Venturi Flow Meter Calculator
Calculate volumetric flow rate from differential pressure in a Venturi meter.
Enter pipe diameter, throat diameter, pressure drop, and fluid density to find flow velocity and rate.
A Venturi meter measures flow rate by constricting a pipe and measuring the pressure drop across the constriction. As fluid speeds up through the narrow throat, pressure drops. The Bernoulli equation relates that pressure difference to velocity.
The derivation. Continuity: A1 v1 = A2 v2. Bernoulli: P1 + 0.5 rho v1^2 = P2 + 0.5 rho v2^2.
Combining and solving for the volumetric flow rate Q:
Q = Cd x A2 x sqrt( 2 x dP / (rho x (1 - (A2/A1)^2)) )
where:
- Cd is the discharge coefficient (typically 0.98-0.99 for well-machined Venturi meters)
- A1 and A2 are the pipe and throat cross-sectional areas
- dP is the differential pressure (P1 - P2)
- rho is the fluid density
The discharge coefficient Cd accounts for viscous friction and flow profile imperfections not captured by ideal Bernoulli theory. A standard Venturi (ISO 5167) has Cd = 0.984 at Re > 200,000.
Advantages over orifice plates. The Venturi has a gradual converging and diverging section, so most of the pressure drop is recovered. A Venturi loses about 10-25% of differential pressure as permanent loss; an orifice plate loses 50-80%. For large flow applications (natural gas pipelines, water treatment), the energy savings justify the higher initial cost.
Cavitation limit. If throat pressure drops below the fluid vapor pressure, cavitation occurs — violent bubble collapse that damages the meter. Check that absolute throat pressure remains above vapor pressure.