What is the difference between Cv and Kv in control valves?
Same Concept, Different Units
Cv and Kv are both flow coefficients that describe how much liquid will pass through a fully open valve at a given pressure drop — but they use different measurement units, and confusing the two is a common and costly specification error on international HVAC projects.
### Definitions
**Cv**: The flow of 60°F (15.6°C) water in US gallons per minute through the valve with a pressure drop of 1 PSI. This is the standard used throughout North America and in many global manufacturers' catalogues aimed at that market.
**Kv**: The flow of water at 5–40°C in cubic metres per hour through the valve with a pressure drop of 1 bar (100 kPa). This is the SI unit used by European, Asian, and Australian manufacturers, and is the default in most ISO-standard valve catalogues.
### The Conversion
**Cv = 1.156 × Kv** **Kv = 0.865 × Cv**
A valve with a Kv of 10 (10 m³/h at 1 bar ΔP) has a Cv of approximately 11.6 (11.6 US GPM at 1 PSI ΔP). This 15.6% difference is not trivial — if an engineer uses a Kv value from a European catalogue as if it were Cv in a North American specification, the valve will be undersized by approximately 14%.
### The Real-World Risk
Consider a project in Southeast Asia where the consulting engineer is Australian (accustomed to Kv), the contractor is local (may use either), and the equipment is sourced from a mix of European and American manufacturers. A chiller bypass valve requiring Cv = 80 per the hydraulic calculation might accidentally be specified from a catalogue that lists Kv = 80. The installed valve would have Cv = 80 × 1.156 = 92.5 — over 15% oversized, operating in its unstable low-stroke region and causing temperature control problems.
The risk runs both ways: specifying Kv when the manufacturer quotes Cv leads to undersizing. The remedy is simple but essential: always confirm which coefficient the manufacturer's selection table uses, and if necessary, convert all values to a single unit system before making the final selection. Most modern electronic catalogues (Belimo SelectPro, Siemens ValvEZ, Honeywell Selector) allow you to toggle between Cv and Kv displays — use this feature deliberately rather than accepting the default.
### Typical HVAC Valve Ranges
In Cv terms: small VAV reheat coil valves range from Cv 0.3 to 2.5; medium FCU and AHU coil valves range from Cv 4.0 to 40; large chiller bypass and plant decoupler valves range from Cv 63 to 400+. In Kv terms, the equivalent ranges are approximately Kv 0.26–2.2, Kv 3.5–35, and Kv 55–350.
Cv vs Kv Conversion Reference Table
Common HVAC control valve flow coefficients in both Cv (US GPM at 1 PSI) and Kv (m³/h at 1 bar) with typical applications.
| Cv (US GPM @ 1 PSI) | Kv (m³/h @ 1 bar) | Typical Pipe Size | Typical Application | Manufacturer Series Example |
|---|---|---|---|---|
| 0.3–1.0 | 0.26–0.87 | DN15 (½ inch) | Small VAV reheat coil | Belimo CCV 0.3–1.0 |
| 1.0–4.0 | 0.87–3.5 | DN15–20 | FCU coil, small AHU heating | Siemens VVP47 1.0–4.0 |
| 4.0–10 | 3.5–8.7 | DN20–25 | Medium AHU chilled water coil | Belimo CCV 4.0–10 |
| 10–25 | 8.7–21.6 | DN25–32 | Large AHU coil (4-row) | Honeywell VGF 10–25 |
| 25–63 | 21.6–54.5 | DN32–50 | Multi-row AHU, small plant bypass | Belimo CCV 25–63 |
| 63–160 | 54.5–138 | DN50–80 | Chiller bypass, plant decoupler | Belimo CCV 63–160 |
| 160–400+ | 138–346+ | DN80–150 | Large plant bypass, district cooling | Siemens VXF 160–400 |
🔑 Key Takeaways
- ✓Cv (US GPM at 1 PSI) and Kv (m³/h at 1 bar) differ by 15.6% — Cv = 1.156 × Kv
- ✓Confusing Kv for Cv causes a 14% undersizing error — always verify the manufacturer's unit system
- ✓Australian engineers typically work in Kv; North American catalogues use Cv — convert deliberately, never assume
- ✓Most electronic valve selection tools (Belimo, Siemens, Honeywell) can toggle Cv/Kv display — use this feature
- ✓For international projects, standardise all valve specifications to a single flow coefficient to eliminate conversion risk