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Control Valve Sizing Calculator (Cv / Kv)

This control valve sizing calculator finds the Cv or Kv a valve needs for a given flow and pressure drop — or start from the coil heat load and water ΔT. It returns the required Cv and Kv, the nearest standard valve size and the pressure drop you will actually get.

Duty point

Two of the three inputs — flow, pressure drop and fluid — are enough to fix the flow coefficient. Everything recalculates as you type.

Flow rate

I know the flow rate Work it out from heat load

Design flow rate The flow at design conditions, with the valve wide open.

Unit

Design pressure drop across the valve A practical target is 30–50 % of the total pressure drop of the coil circuit.

Unit

Advanced

Specific gravity of the fluid Water = 1.00. Glycol mixtures are heavier — 30 % ethylene glycol is about 1.04 and 50 % about 1.07 at 20 °C, so use your supplier's table. The calculation also assumes no flashing or cavitation.

How to use this control valve sizing calculator

Enter the duty point. Type the design flow rate and pick its unit (m³/h, L/s or US GPM). If you know the coil heat load instead, switch to Work it out from heat load and the flow is derived for you.
Set the pressure drop you can afford. The result is only as sound as this number. A practical target is 30–50 % of the total pressure drop of the coil circuit the valve sits in — much below that and the valve loses authority.
Read the required Cv and Kv. Both are shown together with the working, so the arithmetic can be checked against your own sheet.
Match to a standard valve size. The calculator returns the nearest standard body and the pressure drop you will actually get with it — the figure to carry into the schedule.

Everything recalculates as you type and no sign-up is needed. For the coil side of the same selection, pair this with the fan coil flow & ΔT calculator; for the dampers on the same project, use the damper actuator torque calculator.

How the calculation works

For liquids, the flow through a wide-open valve follows the standard sizing equation, where the flow coefficient already bundles up the valve's geometry:

Q = Kv × √( Δp / SG ) · Kv = 0.865 × Cv

In metric units Kv is the flow of water in m³/h through the valve at 1 bar pressure drop; Cv is the same idea in US units, in GPM at 1 psi. The two differ by a fixed factor, so once you know one you know the other. Divide the required Kv by the pressure drop you are prepared to accept and you get the size of valve to order.

Sizing tips that avoid most commissioning calls

Valve authority matters more than nominal size. If the valve drops 100 kPa in a circuit whose total drop is 150 kPa, authority is 0.67 and control is stable. Below roughly 0.3 the valve is oversized for the duty and control becomes erratic.
Never size from pipe diameter alone. Two DN25 valves with different trim can differ several times over in Kv.
Check the minimum controllable Kv. A valve whose required Kv falls below about 10 % of its rated Kv will hunt; for wide load swings, consider a characterised trim or split the load over two valves.
Water above 100 °C or a large drop across the seat can flash or cavitate — those cases need a proper check, not just the flow coefficient.
Two-way on variable flow, three-way on constant flow. Most modern FCU installations use two-way valves with a variable-speed pump.