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Venturi Meter Flow Rate Calculator

Enter the venturi geometry, pressure drop and fluid density to compute the flow rate.

How it is calculated

The tool applies Q = Cd × A2 × √(2·ΔP / (ρ·(1 − (A2/A1)2))). Inputs must be in SI units: inlet diameter d1 and throat diameter d2 in metres, pressure difference ΔP in pascals, density ρ in kg/m³ and a dimensionless discharge coefficient Cd. Areas are computed as A = π·d²/4. The results are the volume flow in m³/s, the same flow in L/min (× 60,000), the throat velocity (Q divided by A2) and the throat area. There is no result if the throat is not narrower than the inlet, if any diameter, density or Cd is zero or negative, or if ΔP is negative. A ΔP of zero gives zero flow. Compressible gases are not corrected for expansion.

Worked example

Water (1000 kg/m³) flows through a venturi with a 0.10 m inlet and a 0.05 m throat, Cd = 0.98, measured pressure drop 20,000 Pa. Areas: A1 = 0.007854 m², A2 = 0.001963 m², so (A2/A1)2 = 0.0625. Under the root: 2 × 20,000 / (1000 × 0.9375) = 42.667, whose square root is 6.532. Then Q = 0.98 × 0.001963 × 6.532 = 0.01257 m³/s, which is 754.14 L/min, with a throat velocity of 6.4013 m/s.

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Common mistakes

  • Entering diameters in millimetres or inches. The engine expects metres: typing 100 and 50 for a 100 mm and 50 mm venturi gives an area millions of times too large and an absurd flow.
  • Entering the pressure drop in kPa, bar or psi. It must be in pascals, so 20 kPa is 20000.
  • Putting the pipe size or the throat in the wrong box. If d2 is not smaller than d1 the tool returns nothing, because a venturi throat is by definition the narrower section.
  • Using the pressure at one tap instead of the difference between inlet and throat taps. The formula needs ΔP between the two.
  • Forgetting that density changes with fluid and temperature. Water is about 1000 kg/m³ but oil or brine differ, and the flow scales with 1/√ρ.

About this calculator

A venturi meter measures flow rate by narrowing a pipe to a throat section, which forces the fluid to speed up and, by Bernoulli's principle, drop in pressure — the bigger that pressure drop, the faster the fluid is moving. The flow rate is recovered from Q = Cd × A₂ × √(2ΔP / (ρ(1−(A₂/A₁)²))), where A₁ and A₂ are the inlet and throat areas, ΔP is the measured pressure differential, ρ is fluid density, and Cd is a discharge coefficient (typically 0.95-0.99) that corrects for the small energy losses a real venturi has that the idealized Bernoulli equation ignores.

This calculator takes your venturi's inlet and throat diameters, the measured pressure drop, fluid density and discharge coefficient, and returns the volumetric flow rate. Process and chemical engineers monitoring flow in a pipeline, HVAC and plumbing engineers sizing metering devices, and instrumentation technicians calibrating a venturi flow meter use it because venturi meters are prized for measuring flow with very little permanent pressure loss compared to an orifice plate, making them a common choice for permanent installations.

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Comments (1)

  • Editorial team

    The flow grows with the square root of the pressure difference, so quadrupling the differential only doubles the flow. With the default water example, 2000 Pa gives 238.48 L/min and 8000 Pa gives 476.96 L/min. If your gauge reads in millimetres of water column, each millimetre is about 9.81 Pa. Cd is your assumption, so check it against the meter's datasheet. Which fluid are you metering?

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