Quick Answer
Valve Cv is the flow coefficient: US gpm of 60 °F water that passes through a fully open valve at exactly 1 psi pressure drop. For liquid sizing, Cv = Q × √(SG / ΔP). For gas and steam, ISA-75.01 / IEC 60534 equations apply with compressibility and choked-flow corrections. Always add a 20–30% margin — select a valve whose rated Cv exceeds your calculated value.
- Liquid: Cv = Q × √(SG / ΔP) — Q in gpm, ΔP in psi, SG relative to water
- Gas (subcritical): ISA equation with absolute pressures and temperature in °R
- Choked flow: occurs when P2 ≤ 0.53 × P1 — Cv cannot increase further by lowering P2
- Sizing margin: select rated Cv ≥ 1.25× the calculated required Cv
Calculate the required valve flow coefficient (Cv) for liquid, gas, or steam service in one tool. Unlike most online Cv calculators, this one handles steam and automatically flags choked (critical) flow, using the ISA-75.01.01 / IEC 60534-2-1 sizing equations. Results include the equivalent metric Kv and a suggested rated Cv with sizing margin.
How valve Cv is calculated
Cv is the flow of 60 °F water (US gpm) that passes through a valve at a 1 psi pressure drop. The sizing equation depends on the fluid:
Metric conversion: Kv = 0.865 × Cv and Cv = 1.156 × Kv (Kv in m³/h at 1 bar). Always use the specific gravity at the operating temperature, and absolute pressure for gas and steam.
Typical Cv values by valve size & type
| Nominal size | Globe | Full-bore ball | Butterfly |
|---|---|---|---|
| 1" | 8–20 | 40–80 | 35–45 |
| 2" | 40–90 | 200–300 | 100–200 |
| 3" | 100–220 | 500–700 | 300–500 |
| 4" | ~195 | 900–1200 | 600–900 |
| 6" | ~435 | ~1150 | 1200–1900 |
Indicative ranges; consult the manufacturer's rated Cv for a specific valve. Pressure-recovery factor FL: globe ≈0.9, ball ≈0.6–0.7, butterfly ≈0.55.
Frequently Asked Questions
What is valve Cv (flow coefficient)?
Cv is the number of US gallons per minute of 60 °F water that will flow through a fully open valve with a pressure drop of 1 psi. A higher Cv means the valve passes more flow for the same pressure drop. It is the standard way to size and compare valve flow capacity.
What is the difference between Cv and Kv?
Cv is the US/imperial flow coefficient (gpm of water at 1 psi drop); Kv is the metric equivalent (m³/h of water at 1 bar drop). They convert directly: Kv = 0.865 × Cv, and Cv = 1.156 × Kv. For a quick two-way conversion, use our Cv ↔ Kv converter.
How do I calculate Cv for a liquid?
Use Cv = Q × √(SG / ΔP), where Q is flow in gpm, SG is the liquid specific gravity (water = 1.0), and ΔP is the pressure drop in psi. For water at 100 gpm and 10 psi drop, Cv = 100 × √(1/10) ≈ 31.6.
What is choked or critical flow in a valve?
For gases, when the outlet pressure falls to about half the inlet absolute pressure, flow reaches sonic velocity at the valve and stops increasing even if you drop the downstream pressure further. This calculator detects that condition and switches to the critical-flow equation automatically.
What specific gravity should I use?
Use the specific gravity at the actual operating temperature, not at ambient. SG changes with temperature, and using the wrong value is one of the most common valve-sizing errors.
How much margin should I leave when sizing a valve?
As a rule of thumb, select the valve so the calculated Cv falls between roughly 20% and 80% of the valve’s fully-open rated Cv. Operating near the extremes causes poor control, noise, and accelerated wear.
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Related Resources
Why Valve Cv Is the Foundation of Every Valve Selection
Choosing a valve by port size alone is one of the most common and costly mistakes in process piping design. A 4″ ball valve from one manufacturer may have a rated Cv of 1,050 while another is rated at 900 — selecting the lower-Cv valve in a high-flow application results in excessive pressure drop, reduced throughput, and cavitation risk. Calculating required Cv first, then matching it to a properly rated valve, is the industry-standard approach mandated by ISA-75.01.01 and IEC 60534-2-1.
Choosing the Right Cv Equation for Your Fluid
| Fluid Type | Equation Form | Key Inputs | Watch Out For |
|---|---|---|---|
| Liquid (non-flashing) | Cv = Q √(SG / ΔP) | Q (gpm), ΔP (psi), SG | Cavitation when P2 near vapor pressure |
| Liquid (flashing) | ISA Eq. 3 with Ff factor | Vapor pressure, critical pressure | Two-phase flow — use two-phase Cv |
| Gas / Vapour (subcritical) | ISA Eq. 8 (P1²−P2² form) | P1, P2 (psia), T (°R), SG | Compressibility Z — use real gas correction for high P |
| Gas (choked / critical) | ISA Eq. 9 (P1 only) | P1 (psia), T (°R), SG | P2 ≤ 0.53 P1 — downstream P irrelevant |
| Saturated steam | ISA Eq. 12 (steam tables) | P1, P2 (psia), steam density | Superheat: use superheated steam density at actual T |
Worked Sizing Example — Liquid Service
A 3″ cooling-water line carries 200 gpm of water (SG = 1.0) with an allowable pressure drop of 5 psi across the isolation valve. Calculate required Cv:
Cv = 200 × √(1.0 / 5) = 200 × 0.447 = 89.4
Add a 25% sizing margin: rated Cv required = 89.4 / 0.8 = 111.8. A 3″ full-bore ball valve with rated Cv ≈ 600 is far more than sufficient; a 2″ reduced-bore valve (Cv ≈ 170) also passes but with less margin. Confirm the selected valve's pressure-temperature rating and material compatibility before specifying.
Worked Sizing Example — Compressed Air (Gas Service)
A nitrogen purge valve must pass 500 SCFH of N₂ (SG ≈ 0.967) from P1 = 120 psia to P2 = 90 psia (subcritical — check: P2/P1 = 0.75 > 0.53). Using the subcritical gas equation:
Cv = 500 / [962 × √((120²−90²) / (0.967 × 530))] ≈ 0.18
Even a ½″ ball valve (Cv ≈ 12) is greatly oversized. This is typical for gas purge applications — the required Cv is much smaller than the mechanical port size, so a throttling needle valve or globe valve with an appropriate rangeability is often a better fit.
Extended FAQ
What does it mean when my required Cv is very large?
A very large required Cv usually indicates a high flow rate, a very small allowable pressure drop, or both. For example, large cooling-water or firewater systems often need Cv values in the thousands, which points toward large full-bore ball valves or butterfly valves (which have higher Cv per pipe diameter than globe valves). If the required Cv exceeds common valve ratings for your pipe size, consider: (1) reducing velocity by increasing pipe diameter; (2) accepting a higher ΔP budget; or (3) using a parallel valve arrangement.
How does specific gravity affect Cv for non-water liquids?
Cv scales with √(SG). A liquid with SG = 0.8 (light hydrocarbon) requires Cv = Q × √(0.8 / ΔP), which is about 11% less than water for the same flow and ΔP. Conversely, a brine with SG = 1.2 requires 10% more Cv than water. The difference matters most when sizing control valves for precise flow regulation.
Should I use ISA or IEC equations — are they the same?
ISA-75.01.01 and IEC 60534-2-1 are technically harmonized and produce the same results. ISA uses Cv with US customary units (gpm, psi, °F); IEC uses Kv with SI units (m³/h, bar, °C). Both standards define the same correction factors (Fp, FR, FF, xt) and flow models. When comparing valves from US and European suppliers, convert to a common coefficient using Kv = 0.865 × Cv before comparing.