Tools · Valve Cv

Control Valve Cv Calculator Hydronics

Q: What is valve Cv?

Cv, the flow coefficient, is the GPM of 60 °F water a valve passes wide open at 1 psi pressure drop. It's the valve's capacity rating — a higher Cv passes more flow at the same drop.

Q: How do you calculate valve Cv?

Cv = Q × √(SG ÷ ΔP), where Q is flow in GPM, SG is specific gravity (1.0 for water), and ΔP is the pressure drop across the valve in psi. The same equation solves in reverse for flow or drop.

Q: How do you convert Kv or Kvs to Cv?

Cv ≈ 1.156 × Kv and Kv ≈ 0.865 × Cv. Kv (and Kvs, the rated Kv of the fully-open valve) is the metric coefficient — m³/h of water at 1 bar drop; Cv is the US coefficient — GPM at 1 psi.

Q: What Cv should I pick for a control valve?

Choose the cataloged valve whose rated Cv sits at or just above the required Cv for the design duty. Slightly under is fine; significantly oversized costs you valve authority, so check authority after sizing.

A control valve is rated by its flow coefficient Cv — the GPM of water it passes at 1 psi drop.

Cv / flow / ΔP solves the sizing equation in any direction: the Cv you need for a flow at a drop, the flow a known valve passes, or the drop it imposes. Valve authority checks the other half of the job — whether the valve takes enough of the circuit's pressure drop to actually control flow.

Inputs are seeded with an example — edit them to your numbers.

Input

Solve for

Flow coefficient Cv

Flow (GPM)

Pressure drop ΔP (psi)

Specific gravity (water = 1)

Output

Required Cv —

Kv equivalent (m³/h·bar^−½) —

Worked example

A coil needs 20 GPM of water (SG 1.0) and the schedule allows a 3.3 psi drop across the control valve at design flow.

The sizing equation: Cv = Q × √(SG ÷ ΔP).
Plug in: Cv = 20 × √(1.0 ÷ 3.3) = 11.0.
Pick the cataloged valve whose rated Cv sits at or just above 11 — slightly under is fine; significantly over costs you authority (next tab).

Flip Solve for to read the other way — how much flow a known-Cv valve passes at a given drop, or what drop a valve imposes at a target flow. The metric coefficient rides along: Kv ≈ 0.865 × Cv (Kv is m³/h at 1 bar).

Two edges bound this number. The √ form assumes turbulent, non-flashing liquid — push a large ΔP and the liquid can flash and cavitate, where a choked-flow (F_L) correction takes over and the square-root relation breaks down. And the Cv solved here is the wide-open rating: the valve’s inherent characteristic, not this figure, sets how flow tracks across the stroke. Size for it, then confirm it claims enough of the circuit’s drop to actually control (next tab).

Input

ΔP across valve, wide open at design flow

ΔP across the rest of the controlled branch

Use any consistent pressure unit — authority is a ratio, so the units cancel. The "rest of the branch" is everything whose drop changes as the valve strokes: the coil, the balancing valve, the fittings on that controlled circuit.

Output

Valve authority β —

Enter both pressure drops to see the verdict.

What authority means

A control valve only controls flow if it claims a healthy share of the circuit's pressure drop. Authority β = ΔP_{valve, open} ÷ (ΔP_{valve, open} + ΔP_rest) compares the drop across the wide-open valve to the total drop across the part of the circuit that varies with the valve. Low authority means the valve does almost all of its throttling in the first sliver of stroke — the loop hunts and the rest of the travel does nothing.

β	Control quality
≥ 0.5	Good — installed characteristic stays close to linear.
0.25 – 0.5	Marginal — distorted, front-loaded stroke.
< 0.25	Poor — little real control, prone to hunting.

An equal-percentage valve is the usual pick precisely because, fighting a circuit's rising resistance, its installed characteristic lands near linear — but only when authority is high enough to begin with. Oversizing on Cv is the quiet killer: a valve far bigger than the duty barely cracks open and authority collapses. The Valve Authority Calculator plots how the installed characteristic distorts as β falls.

Valve Cv FAQ

What is valve Cv?

Cv, the flow coefficient, is the GPM of 60 °F water a valve passes wide open at 1 psi pressure drop. It's the valve's capacity rating — a higher Cv passes more flow at the same drop.

How do you calculate valve Cv?

Cv = Q × √(SG ÷ ΔP), where Q is flow in GPM, SG is specific gravity (1.0 for water), and ΔP is the pressure drop across the valve in psi. The same equation solves in reverse for flow or drop.

How do you convert Kv or Kvs to Cv?

Cv ≈ 1.156 × Kv and Kv ≈ 0.865 × Cv. Kv (and Kvs, the rated Kv of the fully-open valve) is the metric coefficient — m³/h of water at 1 bar drop; Cv is the US coefficient — GPM at 1 psi.

What Cv should I pick for a control valve?

Choose the cataloged valve whose rated Cv sits at or just above the required Cv for the design duty. Slightly under is fine; significantly oversized costs you valve authority, so check authority after sizing.

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