Valve Cv (Flow Coefficient): Formula, Sizing & Port-to-Pipe Matching Guide
What Is Valve Cv?
The flow coefficient (Cv) is the single most useful number for sizing any valve. It is defined as the volume of water in US gallons per minute (GPM) that will flow through a fully open valve with a 1 psi pressure drop, at 60°F (15.6°C).
A higher Cv means the valve passes more flow for the same pressure loss. Cv is the bridge between your process requirement and the physical valve you should install.
Cv Formulas (US and Metric)
For liquids, the governing relation is:
Cv = Q × sqrt(G / ΔP)
where:
- Q = flow rate in GPM
- G = specific gravity of the fluid (water = 1.0)
- ΔP = pressure drop across the valve in psi
In metric units the equivalent coefficient is Kv (m³/h at 1 bar). Conversion:
- Cv = 1.156 × Kv
- Kv = 0.865 × Cv
Step-by-Step Sizing Method
- Determine the required flow rate Q from your process data.
- Estimate the allowable pressure drop ΔP across the valve (typically 10-20% of system pressure).
- Calculate the minimum Cv needed from the formula above.
- Select a valve whose rated Cv meets or slightly exceeds the requirement at your expected travel.
- Check the port-to-pipe match using the table below.
- Verify the pressure drop stays above the cavitation threshold for your trim.
See our valve pressure-temperature rating chart to confirm the body class can survive the upstream pressure.
Port-to-Pipe Diameter Matching Table
| Pipe Size (NPS) | Typical Full-Bore Cv Range | Recommended Valve Port |
|---|---|---|
| 1/2" | 4 - 9 | 1/2" or 3/4" |
| 1" | 12 - 25 | 1" |
| 2" | 45 - 90 | 2" |
| 4" | 180 - 350 | 4" |
| 6" | 450 - 800 | 6" |
| 8" | 900 - 1500 | 8" |
Oversizing a valve (choosing a Cv far above requirement) causes poor control near the seat and accelerates trim wear. Undersizing starves the line and overheats the pump.
Cavitation and Choked Flow
When ΔP exceeds the critical pressure drop, vapor bubbles form and collapse on the trim — cavitation. Always keep the actual pressure drop below the manufacturer's cavitation index. Our valve selection guide explains which valve types tolerate throttling best.
Why Cv Matters for Spec Comparison
Two valves with the same nominal size can have very different Cv. When you compare products in our database of more than 1,000 valve specifications across 90-plus brands at valve brands, the Cv column tells you directly which unit will actually pass your flow.
For dimensionally verified Cv and flow data, browse the ValveSpecs Pro specification database.
Frequently Asked Questions
What is a good Cv value for a 2 inch valve?
A 2 inch full-bore valve typically has a Cv between 45 and 90 depending on type. A ball valve sits at the high end; a globe valve is lower because of its tortuous flow path.
How do I convert Cv to Kv?
Kv is the metric equivalent (m3/h at 1 bar). Cv = 1.156 × Kv, and Kv = 0.865 × Cv.
Can a valve have too high a Cv?
Yes. Oversizing causes the valve to operate near the seat where control is poor and trim wear is high. Size to the calculated requirement, not the largest available.
Does Cv change with fluid type?
Cv is defined for water. For other liquids you correct using specific gravity: Cv = Q × sqrt(G / ΔP). Viscous or slurry fluids need additional correction factors.
Where do I find Cv for a specific product?
Cv is a standard field in manufacturer datasheets. The ValveSpecs Pro database aggregates Cv and flow data for more than 1,000 valve products across 90-plus brands.