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.
About this calculator
A venturi meter measures flow by constricting a pipe at a throat section, converting pressure energy into velocity energy. The resulting pressure drop across the throat is used to back-calculate the flow rate. The governing formula is: Q = Cd × (π × d² / 4) × √(2 × ΔP / ρ), where Cd is the discharge coefficient (typically 0.95–0.99), d is the throat diameter in metres, ΔP is the differential pressure in Pascals, and ρ is the fluid density in kg/m³. The discharge coefficient accounts for real-world friction and flow profile deviations from ideal theory. A higher pressure differential or larger throat area yields a greater volumetric flow rate, measured in m³/s.
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.