Size a control valve, ball valve or regulator from your process conditions. Get the required flow coefficient (Cv or Kv) for liquid, gas or steam, check whether the flow is choked, or find the flow and pressure drop a valve will give.
Last updated:
Fluid
US gpm, or m³/h in metric
psi, or bar in metric
Water 1.0, glycol 50% ≈ 1.07, diesel ≈ 0.85
SCFM (60 °F, 14.696 psia), or Nm³/h (0 °C) in metric
lb/h, or kg/h in metric
psi or bar
psi or bar
Natural gas ≈ 0.6, nitrogen 0.967, CO₂ 1.52
°F, or °C in metric
°F, or °C in metric
Enter your values to see the result.
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How to use the Valve Cv Calculator
Pick the fluid: liquid, gas or steam.
Choose what to find: the required Cv/Kv, the flow for a known valve, or the liquid pressure drop.
Enter the conditions: flow, pressure drop or inlet and outlet pressures, and specific gravity or temperature.
Read the required Cv and Kv and the suggested minimum rated Cv for valve selection.
Use the table and chart to see how flow changes with pressure drop, and download the table.
Valve flow coefficient formulas
Fluid
Required Cv (US units)
Units
Liquid
Cv = Q × √(SG ÷ ΔP)
Q gpm, ΔP psi
Liquid (metric)
Kv = Q × √(SG ÷ ΔP)
Q m³/h, ΔP bar
Gas
Cv = Q ÷ (22.67 × P1 × Y × √(x ÷ (SG × T)))
Q SCFM, P psia, T °R, x = ΔP/P1 ≤ 0.5, Y = 1 − 2x/3
Saturated steam
Cv = W ÷ (2.1 × √(ΔP × (P1 + P2)))
W lb/h, P psia
Steam, critical (P2 < ½P1)
Cv = W ÷ (1.83 × P1)
W lb/h, P1 psia
Conversions: Kv = Cv ÷ 1.156; 1 m³/h = 4.403 gpm; 1 bar = 14.504 psi; 1 Nm³/h (0 °C) ≈ 0.6221 SCFM (60 °F). Specific gravity of a gas is its molecular weight divided by 28.96 (air).
Worked example
A chilled-water coil needs 60 gpm of water (SG 1.0), and the design allows a 4 psi drop across the control valve. Cv = 60 × √(1 ÷ 4) = 30. Choosing a valve with a rated Cv around 38 to 50 means the design flow is reached at roughly 60–80% of capacity, leaving room for control. In metric, 13.6 m³/h at 0.276 bar gives Kv = 13.6 × √(1 ÷ 0.276) ≈ 25.9, which matches 30 ÷ 1.156.
Sizing tips
Valve authority: for good control, the valve's pressure drop should be a sizable share (often 25–50%) of the total drop of the controlled circuit.
Cavitation: high liquid pressure drops close to the vapor pressure can cause cavitation noise and damage; check the manufacturer's FL factor.
Choked gas flow: once flow is choked, only a higher inlet pressure or a larger Cv increases capacity.
Viscous liquids: oils and syrups need a viscosity correction, so the simple formula underestimates the Cv.
Frequently asked questions
What is valve Cv?
Cv (flow coefficient) is the flow of 60 °F water in US gallons per minute that passes through a valve with a 1 psi pressure drop. A valve with Cv 10 passes 10 gpm of water at 1 psi drop. A larger Cv means a higher-capacity valve.
What is the Cv formula for liquids?
Cv = Q × √(SG ÷ ΔP), where Q is flow in gpm, SG is the specific gravity (water = 1) and ΔP is the pressure drop in psi. Example: 100 gpm of water with a 10 psi drop needs Cv = 100 × √(1 ÷ 10) = 31.6. It is valid for non-cavitating, turbulent, low-viscosity liquid flow.
How do I convert Cv to Kv?
Kv is the metric flow coefficient: flow of water in m³/h at a 1 bar pressure drop. Kv = Cv ÷ 1.156 (or Kv ≈ 0.865 × Cv), and Cv = 1.156 × Kv.
How is valve Cv calculated for gas?
This calculator uses the ISA-style formula Q = 22.67 × Cv × P1 × Y × √(x ÷ (SG × T)), with Q in SCFM, P1 in psia, T in °R, x = ΔP ÷ P1 and the expansion factor Y = 1 − 2x/3. When ΔP reaches about half of P1, the flow chokes and further lowering the outlet pressure does not increase flow.
How is a steam valve sized?
A common rule for saturated steam is W = 2.1 × Cv × √(ΔP × (P1 + P2)) lb/h with pressures in psia. When the outlet is below about half the inlet pressure the flow becomes critical and W = 1.83 × Cv × P1. For superheated steam the result is divided by (1 + 0.00065 × degrees of superheat °F).
What Cv should I choose for a control valve?
Pick a valve whose rated (wide-open) Cv is somewhat larger than the required Cv so it controls in the middle of its travel, often aiming for the required flow at about 60–80% open. An oversized valve controls poorly near its seat, while an undersized one cannot reach full flow.
What are the limits of these formulas?
They are simplified sizing equations. They ignore piping geometry factors (Fp), liquid cavitation and flashing (FL), high viscosity, and the exact pressure-drop ratio factor xT of a specific valve. For final selection, use the manufacturer's sizing software or the full IEC 60534 / ISA-75.01 method.
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Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.