Venturi Meter Flow Calculator
Determine the volumetric flow rate of a fluid passing through a venturi meter. Use this when designing pipelines or calibrating flow measurement devices in industrial systems.
Last updated: May 2026
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About this calculator
A venturi meter measures fluid flow rate from the pressure drop across a narrowed throat section. Fluid speeds up through the constriction (continuity) and its pressure drops (Bernoulli). Measure that pressure drop, plug into the venturi equation, and you have the volumetric flow rate — accurate to ±0.5-1% for a properly-installed unit, which is why venturi meters are the reference standard for high-accuracy pipe flow measurement.
The venturi meter equation
Q = Cd · A₂ · √(2 · ΔP / (ρ · (1 − β⁴))), where Cd is the discharge coefficient (~0.98 for a machined venturi), A₂ is the throat cross-sectional area, ΔP is the pressure drop pipe-to-throat, ρ is fluid density, and β = d_throat / d_pipe is the diameter ratio. This calculator handles the geometry and returns Q in m³/s, L/s, and GPM.
The discharge coefficient Cd
A perfect Bernoulli-only calculation would give Cd = 1. Real venturis lose a few percent to friction and to slight flow non-uniformity — machined venturis run 0.98-0.99, cast venturis 0.96-0.98, and orifice plates (a cheaper cousin) sit at 0.60-0.65. ISO 5167 specifies the Cd values for standardised venturi geometries; use the ISO number if you have a certified meter.
Beta ratio design tradeoffs
Higher β (larger throat) = smaller pressure drop = less permanent energy loss but harder-to-measure ΔP signal. Lower β = large clear pressure drop = easy measurement but significant pumping-power cost from the permanent head loss. Common industrial range is β = 0.4-0.7, with 0.5-0.6 being the default for a general-purpose flow meter.
Venturi vs orifice vs pitot
Venturi: highest accuracy, lowest permanent head loss, most expensive to manufacture (machined casting with smooth taper). Orifice plate: simple flat plate with a hole, cheap, higher permanent head loss (~40% of ΔP is unrecoverable), Cd ≈ 0.60. Pitot tube: measures local velocity at a single point, needs a velocity-profile assumption to get total flow — good for large-diameter or open-channel applications where a full venturi is impractical.
How to use
Suppose water (ρ = 1000 kg/m³) flows through a venturi meter with a throat diameter of 0.05 m, a pressure difference of 5000 Pa, and a discharge coefficient of 0.97. Step 1 — compute the throat area factor: π × (0.05)² / 4 = 0.001963 m². Step 2 — compute the velocity term: √(2 × 5000 / 1000) = √10 = 3.162 m/s. Step 3 — multiply: Q = 0.97 × 0.001963 × 3.162 ≈ 0.00602 m³/s (about 6.02 litres per second).
Frequently asked questions
What is a discharge coefficient and why does it matter in a venturi meter calculation?
The discharge coefficient (Cd) is a dimensionless correction factor that accounts for real fluid behaviour such as friction losses, boundary layer effects, and non-uniform velocity profiles. An ideal venturi meter would have Cd = 1, but in practice values typically range from 0.95 to 0.99 for well-machined meters. Using an incorrect Cd directly scales the calculated flow rate, so an accurate value — usually obtained from manufacturer calibration data — is essential for precise measurement. For preliminary estimates, 0.98 is a common default for water in smooth-bore meters.
How does throat diameter affect the flow rate measured by a venturi meter?
Because throat diameter appears squared in the formula (π × d² / 4), even small changes in diameter have a large effect on the computed flow rate. Doubling the throat diameter quadruples the cross-sectional area and therefore quadruples Q for the same pressure differential. This is why precise dimensional measurement of the throat is critical during installation and maintenance. Wear or scaling that alters the throat diameter will introduce systematic errors into every flow reading taken with that meter.
When should I use a venturi meter instead of an orifice plate to measure flow rate?
Venturi meters are preferred when minimising permanent pressure loss is important, since their gradual downstream expansion recovers most of the pressure drop, whereas orifice plates cause a larger irrecoverable loss. They are also better suited to dirty or slurry-laden fluids because there are no sharp edges or pockets to trap particles. The trade-off is cost and size: venturi meters are longer and more expensive to manufacture than orifice plates. For clean fluids in space-constrained installations where low cost matters more than pressure recovery, an orifice plate is usually chosen instead.
What is the venturi meter equation?
Q = Cd · A₂ · √(2ΔP / (ρ · (1 − β⁴))), where Cd is the discharge coefficient (~0.98), A₂ is the throat area, ΔP is the pressure drop, ρ is fluid density, and β is the throat-to-pipe diameter ratio.