PSI, CFM, and HP: understanding air compressor measurements

August 1, 2019

6 minutes

If you’ve been in the air compressor industry for any length of time, you’re likely familiar with the acronyms PSI (pressure), CFM (flow), and HP (power). These principles are essential in determining which size air compressor will fit your application – in fact, PSI, CFM, and HP are some of the most important elements to consider when selecting the best air compressor for your needs.

PSI vs. CFM vs. HP

PSI, CFM, and HP are the three main ratings that showcase exactly what a compressor can do.

PSI, or pounds per square inch, measures the amount of pressure placed on a square inch of space. In compressor terms, PSI is the amount of force that an air compressor can deliver.
CFM is cubic feet per minute, which indicates the compressor’s flow rate - or the amount of air that a compressor can produce at a given pressure level. Typically, compressors that have higher CFM ratings are able to provide more air, which makes them ideal for larger applications.
HP is horsepower, or the amount of work that a motor can perform. HP is not as important as pressure and flow in determining if your compressor will work for you, as newer & more efficient compressors can do more with less HP! Doing more with less HP will save you energy, as well as will give you larger returns throughout the life of the compressor.

What is the difference between PSI and CFM?

While PSI and CFM are both critical air compressor measurements, they describe two completely different characteristics of a compressor's performance, and understanding the difference between the two is key to selecting the right compressor for your needs.

PSI tells you how hard the air is being pushed. It's a measure of pressure, specifically, the amount of force being delivered per square inch. A higher PSI means the compressor is capable of delivering air at greater pressure, which is important for applications that require a strong, forceful output.

CFM, on the other hand, tells you how much air is being delivered. It's a measure of flow, the volume of air a compressor can produce per minute at a given pressure level. A higher CFM means the compressor can supply a greater volume of air, which is essential for applications that require a continuous or high-volume air supply.

Here's a simple way to think about the difference: PSI is the strength of the air, and CFM is the quantity of the air. Both matter, but in different ways depending on your application. A tool or process that requires high pressure but low volume will prioritize PSI, while one that needs a steady, large supply of air will prioritize CFM. Many applications require a careful balance of both.

It's also important to note that PSI and CFM have an inverse relationship in a compressor system. As pressure (PSI) increases, the available flow (CFM) typically decreases, and vice versa. This is why compressor ratings are always specified together, for example, "10 CFM at 100 PSI", because quoting one without the other doesn't give you the full picture of what the compressor can actually deliver.

Things to Consider

No matter your application, it’s critical to understand the PSI, CFM, and HP that your application requires. This will ensure that your application receives sufficient air flow (CFM) at the correct pressure (PSI) – and that the air compressor is providing your pressure and flow as efficiently as possible.

Atlas Copco is always available to assist you in your compressor selection. Our experts will guide you in your selection based on your application’s specific needs and rating requirements. Reach out to us today!

Frequently asked questions

What is the difference between PSI and CFM in an air compressor?

PSI (pounds per square inch) measures the pressure an air compressor delivers, essentially, how forcefully the air is pushed out. CFM (cubic feet per minute) measures the flow rate, how much air the compressor produces per minute at a given pressure. In short, PSI measures the strength of the air and CFM measures the volume. Both are essential ratings that together define a compressor's overall performance.

Is PSI or CFM more important when choosing an air compressor?

Neither is universally more important; it depends on your application. If your tools or processes require high-pressure output, PSI is the priority. If your application demands a continuous, high-volume air supply, CFM takes precedence. In most cases, you'll need to match both the PSI and CFM requirements of your application to ensure optimal performance. That's why it's always best to evaluate both together rather than focusing on one in isolation.

Why do PSI and CFM have an inverse relationship?

PSI and CFM are inversely related because compressing more air into a smaller space (higher PSI) naturally reduces the volume of air available (lower CFM). Think of it like a water hose, if you increase the pressure, the flow rate decreases. This is why air compressor ratings always pair PSI with CFM, and why it's important to understand both when sizing a compressor for your specific application.

Can a compressor have high PSI and high CFM at the same time?

To a degree, yes, larger, more powerful compressors are designed to deliver both higher PSI and higher CFM simultaneously. However, within any given compressor, increasing the output pressure will reduce the available flow rate. If your application requires both high pressure and high volume, you'll need a compressor that is specifically sized to meet both demands, which is something our compressed air experts can help you determine.

How do I know if my application needs more PSI or more CFM?

The best starting point is to check the requirements of the tools or equipment you'll be running. Most pneumatic tools, for example, list both their required PSI and CFM on their specification sheets. If you're running multiple tools or a continuous process, you'll need to add up the total CFM demand and ensure your compressor can meet it at the correct pressure. When in doubt, reach out to an Atlas Copco compressed air expert, we're always happy to help you find the right fit.

How are CFM and PSI inversely proportional in a compressed air system?

In any compressed air system, CFM and PSI are inversely proportional, meaning as pressure increases, the volume of air available decreases, and vice versa. This happens because compressing air into a higher PSI forces the air molecules closer together, reducing the air volume that can flow through the system at any given moment. Understanding this relationship is critical for efficient operation, because pushing a compressor beyond its optimal pressure level to chase higher PSI can result in lower CFM output, poor performance of air powered tools, and unnecessary energy usage. Always size your compressor so that it can comfortably deliver the required CFM at your needed PSI without being pushed to its limits.

What happens if my compressor doesn't have enough CFM for my application?

If your air compressor cannot supply sufficient air flow (CFM) at the correct pressure (PSI), your pneumatic tools and air tools will not operate correctly. Common signs of insufficient CFM include tools losing power mid-operation, pressure drops during use, and the compressor running continuously without meeting demand, which shortens its service life and drives up energy costs. In industrial compressor applications, insufficient CFM can also lead to artificial demand, where the system compensates in ways that cause equipment damage and compromise operational standards. This is why matching your required CFM to your compressor's CFM rating is just as important as matching PSI.

Does air density affect CFM and PSI measurements?

Yes, air density plays a significant role in both CFM and PSI measurements. Air density is affected by factors like altitude, temperature, and humidity, all of which influence how much air a compressor can actually deliver. At higher altitudes, air is less dense, which means the compressor moves a greater volume of air (higher CFM) to deliver the same mass flow, while also working harder to achieve the same PSI levels. At a constant temperature and sea-level conditions, air behaves more predictably, which is why standard reference conditions are used when publishing compressor ratings. For facilities in locations where air density varies significantly, accounting for these factors is essential to ensure accurate measurements and optimal performance.

What role does tank size play in CFM and PSI performance?

Tank size directly affects how your compressor manages the relationship between CFM and PSI in real-world use. A larger tank stores more compressed air, which means the compressor can handle short bursts of high demand, like running multiple air tools simultaneously, without an immediate pressure drop. However, tank size does not increase the compressor's maximum CFM output; it simply provides a buffer. If your application has a high duty cycle or requires a continuous, high volume of air, the compressor's CFM rating is what ultimately determines whether it can keep up with demand. A large tank paired with an undersized compressor will still run out of air if the required CFM consistently exceeds what the compressor can produce.

Is there a conversion formula to help size the right air compressor for my needs?

While there isn't a single universal conversion formula that covers every scenario, a commonly used starting point for sizing is to calculate your total required CFM across all air powered tools and processes, then add a buffer, typically 25 to 30%, to account for leaks, peaks in demand, and future growth. From there, you match that CFM requirement to a compressor that delivers it at your required PSI. For more complex applications involving multiple tools, varying duty cycles, or fluctuating pressure levels, the calculation becomes more involved and factors like initial pressure, final pressure, final volume, and initial volume may all come into play. In these cases, working with a compressed air expert ensures you select the right compressor with confidence, avoiding both poor performance and unnecessary overcapacity.