Valve Sizing Calculator
From your design flow, allowable pressure drop, and fluid specific gravity, get the required Cv and a recommended rated Cv with headroom to spare.
Sizing result
Required Cv = Q × √(SG / ΔP). The recommendation is the next rated Cv step above it, so the valve keeps some travel in reserve rather than running wide open. A valve that has to be nearly shut, or nearly wide open, at design flow controls poorly — aim to operate mid-travel.
Sizing for control, not just capacity
Good valve sizing is a balance. Undersize a valve and it can’t pass the flow you need without an unacceptable pressure drop; oversize it and it spends its whole range crammed into the first sliver of travel, where small movements cause large, twitchy changes in flow. The required Cv fixes the capacity; the margin over it fixes the controllability.
The workflow here is deliberately simple: calculate the Cv the duty demands, then step up to the next standard rating so the valve operates with reserve. For a real selection, cross-check the recommended Cv against the specific valve’s published Cv-versus-travel curve, confirm the flow characteristic (linear or equal-percentage) suits the loop, and check the drop you allocated against the pump and system curve.
Frequently Asked Questions
How do I size a valve from flow and pressure drop?
Compute the required flow coefficient with Cv = Q × √(SG / ΔP), using the design flow in US gpm, the pressure drop the system can allocate to the valve in psi, and the fluid specific gravity. Then choose a valve whose rated Cv sits comfortably above that required value so it has travel in reserve. This tool does both steps and reports the margin.
Why not just pick a valve with exactly the required Cv?
A valve sized to be wide open at design flow has no room to open further when conditions change, and it tends to control poorly near its travel limits. Selecting the next rated Cv step up leaves headroom, so the valve throttles nearer the middle of its range where control is smoother and more accurate.
What margin does the calculator use?
It reports the required Cv, then picks the next value up a ladder of representative rated Cv steps and shows the resulting headroom as a percentage over the required Cv. That percentage tells you how much reserve capacity the recommended valve has — it is a starting point, not a substitute for the manufacturer's actual valve Cv table.
How much pressure drop should I allocate to the valve?
A control valve needs enough drop to have authority over the flow. A common rule of thumb gives the valve roughly 25–33% of the total dynamic system pressure drop at design flow, so it stays in control as the rest of the system's losses change. Too little allocated drop and the valve barely influences flow; too much wastes pump energy.
Does this handle gas, steam, or two-phase flow?
No. This is the incompressible-liquid sizing form. Gas and steam use a compressible-flow equation with expansion factors, and flashing, cavitating, or two-phase service needs choked-flow limits and correction factors from a full sizing standard such as ISA/IEC 60534. Treat this as a first-pass liquid estimate.
Engineering estimates for preliminary sizing — not professional engineering advice; verify with the valve manufacturer’s data and a qualified engineer before finalizing a selection.