Orifice Flow Calculator

What is an Orifice?

An orifice is a sharp-edged opening in a plate or fitting through which fluid flows. The flow rate through an orifice depends on the upstream head (or pressure differential) and the orifice area. Orifice plates are one of the most common flow measurement devices in industry.

Torricelli's Theorem (Tank Discharge)

For a tank with a free surface at height h above the orifice:

v = Cd × √(2gh) Q = Cd × A × √(2gh)

Where:

• Cd = discharge coefficient (typically 0.6–0.65 for a sharp-edged orifice)
• A = orifice area (m²)
• g = 9.81 m/s²
• h = head above orifice centre (m)

Orifice Plate (Differential Pressure Meter)

Q = Cd × A_orifice × √(2ΔP/ρ × (1/(1−(D_orifice/D_pipe)⁴)))

The ratio D_orifice/D_pipe is called the beta ratio (β), typically 0.3–0.75 for accuracy.

Practical Examples

Example 1 — Tank drain: 25mm orifice, Cd=0.62, water head 3m. A = π × 0.0125² = 4.91×10⁻⁴ m². v = 0.62 × √(2 × 9.81 × 3) = 4.75 m/s. Q = 4.91×10⁻⁴ × 4.75 = 2.33 L/s = 140 L/min.

Example 2 — Orifice plate meter: 100mm pipe, 50mm orifice, ΔP = 20 kPa, water. Q = 0.61 × π×0.025² × √(2×20000/1000/(1−0.5⁴)) = ~5 L/s.