CFM Calculator for Compressed Air

Looking for a free and easy CFM calculator to determine how much compressed air your compressor is producing? Our online CFM calculator can help you properly size your air compressor and make sure you’ve got enough airflow to meet your compressed air demand.

In this article, we are going to show you how to calculate the compressed air flow rate of your air compressor using the CFM formula or our easy online calculator. You may also want to calculate your CFM demand to determine whether your compressed air output meets your airflow requirements. 

Use our air compressor CFM calculator below to help you quickly and easily calculate your CFM output!

Why Do You Need to Calculate Your Air Compressor CFM?

Calculating your airflow requirements for compressed air is important for properly sizing your air compressor. If you don’t have enough airflow to power all of your tools and processes, you may find yourself without enough air for efficient operations. On the other hand, paying for more capacity than you need doesn’t make sense, either. Calculating the CFM output of your air compressor is important for several reasons.

• Evaluating compressor performance: By comparing the actual CFM output to the manufacturer's specifications, you can determine if your compressor is operating efficiently and effectively or if there are potential issues affecting its performance.
• Troubleshooting: Knowing the actual CFM output can help you diagnose problems with the compressor or the air delivery system, such as leaks, blockages, or worn components.
• System optimization: Assessing the actual CFM output allows you to optimize your compressed air system by identifying areas for improvement, such as air storage, pressure regulation, or distribution.
• Capacity planning: Understanding the actual CFM output of your compressor can help you plan for future expansion or additional equipment requirements.

What Is CFM in Compressed Air?

CFM is the abbreviation for cubic feet per minute (cu ft/min). It is a measurement of how fast air flows into or out of a space. In compressed air, this means it is a measure of the flow or output rate that air is coming out of your compressor.

What Is the Relationship Between CFM and PSI?

CFM is the “Flow” or the compressor’s ability to continue performing a certain task over time. It expresses the amount of flow needed and depends on the length of time required to complete the task. With insufficient flow, the compressor will require breaks to rebuild pressure in the compressor’s reserve tank.

PSI (Pounds per Square Inch) is the “Pressure”. It expresses the compressor’s ability to perform a specified amount of work at any given point in time. The compressor must provide the right amount of pressure, or force, needed to complete the process.

In compressed air terms, pressure delivers the force, yet horsepower delivers the flow. These two measures are inversely related: as pressure (PSI) increases, the flow rate (CFM) decreases, and vice versa. To achieve the best efficiency and performance from your air compressor, it's crucial to find the right balance between CFM and PSI for your specific application.

Aire Tip: Many plants are operating at higher operating pressures than required for their equipment. Lowering plant pressure can help you save money and energy and get more airflow from your compressor.

Calculating Compressor Capacity

Air compressor sizing is important. Air compressors are rated for a certain CFM (flow) at a certain PSI (pressure). For example, an industrial rotary screw air compressor may be rated for 100 CFM at 100 PSI. Raising or lowering the pressure output will also change the actual CFM delivered by the air compressor. It can be helpful to manually test and calculate the actual CFM output for your specific operating pressure.

Between CFM and PSI, these two measurements essentially define whether a compressor can meet the demand of your equipment. Each piece of production equipment will have a CFM (airflow) requirement. When calculating how much your air compressor’s output rate needs to be, you not only need to consider the requirements of each of your tools but also how often/how long they will run. Another fact to consider is how much of your equipment will be running at the same time.

Free Online Air Compressor CFM Calculator

Utilizing the Tank Pump-Up Time method, let’s test the efficiency & calculate the CFM of your compressed air system. The tank pump-up time method is a practical way to estimate the CFM output of an air compressor, especially when the manufacturer's specifications are not available. The method involves measuring the time it takes for the compressor to fill the air receiver tank from a specific starting pressure to a specific ending pressure.

How Do You Calculate CFM for an Air Compressor?

When customers call us trying to learn about the physics behind compressed air and wanting to understand what is best for them, they ask, “how do you measure the compressed air flow rate?” It may seem complex, but it really is quite easy to do yourself (though our calculator makes it much easier!).

CFM Calculation Formula

The formula to calculate the CFM output of an air compressor using the tank pump-up time method is as follows:

CFM = (V × ∆P) ÷ (T × 14.7)

Where:

• CFM: Cubic Feet per Minute (output of the air compressor)
• V: Volume of the air receiver (tank) in cubic feet
• ∆P: Change in pressure during the pump-up time (ending pressure - starting pressure) in PSI
• T: Time it takes for the air compressor to fill the tank from the starting pressure to the ending pressure, in seconds
• 14.7: Atmospheric pressure in PSI at sea level
  

Instructions for Manual Air Compressor CFM Calculation

To use the tank pump-up time method:

1. Empty the air receiver by opening the tank drain valve and releasing all the air.
2. Close the drain valve and start the air compressor.
3. Measure and record the time (T) it takes for the air compressor to fill the tank from the desired starting pressure (usually 0 PSI) to the ending pressure (often the compressor's maximum working pressure, such as 90 PSI or 100 PSI).
4. Determine the air receiver's volume (V) in cubic feet. This information is typically listed on the tank or can be calculated using the tank's dimensions.
5. Calculate the change in pressure during the pump-up time (∆P) as the difference between the ending pressure and the starting pressure (in PSI).
6. Use the formula to calculate the CFM output of the air compressor.

This method provides an approximate CFM output and may not be as accurate as the data provided by the manufacturer. Additionally, it assumes there are no air leaks in the system, and the compressor's performance is optimal.

Example Air Compressor CFM Calculation

Let's look at an example.

Example: 15 HP Compressor with a 132 Gallon Tank rated for 125 PSI and 60 CFM. Let’s work backward with our formula and see what we get.

Start the compressor and measure the time in seconds of the load and unload cycles on the compressor tank gauge.Using a stopwatch (there is one on your smartphone), time how long it takes from when the compressor starts running to the time it stops running. Also, watch the compressor’s tank gauge during this time and record the PSI, both when the compressor starts and stops running. If you are starting up a compressor that has been off, let the tank fill completely once before starting your timing and recording of results.

If the compressor loads at 80 PSI and unloads at 120 PSI, this equals a 40 PSI difference (also known as differential pressure). Let’s say the time between load and unload is 6 seconds.

So the calculation looks like this:

CFM = (132 gal × 40 PSI) ÷ (6 seconds × 14.7)
CFM = 5280/88.2
CFM=59.8

This is very close to the manufacturer's rating of 60 CFM. The 0.2 difference is most likely because of the rounding that we did.

The easier way to do all this is simply to use our air compressor CFM calculator above and it will do all the work for you.

Using a Flow Meter to Calculate CFM (Airflow)

Another easy way to calculate your CFM is by using an airflow meter. This will give you the amount of flow and the pressure that is coming out of a fitting. Flow meters can be installed on individual pieces of equipment or complete systems to provide an instant reading of actual usage. For a more accurate reading, a receiver tank should be installed near the flow meter to even out peaks and valleys in the data.

There are different types of flow meters that can be used for point-of-use measuring, and in-line measuring. VIP Instruments has a vast array of different flow meters. If you don’t know which one to choose, give us a call and we will help to walk you through what your system may need.

Quick Air Compressor CFM Chart

This handy chart will show you the average cubic feet per minute (CFM) production per horsepower for rotary screw air compressors at different pressure levels. Note that the CFM production per horsepower for rotary screw compressors can vary depending on the compressor's design, efficiency, and the specific manufacturer. Actual values for your air compressor may differ; it is crucial to refer to the compressor manufacturer's specifications to obtain accurate CFM values for a specific compressor model.

Estimated CFM Production Per HP for Rotary Screw Air Compressors
Horsepower (HP)	CFM at 100 PSI	CFM at 120 PSI	CFM at 150 PSI
5	17	14	13
10	34	28	26
15	51	42	39
20	68	56	52
25	85	70	65
30	102	84	78
40	136	112	104
50	170	140	130
60	204	168	156
75	255	210	195
100	340	280	260

Here is an approximate chart for average CFM production per horsepower for reciprocating air compressors. To obtain accurate CFM values for a specific reciprocating air compressor model, always refer to the compressor manufacturer's specifications.

Estimated CFM Production Per HP for Reciprocating Air Compressors
Horsepower (HP)	CFM at 100 PSI	CFM at 120 PSI	CFM at 150 PSI
5	20	17.5	15
7.5	30	26.25	22.5
10	40	35	30
15	60	52.5	45
20	80	70	60
25	100	87.5	75
30	120	105	90
40	160	140	120
50	200	175	150

Need Help Calculating CFM Output or Demand for Your Compressed Air System?

Contact Fluid Air Dynamics at (866)-432-6379 and our air expert will answer all your questions about CFM calculation.