Engineering calculator reviewed for preliminary design use · Last updated: March 2026
Calculate orifice plate flow measurement per ISO 5167. Compute volumetric and mass flow rates from differential pressure, discharge coefficient Cd, fluid density, and orifice diameter. Apply to differential pressure flowmeter design and calibration.
What this calculator is used for
An orifice plate flowmeter uses a restriction inserted in the pipe to create a differential pressure proportional to the square of the flow rate. It is one of the most widely used flow measurement devices in oil, gas, and chemical plants thanks to its simplicity, robustness, and well-documented standards.
Typical engineering use cases
Metering steam, gas, and liquid streams in process piping
Building flow control loops together with DP transmitters
Re-rating an existing plate by revising the beta ratio
Preliminary sizing ahead of custody-transfer metering
Governing equation and methodology
Volumetric flow rate Q is calculated per ISO 5167:
Q = Cd × (π/4) × d² × √(2ΔP/ρ)
where Cd is the discharge coefficient (typically 0.6–0.65), d the bore diameter [m], ΔP the upstream-to-downstream differential pressure [Pa], and ρ the fluid density [kg/m³]. For compressible service an expansibility factor ε and the velocity-of-approach factor are applied.
Engineering assumptions and limitations
Fully developed, single-phase, steady-state flow
Adequate straight run upstream of the plate
Cd depends on the beta ratio (d/D) and Reynolds number
Not valid under cavitating or two-phase conditions
Practical design notes
Avoid using a blanket Cd of 0.61; use calibrated values within the actual Reynolds range. Because ΔP varies with the square of flow, a wide turndown sacrifices low-flow accuracy. Account for the permanent (non-recoverable) pressure loss in the overall hydraulic design.
Worked Example
Given:
Pipe ID D = 100 mm, orifice bore d = 50 mm (β = 0.5)
Interpretation: Cd ≈ 0.62 captures the vena contracta and only holds for liquids; for gas you must add the expansion factor Y or you will over-read flow. Keep β in the 0.2–0.6 range and pipe Re high enough that Cd stays flat — outside that band the coefficient drifts and the measurement loses accuracy.
Common Mistakes & Misuse
Using a liquid Cd for gas without the expansion factor Y, which corrects for density change across the plate and is well below 1 at high ΔP/P.
Operating outside the beta-ratio (typically 0.2–0.75) or pipe-Reynolds range where the ISO 5167 Cd correlation is valid.
Mismatching the tap arrangement (corner, D-D/2, flange) to the Cd used — each tap location has its own coefficient.
Taking the differential at one flow and assuming linearity — orifice flow follows √ΔP, so the meter is non-linear across its range.
Frequently Asked Questions
What is the discharge coefficient Cd?
Cd accounts for the vena contracta effect — the actual flow area is smaller than the orifice bore. Typical Cd values range from 0.60 to 0.65 for sharp-edge orifice plates. It depends on beta ratio and Reynolds number.
Is this accurate for gas flow measurement?
For gas flow, an expansion factor Y must be applied. This calculator provides liquid flow calculation. For gas, use ISO 5167 Part 2 with appropriate expansion factor correction.
For preliminary estimation and educational use only. Results may depend strongly on assumptions, input data, fluid or material properties, and the range of validity of the underlying equation. Verify critical calculations independently and follow the applicable code, specification, and formal engineering review process before using any result for design, procurement, fabrication, operation, or safety decisions.