Liquid valve Cv calculator
Uses the standard liquid flow-coefficient equation. Enter volumetric flow, pressure drop across the valve and the fluid's specific gravity.
Cv = Q × √(SG / ΔP) · Q in GPM · SG vs water · ΔP in psi
Optional: flashing / cavitation screening
Enter upstream pressure and the liquid's vapor pressure for a simplified flashing check. Leave blank to skip.
Reading the result: pick a valve whose published full-open Cv is above your calculated Cv — control valves ideally operate at 20–80% of rated Cv. Never size exactly at the calculated number; add margin for fouling and future demand.
Frequently asked questions
What is Cv in a valve?
Cv is the flow coefficient of a valve: the number of US gallons per minute of water at 60°F that flows through a fully open valve with a 1 psi pressure drop across it. A higher Cv means more flow capacity. It is the standard way to compare valve sizes regardless of brand or type — a Cv 100 valve passes twice the water of a Cv 50 valve at the same pressure drop.
What is the Cv formula for liquids?
For liquids, Cv = Q × √(SG / ΔP), where Q is the flow rate in GPM, SG is the specific gravity of the liquid relative to water, and ΔP is the pressure drop across the valve in psi. Example: 100 GPM of water (SG 1.0) with a 5 psi drop needs Cv = 100 × √(1/5) = 44.7. Heavier liquids (higher SG) need a larger Cv for the same flow and drop.
How do I calculate Cv for gas or air?
Gas Cv uses a different equation because gas is compressible: Cv = Q / (1360 × P1 × √((P1 − P2) / (G × T))), where Q is flow in SCFH, P1 and P2 are upstream and downstream pressures in psia (absolute), G is the specific gravity of the gas relative to air, and T is the absolute temperature in Rankine. This form is valid when P2 is greater than P1/2 (subsonic flow). Below that, the flow is choked and ΔP is capped at P1/2.
What is choked flow and how does it change the Cv calculation?
Choked (sonic) flow happens when the pressure drop across the valve is so large that the gas reaches sonic velocity at the vena contracta — roughly when P2 falls below P1/2. Past that point, lowering downstream pressure further does not increase flow. For sizing, the effective pressure drop is capped at P1/2, so Cv = Q × √(2 × G × T) / (1360 × P1^1.5). This calculator detects the choked condition automatically.
How do I convert Cv to Kv and back?
Kv is the metric flow coefficient: the m³/h of water at 4°C flowing through a valve with a 1 bar pressure drop. The conversion is Kv = 0.865 × Cv and Cv = 1.156 × Kv. For example, Cv 44.7 equals Kv 38.7. This calculator converts both directions instantly in the Kv converter tab.
What size valve do I need for a given Cv?
Valve size depends on both Cv and valve type. As a rough guide for full-open valves: a 1-inch ball valve typically has Cv around 26, a 2-inch around 100, a 4-inch around 480, and a 6-inch around 1100. Globe valves give roughly half the Cv of a same-size ball valve; butterfly valves sit between. Always check the manufacturer's published Cv table for the exact valve you are buying — this page includes a typical Cv chart as a starting point.
Is Cv the same for every valve of the same size?
No. Flow path geometry matters a lot: a full-port ball valve passes much more flow than a globe valve of the same nominal size because the globe valve forces the flow through a tortuous S-shaped path. Reduced-port valves, trim designs, and throttling positions all change the effective Cv. Never assume size equals Cv — use the datasheet.
What is the difference between Cv and flow rate?
Flow rate (Q) is how much fluid is moving right now — it changes with system conditions. Cv is a property of the valve itself: its capacity, measured as the GPM of water it passes at a 1 psi drop when fully open. You use Cv to select the valve, and then flow rate and pressure drop tell you what the valve will do in your system via the same equation rearranged: Q = Cv × √(ΔP / SG).
What units does Cv use?
Cv is defined in US customary units: US gallons per minute (GPM) of water at 60°F with a 1 psi pressure drop. Kv is the metric equivalent: cubic meters per hour (m³/h) of water at 4°C with a 1 bar pressure drop. Both are dimensionless ratios in a sense, but they are only interchangeable with the 0.865 conversion factor.
How does specific gravity affect valve sizing?
Denser fluids are harder to push through a valve: required Cv scales with √(SG). A fluid with SG 2.0 needs √2 ≈ 1.41 times the Cv of water for the same flow and pressure drop. For gases, SG is measured relative to air, and hydrogen (SG 0.07) needs much less valve capacity than propane (SG 1.52). Using SG = 1.0 for everything is a common sizing mistake.
Can I use the same Cv value for gas and liquid service?
No. The Cv of the valve itself is the same, but the sizing equations differ: liquids use Cv = Q × √(SG/ΔP) while gases need the compressible-flow equation with absolute pressures and temperature. A valve that passes 100 GPM of water easily may be far too small — or far too big — for an equivalent mass flow of gas. Use the gas tab on this page for compressible service.
What is critical pressure drop for a valve?
Critical pressure drop is the ΔP at which flow chokes and stops increasing — for gases the simple rule of thumb is ΔP_critical ≈ P1/2 (downstream absolute pressure equal to half the upstream absolute pressure). The exact value depends on the valve's pressure-recovery characteristics (FL and xT factors in ISA 75.01), but P1/2 is the conservative simplified screening value used in this calculator.
How accurate are simplified Cv gas formulas?
Simplified gas Cv formulas typically agree with the full ISA 75.01 equations within about 10–20% for ordinary subsonic conditions. That is fine for preliminary sizing and budget estimates, but final selection for control valves — especially with choked flow, mixed phases, or high pressure ratios — should use the full standard or manufacturer sizing software. This calculator is an engineering estimate, not a substitute for datasheets.
What happens if my control valve is oversized?
An oversized valve operates nearly closed for most of its range, which causes hunting (the valve constantly cycling open and closed), poor control resolution, and accelerated seat wear from high-velocity jets. As a rule of thumb, a control valve should be sized so normal operation sits between roughly 20% and 80% of its travel — if your calculated Cv is below 20% of the valve's rated Cv, pick a smaller valve or a reduced trim.
Do I use psia or psig in the gas Cv formula?
Always absolute pressure (psia) — add 14.7 psi to gauge readings (psig). Using gauge pressure directly is one of the most common Cv calculation errors and it always under-sizes the valve, because the formula ratios depend on absolute pressure. This calculator has a psig toggle that adds atmospheric pressure for you.
Where do I find the Cv value of a valve?
Manufacturers publish Cv (or Kv) tables in their valve datasheets and catalogs — look for the full-open Cv for on/off valves, and the Cv-versus-travel curve for control valves. Reputable brands list Cv per nominal size and per trim option. If a supplier cannot provide a Cv value, treat the product as un-engineered and pick a valve whose published Cv covers your calculated requirement with margin.