Pneumatic Valve Sizing Calculator

The calculator divides outlet absolute pressure by inlet absolute pressure and compares that ratio to 0.528, the critical pressure ratio for air. If the ratio is at or below 0.528 — meaning the pressure drop is large relative to inlet pressure — flow is choked and the simplified critical-flow formula is used; above that threshold, the calculator switches to the sub-critical ISA/IEC-style formula that also factors in the average absolute pressure across the valve.

After computing the exact required Cv (with the safety factor applied), the calculator searches a list of common commercial Cv sizes and picks the smallest one that meets or exceeds it, since valves are sold in standard catalog sizes rather than custom orifices. If your required Cv exceeds every value on that list, it falls back to rounding up to the next multiple of 10.

Both critical and sub-critical Cv formulas include a factor of √(specific gravity × absolute temperature in Rankine / 520), which corrects the sizing away from the standard reference condition of air at 60°F. A lighter gas (lower specific gravity) or cooler operating temperature reduces the required Cv for the same SCFM, since the gas is denser and easier for the valve to pass at a given pressure drop.

The 25% default adds proportional margin on top of the exact calculated Cv before the calculator rounds up to a standard valve size, guarding against underestimated flow, future capacity increases, or valve wear over time. Lowering it saves cost by allowing a smaller valve to qualify, but it also shrinks the margin against choked or restricted flow if actual demand runs higher than expected — 25% is a common industry default, not a fixed requirement.