Cv Flow Calculator

This Cv flow calculator will help you understand how the capacity of a valve to let fluid flow through is measured.

Flow37.947 US gpm143.65 litres a minute, 8.619 m³ an hour

Flow coefficient Cv12defined as the gpm of water at 60 °F that the valve passes with a 1 psi drop — which is why this page works in gpm and psi rather than SI

Pressure drop10 psi68.948 kPa, 0.6895 bar

Kv equivalent10.381the metric coefficient, in m³/h at 1 bar — Cv ÷ 1.156

At four times the pressure drop75.895 gpmonly double the flow — Q goes as the SQUARE ROOT of the drop, so throttling harder buys progressively less

Drop needed to double the flow40 psifour times the pressure, for twice the flow

On a fluid of SG 1.530.984 gpm18.35% less than water — a denser fluid passes more slowly at the same pressure drop

Cavitation warningcheck the downstream pressurethis relation assumes no flashing or cavitation. Once the pressure at the vena contracta falls to the fluid's vapour pressure the flow chokes and stops rising with further pressure drop, however much you increase it

The formula

Q = C_v√(ΔP ÷ SG), in US gpm and psi

Square root, so throttling gets inefficient

Flow through a valve goes as the square root of the pressure drop, so quadrupling the drop only doubles the flow. Control valves therefore waste an increasing share of the available pressure as they open, which is the fundamental reason variable-speed pumping beats throttling on energy.

Cv is defined empirically: the US gallons per minute of water at 60 °F that a valve passes with a one-psi drop. It is a definition rather than a derived quantity, which is why this page stays in gpm and psi — an SI conversion would produce a number no valve datasheet contains. The metric equivalent Kv, in m³/h at one bar, is Cv divided by 1.156.

A denser fluid passes more slowly at the same pressure drop, by the reciprocal square root of specific gravity. That correction is small for most liquids and large for slurries and brines.

The relation fails once the fluid cavitates. When pressure at the narrowest point falls to the vapour pressure, vapour forms and the flow chokes: further increases in pressure drop produce no further flow, and the collapsing bubbles erode the valve trim. Sizing a valve on the Cv relation alone, without checking the downstream pressure, is how control valves get destroyed.