Calculate valve Cv, flow rate, or pressure drop for control valves. Convert between Cv (US) and Kv (metric) with specific-gravity correction.
The valve flow coefficient Cv is the standard measure of a valve's flow capacity. It is defined as the number of US gallons per minute of water (at 60°F) that will flow through the valve with a pressure drop of 1 psi. The metric equivalent Kv represents cubic metres per hour with a 1 bar drop. The conversion is Kv = 0.865 × Cv.
The fundamental equation Q = Cv √(ΔP / SG) relates flow rate, pressure drop, valve size, and fluid specific gravity. By rearranging, you can solve for any one of the three unknowns: Q (flow rate), Cv (required valve size), or ΔP (expected pressure drop). This makes Cv the single most important parameter in control-valve selection.
This calculator handles all three modes, accepts input in multiple units, and includes Cv presets for common ball and butterfly valve sizes. The flow-vs-ΔP table illustrates the square-root relationship between pressure and flow, essential for understanding valve authority and rangeability.
Cv sizing is the first step in every control-valve selection. Get it wrong and the valve either can't pass enough flow or operates at a tiny opening with poor control. This calculator handles the full liquid Cv equation with unit conversions. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain.
Liquid flow: Q = Cv × √(ΔP / SG) Solve for Cv: Cv = Q / √(ΔP / SG) Solve for ΔP: ΔP = SG × (Q / Cv)² Metric: Kv = 0.865 × Cv Where: • Q = flow rate (US gpm) • ΔP = pressure drop across valve (psi) • SG = specific gravity (water = 1.0)
Result: Q = 94.9 gpm (359 L/min)
Q = 30 × √(10/1.0) = 30 × 3.162 = 94.9 gpm. For a 1″ ball valve at 10 psi drop, you get about 95 gallons per minute.
Use consistent units, verify assumptions, and document conversion standards for repeatable outcomes.
Most mistakes come from mixed standards, rounding too early, or misread labels. Recheck final values before use. ## Practical Notes
Use concise notes to keep each section focused on outcomes. ## Practical Notes
Check assumptions and units before interpreting the number. ## Practical Notes
Capture practical pitfalls by scenario before sharing the result. ## Practical Notes
Use one example per section to avoid misapplying the same formula. ## Practical Notes
Document rounding and precision choices before you finalize outputs. ## Practical Notes
Flag unusual inputs, especially values outside expected ranges. ## Practical Notes
Apply this as a quality checkpoint for repeatable calculations.
Cv uses US gallons/min and psi (imperial). Kv uses m³/h and bar (metric). The relationship is Kv = 0.865 × Cv. Always check which system the valve manufacturer uses.
Calculate the required Cv at maximum design flow, then select a valve whose rated Cv is 1.2–1.5× the calculated value. The valve should operate between 10–80% open at normal conditions for good controllability.
For gases, Cv equations are different and depend on inlet pressure, temperature, and the critical pressure ratio. This calculator covers liquid flow only.
Authority (N) is the ratio of valve ΔP to total system ΔP. For good control, N should be ≥ 0.5. Low authority means the valve has little influence on flow.
The Bernoulli principle gives ΔP ∝ V² ∝ Q². Inverting: Q ∝ √ΔP. Doubling the flow rate requires quadrupling the pressure drop.
Heavier fluids (SG > 1) flow slower at the same ΔP because more force is needed to accelerate the denser fluid. The SG correction is inside the square root: Q = Cv√(ΔP/SG).