Venturi Meter Calculator
Determine the volumetric flow rate through a pipe using upstream and throat pressure readings from a Venturi meter. Ideal for engineers verifying flow in water treatment, HVAC, and chemical process pipelines.
About this calculator
A Venturi meter measures flow by accelerating fluid through a converging throat section and measuring the resulting pressure drop. The theoretical flow rate is Q = Cd × A_t × √(2ΔP / ρ) × 1/√(1 − β⁴), where A_t = π·d_t²/4 is the throat area, β = d_t/d_p is the diameter ratio, ΔP is the differential pressure in Pa, ρ is fluid density in kg/m³, and Cd is the discharge coefficient accounting for real-fluid losses. Typical Cd values range from 0.95 to 0.99 for machined Venturis and 0.92–0.96 for rough-cast versions. The term 1/√(1 − β⁴) is the velocity-of-approach factor, which corrects for the non-negligible upstream velocity. This formula is standardised in ISO 5167.
How to use
Given: throat diameter d_t = 0.05 m, pipe diameter d_p = 0.10 m, ΔP = 5000 Pa, ρ = 1000 kg/m³, Cd = 0.98. Step 1 — β = 0.05/0.10 = 0.5. Step 2 — A_t = π × 0.05²/4 = 0.001963 m². Step 3 — velocity-of-approach factor: 1/√(1 − 0.5⁴) = 1/√(0.9375) = 1.033. Step 4 — Q = 0.98 × 0.001963 × √(2 × 5000/1000) × 1.033 = 0.98 × 0.001963 × 3.162 × 1.033 ≈ 0.00629 m³/s (6.29 L/s). Enter your own pipe dimensions and pressure reading to find flow instantly.
Frequently asked questions
What discharge coefficient should I use for a Venturi meter calculation?
The discharge coefficient Cd corrects for friction, flow separation, and non-uniform velocity profiles that cause the real flow to differ from the ideal Bernoulli prediction. For a classical machined (smooth) Venturi conforming to ISO 5167, Cd is typically 0.995. Rough-cast or welded Venturis use Cd values around 0.92–0.96. The correct Cd depends on the Reynolds number, throat-to-pipe diameter ratio, and manufacturing quality; manufacturers provide calibration curves for precision applications. Using a Cd that is too high will overestimate flow rate and lead to process errors.
How does the diameter ratio beta affect Venturi meter accuracy?
The diameter ratio β = d_t/d_p controls sensitivity and pressure loss. A smaller β (narrower throat) produces a larger ΔP for the same flow rate, improving measurement sensitivity and reducing uncertainty at low flows. However, a very small β increases the permanent pressure loss in the system, consuming pumping energy. ISO 5167 recommends β between 0.3 and 0.75 to balance sensitivity and acceptable head loss. Outside this range, the velocity-of-approach factor and Cd correlations become less reliable, reducing measurement accuracy.
When should I use a Venturi meter instead of an orifice plate for flow measurement?
Venturi meters are preferred when permanent pressure loss must be minimised, such as in large water mains or energy-intensive chemical plants, because they recover 90–95% of the differential pressure. Orifice plates are cheaper and easier to install but cause two to three times more permanent head loss for the same ΔP. Venturis handle slurries and dirty fluids better because the smooth converging-diverging profile avoids sharp edges that trap solids. For highly accurate custody-transfer metering where fouling is not a concern, orifice plates with corner taps are often preferred due to their simpler calibration traceability.