Calculate CFM From PSI: What You Need to Know

You generally cannot calculate CFM (cubic feet per minute) from PSI (pounds per square inch) alone: pressure measures force per area, while CFM measures airflow. To estimate or determine airflow, you also need information such as the orifice or hose size, the pressure conditions, and the gas or equipment characteristics; this calculator uses those inputs to estimate flow only when they are specified.

Formula

This calculator uses a simplified incompressible-orifice equation to estimate airflow through a circular opening:

Q = C_d × A × √(2ΔP / ρ)

For the calculator’s units, this becomes CFM = C_d × π(d/24)² × 60 × √(2 × ΔP × 144 / 0.0765).

• CFM is estimated volume flow in cubic feet per minute.
• C_d is the dimensionless discharge coefficient, describing losses as air passes through the opening.
• d is the opening diameter in inches; dividing by 24 converts its radius to feet.
• ΔP is the pressure difference across the opening in psi; 144 converts pounds per square inch to pounds per square foot.
• ρ is assumed air density, 0.0765 pounds per cubic foot, a near-standard reference value.
• 60 converts cubic feet per second to cubic feet per minute.

The equation is an estimate under the stated assumptions, not a universal PSI-to-CFM conversion. The calculator has no inputs for temperature, humidity, downstream pressure, or compressibility, so its result should not be treated as a certified compressor rating or a precise prediction for every pneumatic system.

Worked Example

Using the defaults—10 psi pressure difference, a 0.25-inch opening, and a discharge coefficient of 0.65—first find the opening area. Its radius is 0.25 ÷ 24 = 0.0104167 feet, so the area is π × 0.0104167² ≈ 0.0003409 square feet.

Next, the idealized air velocity term is √(2 × 10 × 144 ÷ 0.0765) ≈ 193.96 feet per second. Multiplying area by velocity and by 60 gives about 3.97 cubic feet per minute before applying the coefficient. Finally, 3.97 × 0.65 ≈ 2.58 CFM. The calculator rounds to two decimal places.

Why PSI Alone Does Not Determine CFM

Pressure and flow describe different properties. A pressure gauge reports the force per area at its measurement point. Flow describes how much gas passes a point over time. A sealed tank can remain at a high PSI while its airflow is zero, because no gas is moving. Conversely, an open pipe can carry substantial flow with only a small pressure difference.

To predict flow, a model needs a restriction or passage size and a pressure difference that drives air through it. The actual result also depends on geometry, friction, air density, temperature, and whether the flow is turbulent or compressible. For air flowing from a compressor, “PSI” may mean gauge pressure, absolute pressure, regulated outlet pressure, or pressure measured at another location. These are not interchangeable.

How to Use the Estimate

• Enter the pressure drop across the opening, not simply the compressor’s maximum tank pressure. The upstream-to-downstream difference is what drives flow.
• Enter the opening’s actual diameter in inches. A hose’s outside diameter is not its inside diameter, and fittings can have narrower passages than the hose.
• Choose a discharge coefficient that represents the opening. The default 0.65 is a rough illustrative assumption, not a guaranteed value for a particular fitting.
• Read the output as an approximate flow in CFM under the simplified assumptions. Do not interpret it as a compressor’s free-air-delivery rating.

If the opening is not circular, or the air path includes several fittings, valves, and long hoses, this single-orifice model may not represent the system well. Use component flow data or an appropriate engineering calculation for those cases.

Compressor Ratings and Common Mix-Ups

Compressor capacity is commonly given as CFM at a stated pressure. That rating is useful because it reports airflow under a specified operating condition; PSI by itself is not a capacity rating. A compressor may reach a certain tank pressure but deliver too little air at the working pressure to support a tool continuously. Tank volume, meanwhile, is stored-air capacity, not a rate of delivery.

CFM figures also depend on how the flow is defined. SCFM and other standardized ratings refer to flow corrected to specified reference conditions, while actual CFM describes volume at the current pressure and temperature. Rating conventions can vary, so compare values only when their conditions and definitions match.

A frequent error is to put a compressor’s gauge pressure directly into a formula that expects a pressure difference across an opening. Another is assuming that doubling PSI doubles CFM. In this simplified equation, flow varies with the square root of pressure difference: doubling the pressure difference increases the estimate by about √2, or 1.41 times, if all other assumptions remain unchanged. Real compressed-air flow can depart from this relationship.

Limits, Units, and Better Measurements

The model assumes a fixed air density and treats the flow as incompressible. Air is compressible, and at substantial pressure ratios its density changes through the restriction; flow can also become choked, so increasing upstream pressure no longer raises mass flow in the simple way this equation suggests. The calculator should therefore be used for a rough, low-pressure estimate, not equipment selection, performance guarantees, or pressure-system design.

For better information, measure flow with a suitable airflow meter or use the equipment maker’s published flow curve at the relevant inlet pressure. A calibrated flow meter gives a direct measurement; a compressor rating gives a useful comparison when its rating conditions match the application. Keep units consistent: 1 cubic foot equals about 28.3168 liters, so 1 CFM is about 28.3 liters per minute. One psi is about 6.895 kPa. Converting units does not remove the need to know the restriction and operating conditions.

Frequently Asked Questions

Can you convert PSI directly to CFM?
No. PSI alone does not specify airflow. You need at least a flow path or opening size and pressure conditions, along with assumptions or data about the gas and system.

Does higher PSI always mean more CFM?
Not in a fixed, predictable proportion. More pressure difference can increase flow, but the change depends on the restriction and flow regime; compressed air can also become choked.

What is the difference between PSI and CFM?
PSI is pressure, or force per unit area. CFM is volume flow per unit time. Pressure can exist with no flow, such as in a sealed tank.

Is compressor CFM the same as tank capacity?
No. CFM is a delivery rate. Tank capacity is the volume of the storage vessel. A large tank can store more air without increasing the compressor’s sustained delivery rate.

What does SCFM mean?
SCFM means standard cubic feet per minute: a flow value referenced to specified standard temperature and pressure conditions. Check the rating’s stated standard before comparing it with another figure.

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in p | Pressure difference | psi | 10 | 0
in d | Opening diameter | in | 0.25 | 0
in c | Discharge coefficient | - | 0.65 | 0
out cfm | Estimated airflow | ft³/min | c * pi * (d / 24)^2 * 60 * sqrt(2 * p * 144 / 0.0765) | 2