Orifice Plate Flow Calculator
By Santaji GadepatilinUpdated October 27, 2026
The orifice plate is the most widely used flow element in industry: a thin plate with a hole that creates a pressure drop proportional to flow squared. Learn about orifice plate flow measurement, the orifice plate sizing guide and how it compares in flow nozzle vs orifice plate vs Venturi.
Flow rate (m³/h) vs Differential pressure (mbar) — other inputs held at your current values.
How to use the orifice plate flow calculator
Enter the pipe inner diameter, orifice bore, discharge coefficient, differential pressure and liquid density to get the flow rate. To size a DP transmitter range, press × on Differential pressure and type the maximum flow.
The formula (liquids)
Q = Cd × Ao × √(2ΔP ÷ (ρ(1 − β⁴)))
- Ao = π d² ÷ 4 — orifice bore area
- β = d ÷ D — beta ratio
- Cd — discharge coefficient (about 0.61 for sharp-edged plates)
Worked example
Water (998 kg/m³) in a 100 mm pipe with a 50 mm bore (β = 0.5), Cd = 0.61, ΔP = 250 mbar:
- Ao = 0.001963 m²
- 1 − β⁴ = 0.9375
- Q = 0.61 × 0.001963 × √(2 × 25,000 ÷ (998 × 0.9375)) = 0.00876 m³/s = 31.5 m³/h
Beta ratio effects
| β (d/D) | 1 − β⁴ | Velocity of approach factor 1/√(1 − β⁴) |
|---|---|---|
| 0.2 | 0.9984 | 1.0008 |
| 0.3 | 0.9919 | 1.0041 |
| 0.4 | 0.9744 | 1.0131 |
| 0.5 | 0.9375 | 1.0328 |
| 0.6 | 0.8704 | 1.0719 |
| 0.7 | 0.7599 | 1.1472 |
| 0.75 | 0.6836 | 1.2095 |
ISO 5167-2 limits β to between 0.10 and 0.75 for standard orifice plates.
Permanent pressure loss
An orifice plate recovers only part of the differential pressure downstream. The permanent loss is roughly:
Δω ≈ (1 − β1.9) × ΔP
At β = 0.5, about 73% of the measured DP is lost permanently, which costs pumping energy. Venturi tubes recover far more.
Straight-run requirements
Orifice plates need undisturbed flow. Typical upstream straight lengths are 10–40 pipe diameters depending on the fittings and β ratio, plus about 5–8 diameters downstream. ISO 5167 tables give exact values; flow conditioners can shorten them.
Tapping arrangements
| Taps | Location |
|---|---|
| Flange taps | 25.4 mm (1 in) each side of the plate |
| Corner taps | At the plate faces |
| D and D/2 taps | 1 D upstream, 0.5 D downstream |
The discharge coefficient depends on the tapping type, so use the matching correlation (Reader-Harris/Gallagher in ISO 5167).
Good practice
- Keep β between about 0.2 and 0.75.
- Provide enough straight pipe upstream (often 10–40 D depending on fittings) and downstream (about 5 D).
- Turndown is limited to about 3:1 to 4:1 with one DP transmitter, because flow follows the square root of DP. See the DP square root calculator.
- For gases and steam, an expansibility factor is also needed.
Frequently asked questions
What is the beta ratio?
The ratio of the orifice bore to the pipe's inner diameter (d/D). Most designs keep it between 0.2 and 0.75 for good accuracy.
Why is flow proportional to the square root of DP?
Bernoulli's equation links velocity to the square root of pressure drop. Doubling the flow needs four times the differential pressure.
Is this accurate enough for custody transfer?
No. This uses a fixed discharge coefficient for quick sizing and checks. Fiscal metering uses the full ISO 5167 method with Reynolds-dependent coefficients and expansibility for gases.
What is a typical orifice discharge coefficient?
About 0.60–0.62 for sharp-edged concentric orifices in turbulent flow.
What beta ratio should I use?
Typically 0.3–0.7. Lower beta gives more DP and better low-flow measurement but higher permanent pressure loss.