Orifice Flow to SCFM Calculator
The orifice flow equation for compressed air uses the compressible Bernoulli principle with a discharge coefficient correction. The mass flow rate through a sharp-edged orifice at...
Formula
Source: Engineering Toolbox, Compressed Air Challenge | Last reviewed: June 8, 2026
Examples
0 SCFM
= 90.6 SCFM
- orifice_dia = 0.25
- P_up = 100
- P_down = 0
1/4" hole at 100 psig leaks ~91 SCFM
0 SCFM
= 22.7 SCFM
- orifice_dia = 0.125
- P_up = 100
- P_down = 0
1/8" hole = 22.7 SCFM leak
0 SCFM
= 256 SCFM
- orifice_dia = 0.5
- P_up = 100
- P_down = 50
1/2" orifice with 50 psig back-pressure
Where is this used?
Relief valve and safety valve flow capacity estimation.
Flow metering orifice sizing for differential pressure flow measurement.
Pneumatic system troubleshooting and energy audit calculations.
In the DOE Compressed Air Challenge and CAGI Air Audit programs, the orifice flow equation is the primary tool for quantifying the cost of compressed air leaks in a plant audit — the ultrasonic leak detector (UE Systems UltraTrak, SDT 270, etc.) gives a reading in dB that correlates with the leak rate in SCFM through the orifice flow equation.
A 1/4-inch leak at 100 psig wastes about 91 SCFM of compressed air, costing $11,800/year at $0.10/kWh for a continuously running compressor.
Even a tiny 1/16-inch leak (about 5.7 SCFM at 100 psig) costs $740/year — the cumulative cost of 50 such small leaks in a typical unmaintained plant is $37,000/year, illustrating why ultrasonic leak detection surveys typically have a payback of weeks.
Pressure relief valve and safety valve capacity calculation per ASME BPVC Section XIII and API 520/521 uses the orifice flow equation to determine the required orifice area (orifice letter designation, e.g., API 'D' orifice, 'E' orifice, 'F' orifice, 'G' orifice, 'H' orifice, 'J' orifice) for the design relieving capacity.
The valve manufacturer publishes the rated capacity (in SCFM or lb/hr of saturated steam, or in SCFM of air at the rated set pressure) for each orifice size, and the orifice flow equation is the basis of this rating.
Differential pressure flow measurement in process industries (orifice plates, flow nozzles, Venturi tubes) per ISO 5167 uses the orifice flow equation to relate the measured differential pressure across the primary element to the mass flow rate, with the discharge coefficient and the expansibility factor as the two key parameters.
Compressed air system troubleshooting and energy audits use the orifice flow equation to estimate the flow through pneumatic valves, fittings, hoses, and quick-connects that may be undersized, leaking, or worn, with the field measurement of pressure differential and component dimensions giving an estimate of the flow in SCFM.
Pneumatic system component selection (orifice-type silencers, quick exhaust valves, flow control valves) uses the orifice flow equation to size the orifice diameter for the desired flow capacity at a given pressure differential.
Frequently Asked Questions
What is the typical cost of a compressed air leak?
A 1/4" leak at 100 psig (~91 SCFM) costs approximately $11,800/year in electricity at $0.10/kWh. Even a tiny 1/16" leak (~5.7 SCFM) costs ~$740/year. This is why leak detection and repair programs have rapid payback.
Does the orifice shape matter?
Yes. Sharp-edged orifices have a discharge coefficient Cd ≈ 0.62, which is built into this formula. Rounded or bell-mouth inlets flow more (Cd up to 0.98). Irregular holes from corrosion may flow differently. This assumes a clean sharp-edged orifice.
What about choked flow conditions?
When the pressure ratio P_down/P_up is below the critical ratio (~0.53 for air), flow becomes choked (sonic velocity at the orifice). This formula approximates both subsonic and choked flow regimes for compressed air service.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.