CFM from PSI Calculator
An essential tool for engineers and technicians to calculate air flow rate from pressure.
Calculator
Enter the pressure in Pounds per Square Inch.
Enter the diameter of the opening through which the air flows.
A dimensionless number that accounts for friction and turbulence. Ranges from 0.6 to 0.98.
What is CFM and PSI?
Cubic Feet per Minute (CFM) is a measure of the volume of air that flows in a minute, while Pounds per Square Inch (PSI) is a measure of the pressure exerted by that air. Understanding how to calculate CFM using PSI is crucial in many engineering and industrial applications, especially when dealing with compressed air systems, HVAC, and pneumatic tools. A direct conversion isn’t possible without considering other factors, as they measure different properties. However, with the right formula and inputs, you can accurately estimate the flow rate.
CFM from PSI Formula and Explanation
The calculation of CFM from PSI is based on principles of fluid dynamics. The formula used in this calculator is a widely accepted equation for flow through an orifice:
CFM = 22.67 * C * A * √(P)
Where:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| CFM | Cubic Feet per Minute | ft³/min | Varies with application |
| C | Flow Coefficient | Dimensionless | 0.6 – 0.98 |
| A | Area of the Orifice | in² | Depends on orifice size |
| P | Pressure | PSI | 10 – 200+ |
This formula is an adaptation of Bernoulli’s principle for compressible fluids like air. To learn more about airflow calculations, you might find our guide on HVAC Calculations useful.
Practical Examples
Example 1: Small Air Tool
Let’s say you have an air tool connected to a line with the following parameters:
- Inputs:
- Pressure (P): 90 PSI
- Orifice Diameter: 0.125 inches
- Flow Coefficient (C): 0.9
- Calculation:
- Area (A) = π * (0.125/2)² = 0.0123 in²
- CFM = 22.67 * 0.9 * 0.0123 * √(90) ≈ 2.38 CFM
- Result: Approximately 2.38 CFM
Example 2: Industrial Application
Consider a larger industrial process with these values:
- Inputs:
- Pressure (P): 120 PSI
- Orifice Diameter: 0.5 inches
- Flow Coefficient (C): 0.95
- Calculation:
- Area (A) = π * (0.5/2)² = 0.1963 in²
- CFM = 22.67 * 0.95 * 0.1963 * √(120) ≈ 46.25 CFM
- Result: Approximately 46.25 CFM
For more examples, check out our article on air compressor sizing.
How to Use This CFM from PSI Calculator
- Enter Pressure: Input the pressure of your system in PSI.
- Enter Orifice Diameter: Provide the diameter of the hole or pipe in inches.
- Enter Flow Coefficient: Use the default or enter a specific value if you know it.
- Interpret the Results: The calculator will instantly display the calculated CFM, along with intermediate values.
Key Factors That Affect CFM Calculation
- Pressure (PSI): Higher pressure results in higher CFM, as it forces more air through the opening.
- Orifice Size: A larger orifice area allows more air to flow, increasing the CFM.
- Flow Coefficient (C): This accounts for energy losses due to friction and the shape of the orifice. A perfectly smooth, rounded orifice has a higher C value than a sharp-edged one.
- Temperature: While not a direct input in this simplified formula, temperature affects air density, which can influence the actual flow rate.
- Upstream and Downstream Conditions: The piping and components before and after the orifice can create turbulence and affect the flow. For details on system design, see our fluid dynamics basics guide.
- Altitude: At higher altitudes, the air is less dense, which can impact the mass flow rate for a given CFM.
Frequently Asked Questions (FAQ)
- Is there a direct CFM to PSI conversion?
- No, you cannot directly convert CFM to PSI because they measure different things (flow volume vs. pressure). You need additional information like orifice size to establish a relationship. A related concept is explained in our SCFM vs ACFM article.
- What is a typical flow coefficient?
- For a sharp-edged orifice, a flow coefficient of 0.62 is common. For a well-rounded nozzle, it can be as high as 0.98. The default of 0.95 in our calculator assumes a reasonably efficient opening.
- Why is my calculated CFM different from my compressor’s rating?
- Air compressors are often rated in Standard CFM (SCFM), which is measured under specific standard conditions of temperature, pressure, and humidity. Your actual CFM will vary based on your operating conditions. Our guide to understanding compressor specs can help clarify this.
- How does pipe length affect the calculation?
- Longer pipes increase friction losses, which can reduce the pressure at the orifice and thus lower the CFM. This calculator assumes the pressure is measured directly at the orifice.
- Can I use this calculator for liquids?
- No, this formula is specifically for compressible gases like air. Calculating liquid flow requires a different formula that accounts for the fluid’s density.
- What is the difference between PSI and PSIG?
- PSI (Pounds per Square Inch) is often used interchangeably with PSIG (Pounds per Square Inch Gauge), which is pressure relative to atmospheric pressure. PSIA (Pounds per Square Inch Absolute) is gauge pressure plus atmospheric pressure (approx. 14.7 PSI at sea level).
- Does the shape of the orifice matter?
- Yes, the shape significantly impacts the flow coefficient (C). A smooth, tapered entry (like a nozzle) will have a much higher C value and allow more flow than a sharp, flat-edged hole of the same diameter.
- How accurate is this calculation?
- This calculator provides a very good estimate for many common applications. However, for high-precision scientific or engineering work, more complex formulas and factors may be needed.
Related Tools and Internal Resources
- Pipe Flow Calculator: For more detailed pipe and duct flow calculations.
- HVAC Load Calculator: Determine cooling and heating requirements for a room.
- Air Velocity Converter: Convert between different units of air speed.