How to Use the Valve Cv/Kv Calculator (With Worked Examples)

The Cv/Kv calculator takes a duty (flow, pressure drop, fluid) and returns the flow coefficient a valve needs to pass it. This page explains what the numbers mean, how to fill the form, and where the calculation stops being reliable.

What Cv actually means

Cv is the number of US gallons per minute of water at 60°F that will pass through the valve with a pressure drop of 1 psi. That is the whole definition — no correction factors, no unit tricks. A valve with Cv = 10 passes 10 gpm of water at 1 psi drop, or 31.6 gpm at 10 psi drop, because flow rises with the square root of pressure drop.

Kv is the same idea in metric units: cubic metres per hour of water at 1 bar drop. Kv = 0.865 × Cv — the calculator shows both so you can compare against a datasheet written in either convention.

Liquid: three inputs, one formula

Enter flow (US gpm or m³/h), specific gravity (water = 1.0, diesel about 0.85) and the pressure drop in psi. The formula is Cv = Q × √(SG / ΔP).

Worked example: 100 gpm of water, 10 psi available drop. Cv = 100 × √(1/10) = 31.6, Kv = 27.4. Looking at typical Cv values, that lands on a 1.5″ (DN40) valve — which is what the calculator's size hint suggests.

Change the drop to 25 psi and the same flow needs only Cv = 20, a smaller valve. This is why the available pressure drop, not the line size, usually decides the valve size.

Gas: use absolute pressure

For gas, enter flow at standard conditions (Nm³/h or scfh), specific gravity relative to air (natural gas about 0.60, hydrogen 0.07), upstream and downstream pressure in psia, and temperature.

Worked example: 300 Nm³/h of air, 100 psig inlet (114.7 psia), 50 psig outlet (64.7 psia), 20°C. The calculator returns Cv ≈ 0.22 and reports subcritical flow.

Choked flow — the part most spreadsheets miss

Once the pressure ratio passes a limit, flow stops increasing no matter how low the downstream pressure goes. The limit is roughly ΔP_allow = F_L² × P₁, with F_L around 0.9 for many ball and DBB designs. The calculator applies this limit and shows a warning when you cross it; without that correction you will size a valve that is too small for gas service.

Five mistakes that give the wrong answer

Using gauge pressure in the gas formula. P₁ and P₂ must be absolute: add 14.7 to psig.
Sizing at the pump's maximum flow. Size for the range, not one point — a valve sized at maximum flow is oversized at normal flow and hunts.
Mixing Cv and Kv. A datasheet quoting Kv = 20 is not a Cv = 20 valve; it is Cv ≈ 23.
Ignoring flashing and cavitation. Liquids near vapour pressure can choke too; check P_vapour against P₁ and P₂.
Treating the result as final. Noise limits, turndown, actuator sizing and the manufacturer's actual Cv table all come after.

Where the calculator stops

This is preliminary sizing. For DBB, instrument and pipeline valves we size against the real product Cv table, check seat and body ratings, and confirm actuator torque at the specified differential pressure. Send the duty — fluid, flow, pressures, temperature — to websale@sawye.com and we will return the selection with the Cv, the size and the price in one document.