Valve Authority Calculator (Control Valve Authority N)
By Santaji GadepatilinUpdated November 5, 2026
Valve authority is the share of the circuit's variable pressure drop taken by the control valve at full flow — too low and the valve loses control at small openings. This valve authority calculator works out N from the valve and system pressure drops and rates the result, a useful check when choosing a control valve flow coefficient.
How to use the valve authority calculator
Enter the pressure drop across the valve when fully open at design flow and the pressure drop of the rest of the circuit (coil, branch pipe and fittings) at the same flow. The calculator returns the authority N and rates it.
Valve authority formula
N = Δpvalve ÷ (Δpvalve + Δpcircuit)
Worked example
A valve dropping 0.5 bar in a branch whose coil and pipework drop 1.0 bar:
- N = 0.5 ÷ (0.5 + 1.0) = 0.333 — acceptable, but a smaller valve (higher Δp) would control better.
Authority for different pressure drops
| Valve Δp | Circuit 0.5 bar | Circuit 1 bar | Circuit 2 bar |
|---|---|---|---|
| 0.1 bar | 0.17 | 0.09 | 0.05 |
| 0.25 bar | 0.33 | 0.20 | 0.11 |
| 0.5 bar | 0.50 | 0.33 | 0.20 |
| 1 bar | 0.67 | 0.50 | 0.33 |
| 1.5 bar | 0.75 | 0.60 | 0.43 |
| 2 bar | 0.80 | 0.67 | 0.50 |
| 3 bar | 0.86 | 0.75 | 0.60 |
Notes
- Low authority distorts an equal-percentage characteristic toward quick-opening, so most flow change happens in the first part of the stroke.
- To raise authority choose a valve with a smaller Kv / Cv — check it with the control valve Kv/Cv calculator.
Frequently asked questions
What is valve authority?
N = Δp across the fully open valve ÷ (that Δp + the pressure drop of the rest of the controlled circuit at design flow). It ranges from 0 to 1.
What is a good valve authority?
Between 0.5 and about 0.7 is the usual design target for HVAC two-way valves; 0.25 is generally the lowest acceptable value.
Why not make the authority close to 1?
Because the valve would then take almost all the available pressure, which wastes pump energy. Authority around 0.5 balances control quality and pumping cost.
What pressure drops should be included?
Only the part of the circuit whose flow the valve varies — the coil, branch piping and fittings — not the constant-pressure parts upstream of a differential pressure controller.