What is Cv for a pneumatic valve?

Cv is a flow coefficient: the US gallons per minute of 60 °F water a device passes with 1 psi of pressure drop. For air it is used through a compressible-flow equation to relate inlet pressure, pressure drop and SCFM.

What is choked flow in an air valve?

When the downstream absolute pressure falls below roughly 53% of the upstream absolute pressure, velocity at the restriction reaches sonic and flow stops increasing with further pressure drop. Beyond that point only inlet pressure and Cv set the flow.

How do I convert Cv to the metric flow factor Kv?

Kv is approximately Cv divided by 1.156, and Cv is approximately Kv multiplied by 1.156. Some catalogs instead publish a sonic conductance C in dm³/(s·bar) with a critical pressure ratio b, which is a different and more precise model.

Valve Cv Calculator for Compressed Air — Flow & Sizing

All calculators

Formula

Subsonic: Q = 22.67·Cv·√[ΔP·(P₁+P₂)/(SG·T)] · Choked: Q = 0.5·22.67·Cv·P₁·√(1/SG·T)

What do you know?

Required flow

scfm

Inlet pressure

psig

Allowable ΔP

psi

Air temperature

°F

Required Cv

1.27

Kv 1.1 · C 5.1 dm³/s·bar

Flow

40scfm

1,133 Nl/min

Flow regime

Subsonic flow — ΔP is 4.8% of inlet absolute

Normal sizing region. Keep ΔP under about 10% of inlet pressure for tools and actuators so speed does not sag under load.

Inlet absolute: 104.7 psia
Outlet absolute: 99.7 psia
Equivalent sharp-edged orifice: 0.237 in
Maximum (fully choked) flow at this Cv: 66 scfm

Typical catalog Cv — sanity check your selection

1/8" NPT ball valve, full port	4 – 5
1/4" NPT ball valve, full port	8 – 12
1/2" NPT ball valve, full port	24 – 30
3/4" NPT ball valve, full port	40 – 55
1" NPT ball valve, full port	75 – 95
1/4" 5/2 solenoid valve (typical)	0.6 – 1.2
3/8" 5/2 solenoid valve (typical)	1.2 – 2.2
1/2" 5/2 solenoid valve (typical)	2.5 – 4.0
1/4" flow control / speed control	0.3 – 0.9
3/8" FRL unit (filter-reg-lube)	2.0 – 3.5
1/2" FRL unit	4 – 7
1/4" quick coupler (industrial)	1.4 – 2.0
3/8" quick coupler (high flow)	3.0 – 4.5

Sizing a whole circuit

Series components add as 1/Cv² : Cv_total = 1 ÷ √(1/Cv₁² + 1/Cv₂² + …). One 1.0 Cv coupler in front of a 4.0 Cv valve gives about 0.97 — the coupler is the whole restriction.
Parallel paths simply add: two 1.5 Cv valves feeding one manifold = 3.0 Cv.
Quick-disconnect couplers are the most common hidden choke point on a shop drop. High-flow bodies are typically 2–3× the Cv of an industrial-interchange coupler.
For cylinder speed, size the valve and the tubing together — the tubing ID often limits the circuit before the valve does.

Cv here is for air with a specific gravity of 1.0. For nitrogen use SG 0.97, for CO₂ 1.52. Catalog Cv values are measured at a specified ΔP — a valve rated “Cv 2.0” at 5 psi drop is not the same device as one rated at 15 psi drop.

How to use this calculator

Convert between valve flow coefficient and SCFM at real inlet and outlet pressures, including detection of choked (sonic) flow.

Enter inlet pressure, outlet pressure and air temperature at the valve.
Enter either the required flow in SCFM to get a needed Cv, or a known Cv to get available flow.
Check the flow regime indicator — subsonic uses the pressure-drop equation, choked flow depends only on inlet pressure.
Compare the required Cv against the typical Cv reference table for common valve sizes and port styles.
Add margin for fittings, mufflers and hose, which often dominate the total restriction.

Frequently asked questions

What is Cv for a pneumatic valve?: Cv is a flow coefficient: the US gallons per minute of 60 °F water a device passes with 1 psi of pressure drop. For air it is used through a compressible-flow equation to relate inlet pressure, pressure drop and SCFM.
What is choked flow in an air valve?: When the downstream absolute pressure falls below roughly 53% of the upstream absolute pressure, velocity at the restriction reaches sonic and flow stops increasing with further pressure drop. Beyond that point only inlet pressure and Cv set the flow.
How do I convert Cv to the metric flow factor Kv?: Kv is approximately Cv divided by 1.156, and Cv is approximately Kv multiplied by 1.156. Some catalogs instead publish a sonic conductance C in dm³/(s·bar) with a critical pressure ratio b, which is a different and more precise model.