Calculate orifice diameter, liquid flow rate, or pressure drop from flow, fluid density, and discharge coefficient with metric and imperial units.

For incompressible liquid with negligible upstream approach velocity.

Use the positive pressure difference across the opening.

Water estimate is near 20°C; other presets vary with composition and temperature.

Use a tested value for your opening and operating conditions; the estimate is not a universal constant.

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Orifice Size Formula

The following formula is used to calculate the orifice size.

A = Q / [Cd √(2 × ΔP / ρ)]

Variables:

  • A is the orifice opening area (square meters)
  • Q is the flow rate (cubic meters per second)
  • ΔP is the pressure drop across the orifice (pascals)
  • ρ is the density of the fluid (kg/m³), assumed to be 1000 kg/m³ for water
  • C_d is the discharge coefficient (≈ 0.60 – 0.63 for a sharp-edged orifice)

To calculate the opening area, divide the flow rate by the discharge coefficient multiplied by the square root of twice the pressure drop divided by the liquid density. For a circular opening, convert area to diameter with d = 2√(A/π).

What is an Orifice Size?

An orifice size refers to the area of the opening in an orifice plate, which is used to measure flow rate or reduce pressure within a piping system. The size of the orifice determines the amount of fluid that can pass through and is an important factor in the design and operation of flow control systems.

How to Calculate Orifice Size?

The following steps outline how to calculate the Orifice Size.

  1. First, determine the flow rate (Q) in cubic meters per second.
  2. Next, determine the pressure drop (ΔP) across the orifice in pascals.
  3. Use the liquid density (ρ) at the relevant conditions. Water is approximately 998 kg/m³ near 20°C; 1000 kg/m³ is a common rounded estimate.
  4. Choose an applicable discharge coefficient (C_d), then use A = Q / [C_d × √(2 × ΔP / ρ)].
  5. Calculate the opening area (A) in square meters. For a circular opening, d = 2√(A/π) gives the diameter in meters.
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Flow rate (Q) = 0.01 cubic meters per second

Pressure drop (ΔP) = 5000 pascals; assume liquid density ρ = 1000 kg/m³ and C_d = 0.62. The simplified estimate gives A ≈ 0.00510045 m² and circular diameter d ≈ 80.5859 mm.