Required balancing valve Cv from flow and pressure drop, with a suggested valve size and a minimum-ΔP check.
Free
No sign-up
Runs in your browser
Excel & CSV export
Valve Inputs
Results
Required Cv
—
GPM / √psi
Suggested Size
—
smallest with full-open Cv ≥ required
Full-Open Cv
—
of suggested valve
Valve Share of Branch ΔP
—
ΔPvalve ÷ (ΔPvalve + rest of branch)
Typical Balancing Valve Full-Open Cv
Valve Size
Typical Full-Open Cv
Flow at Full Open, 2 psi (GPM)
Match
½"
2.9
4.1
¾"
4.7
6.6
1"
8.5
12.0
1¼"
14.0
19.8
1½"
20.0
28.3
2"
37.0
52.3
2½"
62.0
87.7
3"
102.0
144.2
4"
175.0
247.5
Representative full-open Cv for globe-pattern balancing valves (circuit setters) — the same table the AIM Works app uses for its valve schedule. Pick the smallest valve whose full-open Cv is at least the required Cv; it is then set partly closed at balancing. An oversized valve ends up set nearly closed, where its setting and flow readout are least accurate. The manufacturer’s Cv data governs.
Balancing Valve vs Control Valve
Valve
How It Works
What to Check
Manual balancing valve
Set once during balancing to absorb a branch’s excess pressure
Passes design flow at the allocated ΔP; keeps at least ~1 ft of head across it so the flow can be read and set
Modulating control valve
Throttles continuously to follow load
Valve authority β = ΔPvalve ÷ (ΔPvalve + ΔPcoil+branch), typically 0.25–0.5 — see the Control Valve Cv tool
Authority describes how a modulating valve’s characteristic is distorted by the rest of the circuit, so it is not a sizing criterion for a manual balancing valve. The “share of branch ΔP” above is information: when most branches burn a large share in their balancing valves, the pump has excess head that an impeller trim or a lower pump speed would save.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This balancing valve Cv calculator finds the required flow coefficient for a manual hydronic balancing valve from the design flow and the pressure drop you allocate across the valve. Enter the flow in GPM, the valve ΔP in psi, and the fluid; the tool returns the required Cv, the smallest typical valve size that passes the flow, that valve’s full-open Cv, and the share of the branch pressure drop the valve takes.
Cv (the valve flow coefficient) is the GPM of 60°F water a valve passes at 1 psi drop. A glycol fluid raises specific gravity, which the tool applies automatically. It also warns when the valve ΔP is too small to read and set accurately.
Formula & Method
Required Cv
Cv = GPM ÷ √(ΔP ÷ SG)
Suggested size
smallest valve with full-open Cv ≥ required Cv
Share of branch ΔP
ΔPvalve ÷ (ΔPvalve + ΔPrest of branch)
Cv is the GPM of 60°F water a valve passes at a 1 psi drop, so for a liquid Cv = GPM ÷ √(ΔP ÷ SG) — the basic incompressible-flow valve equation used in manufacturer Cv data. SG comes from the fluid dropdown: Dow typical densities for inhibited glycol at about 45°F, relative to 60°F water (water 1.00, 50% ethylene glycol 1.081). The size table is representative globe-pattern balancing-valve data; the minimum-ΔP warning (about 1 ft of head, 0.43 psi) matches the AIM Works app’s balancing-valve check. Confirm the final size and setting with the manufacturer’s chart.
Frequently Asked Questions
How is the required Cv for a balancing valve calculated?
Required Cv equals the design flow in GPM divided by the square root of the pressure drop across the valve in psi, with a specific-gravity correction for glycol: Cv = GPM divided by the square root of (Delta-P divided by SG). For water the specific gravity is 1.0. This tool then suggests the smallest typical valve whose full-open Cv is at least that value.
Does a balancing valve need valve authority?
No. Valve authority, the valve pressure drop divided by the valve plus the rest of the circuit, describes how a modulating control valve’s flow characteristic is distorted by the circuit around it. A manual balancing valve is set once and does not modulate, so it has no authority target. What matters is that it passes design flow at the allocated pressure drop and keeps enough pressure drop (roughly 1 ft of head or more) to read and set the flow accurately. This tool shows the valve’s share of the branch pressure drop for information only.
How do I pick the balancing valve size?
Pick the smallest valve whose full-open Cv is at least the required Cv. It can then pass design flow at the allocated pressure drop and is set partly closed to absorb the rest. A valve that is much too large ends up set nearly closed, where its setting and flow readout are least accurate. Read the actual handwheel setting from the manufacturer’s Cv-versus-setting chart; the percentage of full-open Cv this tool shows is not the handwheel position.
How does glycol change the required Cv?
Glycol raises the fluid specific gravity above 1.0, which slightly increases the required Cv for the same flow and pressure drop because Cv is divided by the square root of (Delta-P divided by SG). Select your fluid from the dropdown and the tool applies the specific gravity automatically. The larger effect of glycol is usually its higher viscosity, which raises system pressure drop separately.
Connect the calculation to the rest of the project.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
Design Review that cites the check behind every finding
An AI co-pilot that drafts changes — you approve each one
Duct and pipe distribution sizing is planned for V2 — at launch, keep using this calculator for that step.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.