Valve Pressure Drop Calculator
Enter a flow rate, the valve’s Cv, and the fluid specific gravity to estimate the pressure drop the valve imposes — in psi and bar.
Pressure drop across the valve
ΔP = SG × (Q / Cv)². Drop rises with the square of flow, so doubling the flow through a fixed valve roughly quadruples the pressure loss.
Reading the pressure drop
The pressure a valve consumes is pressure the rest of the system doesn’t get. Knowing that drop lets you check that a pump can still deliver the flow you need against everything downstream, that a control valve keeps enough authority to actually control, and that you are not throwing away energy across a valve that is too small for the duty.
Because the drop follows the square of flow, a valve that looks comfortable at normal flow can become the limiting element at peak demand. If the calculated drop is a large share of the total system pressure, the valve has plenty of control authority but may be costing pumping energy; if it is a tiny fraction, the valve may struggle to control flow smoothly. Both are worth catching before the valve is installed.
Frequently Asked Questions
How is pressure drop across a valve calculated?
For a liquid, the Cv equation rearranges to ΔP = SG × (Q / Cv)², where Q is the flow in US gpm, Cv is the valve's flow coefficient, and SG is the specific gravity relative to water. This calculator applies that formula and reports the drop in both psi and bar.
Why does pressure drop rise so fast with flow?
Because flow appears squared. Drop is proportional to (Q/Cv)², so doubling the flow through a fixed valve roughly quadruples the pressure loss, and tripling it multiplies the loss about nine times. That square-law is why an undersized valve becomes a serious bottleneck as demand climbs.
Which Cv should I enter — fully open or throttled?
Enter the Cv at the opening the valve actually runs at. A control valve throttled part-way has a much smaller effective Cv than its fully-open rating, so using the wide-open figure will badly under-estimate the real pressure drop. The valve's inherent flow characteristic (linear, equal-percentage, quick-opening) sets how Cv varies with travel.
Does specific gravity affect the drop?
Yes, directly and proportionally. A fluid with SG 1.3 produces 30% more pressure drop than water at the same flow and Cv, because ΔP scales linearly with specific gravity. Use the value at operating temperature, since density shifts as fluids warm or cool.
Does this cover gas, steam, or cavitating flow?
No. ΔP = SG × (Q/Cv)² is the incompressible-liquid form for turbulent, non-choked flow. Compressible gas and steam use a different sizing equation, and flashing or cavitating liquids are limited by choked-flow behaviour, where increasing the downstream drop stops increasing the flow. Those need a full sizing standard.
Engineering estimates for preliminary sizing — not professional engineering advice; verify with the valve manufacturer’s data and a qualified engineer before finalizing a selection.