Venturi Meter and Orifice Plate Flow Calculator

A venturi meter is a flow-measurement device with a smoothly converging section leading into a narrow throat, followed by a gradually diverging section back to the original pipe diameter. The pressure drop between the pipe and the throat, caused by the fluid speeding up through the constriction, is used to calculate flow rate.

An orifice plate is a thin plate with a precisely machined circular hole (the orifice) inserted into a pipe, forcing all flow through the smaller opening. It is far cheaper and simpler to install than a venturi meter, but its sharp edge creates more turbulence and a larger permanent pressure loss.

Q = Cd x A2 / sqrt(1 - beta^4) x sqrt(2 x deltaP / rho), where Cd is the discharge coefficient, A2 is the throat or orifice area, beta is the diameter ratio d2/d1, deltaP is the measured differential pressure, and rho is the fluid density.

Beta (β) is the throat or orifice diameter divided by the upstream pipe diameter, d2/d1. It must stay strictly between 0 and 1 because the throat or orifice must be smaller than the pipe itself, and the formula's 1-beta^4 term becomes zero or undefined outside that range.

A classical venturi meter with a smooth machined throat typically uses Cd around 0.95 to 0.99 (0.98 is a common default). A concentric, sharp-edged orifice plate typically uses Cd around 0.60 to 0.62 (0.61 is a common default). Precise metering applications should use a coefficient calibrated per ISO 5167 for the exact geometry.

A venturi meter's gradual, smooth contraction produces a high discharge coefficient and recovers most of the pressure drop downstream, giving very low permanent head loss. An orifice plate's sharp-edged hole is cheap and simple to install but creates more turbulence, a lower discharge coefficient, and a significantly larger permanent pressure loss.

The relationship comes from applying Bernoulli's equation and continuity across the constriction, the kinetic energy gained by the fluid as it accelerates through the throat equals the pressure energy lost, and kinetic energy depends on velocity squared, so velocity (and hence flow rate) is proportional to the square root of the pressure drop.

Yes, the same square-root relationship applies to both liquids and gases at typical industrial flow measurement conditions, as long as you use the correct fluid density for the operating pressure and temperature. For compressible gas flows at high velocity, an additional expansibility factor is normally applied, which this calculator does not include.

Pipe and throat/orifice diameters are entered in millimetres, differential pressure in kilopascals, and fluid density in kilograms per cubic metre. Flow rate results are shown in cubic metres per hour, litres per second, and cubic metres per second.

For a venturi meter, taps are placed at the inlet (upstream of the contraction) and at the throat, per the manufacturer's design. For an orifice plate, common tap arrangements include flange taps, corner taps, and D and D/2 taps, each measuring differential pressure slightly differently, so the discharge coefficient used must match the tap arrangement.

A beta ratio too close to 1 means the throat or orifice is nearly the same size as the pipe, producing a very small, hard-to-measure differential pressure and poor sensitivity. A beta ratio too close to 0 creates a very small opening, causing excessive pressure loss, high fluid velocity, and increased wear or cavitation risk, most designs keep beta between about 0.2 and 0.75.

The orifice plate's sharp edge causes the flow to separate abruptly and form turbulent eddies downstream that dissipate energy as heat, and this energy is never recovered. The venturi meter's gradually diverging outlet section lets the flow decelerate smoothly, recovering most of the kinetic energy back into pressure, so far less energy is permanently lost.