute pressure. Gauge readings use 14.7 psia atmospheric pressure.
- Calculate and check the displayed flow regime against manufacturer data.
Mathematical Equations
The gas model follows the Swagelok Valve Sizing Technical Bulletin. In US units, let x = (P₁ − P₂)/P₁ and u = min(x, 0.5). Then Q = 22.67 Cv P₁ (1 − 2u/3) √(u/(G T)). Solve for Cv by dividing the required Q by the flow per unit Cv.
P₁ and P₂ are absolute pressures in psia, T is upstream temperature in °R (°F + 459.67), G is gas specific gravity relative to air, and Q is SCFM. At x ≥ 0.5, this simplified model caps flow. The exact boundary factor (2/3)√0.5 is used instead of the bulletin's rounded 0.471 so the two branches meet continuously. Real valves may choke at a different pressure ratio.
Metric inputs are converted to this same basis: 1 ft³ = 28.316846592 L, 1 psi = 0.06894757293168 bar, and °R = 1.8(°C + 273.15). Gas SCFM and standard L/min are equivalent reference volumes, not an air-to-liquid conversion.
For the separate liquid mode: Cv = Q √(SG/ΔP), with Q in US GPM and ΔP in psi. SG is relative to water. It excludes viscous-flow corrections, cavitation and flashing.
Air example
Try Example uses 50 SCFM of air at 70°F, 100 psig inlet and 85 psig outlet. The absolute pressures are 114.7 and 99.7 psia. The required Cv is approximately 1.34064 in the subcritical branch. No equivalent-liquid conversion is used.
Complete Technical Guide to Pneumatic Valve Cv Calculations
Gas sizing depends on pressure level as well as pressure drop because density changes with pressure. Use upstream temperature and standard volumetric flow. Equal inlet and outlet pressures cannot support a positive forward flow in this model.
The approximation is for gases near ideal behavior. Manufacturer gas-flow curves and valve-specific sizing coefficients take precedence, especially for unusual gases, high pressures, condensation or critical service. Include piping losses separately and check pressure ratings, material compatibility and operating range.
For equal flow and pressure conditions, higher specific gravity increases the required Cv in both models. A lighter liquid requires a smaller Cv, not a larger one. Temperature affects the gas result through absolute temperature.
Frequently Asked Questions
Can I use the liquid formula for air? No. Select the gas model and enter standard flow and temperature.
What does choked mean here? The simplified model caps flow when outlet absolute pressure is at most half the inlet absolute pressure. Confirm the actual valve behavior with its manufacturer.
Are pressures gauge or absolute? Choose the matching setting. Gauge mode adds 14.7 psi; absolute mode uses your readings directly.
Does changing units convert my inputs? Yes. Changing between gas and liquid clears the flow value because the two types of flow are not interchangeable.
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About the Author
Robbie Dickson
Chief Engineer & Founder, FIRGELLI Automations
Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.
📹 Video Walkthrough — How to Use This Calculator
📹 Video Walkthrough — How to Use This Calculator
Calculation method updated 7 September 2026.
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