How to Use the Valve Flow Coefficient (Cv) Calculator
Sizing a control valve or ball valve correctly requires knowing its flow capacity — the flow coefficient. This calculator takes flow rate (GPM), the pressure drop across the valve (psi), and fluid specific gravity to calculate Cv, the flow coefficient used throughout US valve specifications.
The formula is Cv = Flow (GPM) x sqrt(specific gravity / pressure drop (psi)). For the same flow rate, a smaller pressure drop means the valve is restricting flow less, which requires a larger Cv. If you don't enter a specific gravity, the calculator defaults to water (SG = 1.0).
Valve datasheets from Europe and Asia often list Kv instead, using metric units (m3/h, bar) — the same physical concept in a different unit system. Since Cv ≈ Kv x 1.156, this calculator also shows the equivalent Kv value for reference. Always confirm final valve selection against the manufacturer's datasheet and a piping engineer's review.
Frequently Asked Questions
Cv is the valve flow coefficient in US customary units (GPM, psi), while Kv is the same physical concept expressed in SI/metric units (m3/h, bar) common in Europe and Asia. They convert as Cv ≈ Kv x 1.156 (or Kv ≈ Cv x 0.865).
It's the upstream pressure minus the downstream pressure across the valve. Pull these values from your piping design drawings or measure them directly with pressure gauges on each side of the valve.