What Is an Orifice and Orifice Flow Rate?
An orifice is a flat plate with a hole or cutout that is fixed inside a pipe or at an outlet to create a controlled restriction in the fluid flow. The orifice, usually circular and of known diameter, is used to measure, regulate, or restrict the flow of liquids in pipes, tanks, sewage systems, weirs, and spillways. As fluid passes through the orifice, pressure drops across the plate — a phenomenon also exploited in flow measurement devices called orifice meters.
How to Calculate Orifice Flow Rate
The orifice flow equation gives the volumetric discharge (flow rate) Q through a circular orifice:
where:
- Q — volumetric flow rate (m³/s or ft³/s)
- Cd — coefficient of discharge (dimensionless; typically 0.60–0.65 for a sharp-edged orifice, up to 0.98 for a well-rounded nozzle)
- A — cross-sectional area of the orifice (m² or ft²), calculated as A = π × d² / 4
- g — gravitational acceleration (9.81 m/s² or 32.174 ft/s²)
- H — hydraulic head (the vertical distance from the water surface to the center of the orifice, in m or ft)
Step-by-Step: Using the Orifice Flow Calculator
- Select your unit system: Metric (mm, m) or American (in, ft).
- Enter the orifice diameter d. The calculator derives the orifice area A = π × d² / 4 automatically.
- Enter the coefficient of discharge Cd. For a sharp-edged orifice the standard value is 0.62.
- Enter the hydraulic head H — the water-surface-to-center-of-orifice distance.
- Click Calculate. Results appear instantly in multiple units: m³/s, L/s, L/min, m³/h (metric) or ft³/s, GPM, ft³/min (American).
Worked Example
Find the discharge through an orifice with a diameter of 50 mm, a head of 200 mm (0.2 m), and Cd = 0.80.
- Orifice area: A = π × (0.050)² / 4 = 0.001963 m²
- Flow rate: Q = 0.80 × 0.001963 × √(2 × 9.81 × 0.2) = 0.00310 m³/s (3.10 L/s)
Coefficient of Discharge (Cd) Reference
| Orifice Type | Typical Cd |
|---|---|
| Sharp-edged circular orifice (standard) | 0.60–0.65 |
| Short tube / pipe orifice | 0.80–0.85 |
| Well-rounded nozzle (entrance) | 0.95–0.98 |
| Borda's mouthpiece (re-entrant) | ~0.51 |
Frequently Asked Questions
What is an orifice?
An orifice is a flat plate with a cutout fixed inside a pipe or at an outlet to create a pressure differential in fluid flow. It is used to measure or control flow rate in hydraulic systems, compressors, water tanks, and irrigation networks.
What is orifice flow rate?
Orifice flow rate is the volume of fluid passing through the orifice per unit time. It depends on the orifice area, the gravitational head, and the discharge coefficient of the particular orifice geometry.
What does the coefficient of discharge represent?
Cd accounts for real-world losses due to flow contraction (vena contracta) and viscous friction. A value of 1.0 would represent a perfect, loss-free orifice; real orifices always have Cd < 1.
How does head (H) affect flow rate?
Flow rate increases with the square root of the head. Doubling the head increases Q by a factor of √2 ≈ 1.41 (about 41% more flow).
Can I use this calculator for gas or air flow?
This calculator is designed for incompressible liquids (water, oil, etc.). For compressible gas flows, additional correction factors are required.