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Exploring the air compressor

If you’re one of those people whose confused by the jargon or just daunted by the sheer range of compressors on the market, don’t worry, you’re not alone. Far from it in fact.

With this in mind, we’ve decided to help you out. This guide will go through the many important features of a compressor as well as the things you need to consider when buying one. The end aim is to help you get the air supply you need, first time.

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Understanding Compressor Jargon

Before we get started, here is a brief look at the jargon and terminology frequently used when talking compressors.

  • Air Flow (output) – The volume of compressed air pushed out by the compressor – also called Free Air Delivery (FAD). Measured in Cubic feet Per Minute (CFM) or Litres per minute (L/min).
  • Pressure – This is the force of the compressed air. It is expressed in Pounds per Square Inch (Psi) or Bar.
  • Displacement (volume) –  This is the theoretical capacity of a compressor. Usually worked out by the size of the pumps cylinders and the speed that it operates. Also measured in CFM, this should not be used as an indicator of the power of the compressor.

It’s important to remember that you should not buy a compressor based on its displacement. This figure only represents what it works at if it were 100% efficient. Whereas FAD is what will be discharged.

As a general rule CFM (FAD) is a third less than CFM (displacement). So if a compressor has a displacement of 14.5CFM, the FAD will be around 10CFM.

Air Flow Required

Before you do anything, you must find out how much air you need to power your tools. If the compressor you buy doesn’t have the capacities to match your tools, you will find yourself constantly waiting for the tank to fill up. How do you do this?

  • Find the required CFM of the tool you will be using. This is usually found in manual or if you’re buying new it should be found in the specification.
  • If you’re using more than one tool at the same time, you need to add up the required CFM for each tool.
  • Once you’ve added up the CFM of the tools you will be using, it’s good practice to add in a safety buffer. This is so the compressor won’t be working at its maximum capacity all the time. So multiply the final number by 1.2 and this will give you your airflow requirement.

For example, you’re going to be using an air nailer (3.5CFM) and an impact wrench (4CFM) at the same time.

7.5CFM X 1.2 = A compressor with at least 8.7 CFM (FAD).


You will find that all tools air flow is measured at a certain pressure. So you will need a compressor to match that number. If the pressure is too low, tools won’t work, too high and it can cause unnecessary wear.

If you are doing DIY work, most tools will have a maximum operating pressure of 90 PSI (6 Bar). Many compressors will have a working pressure of at least 110 PSI (7.5 Bar) so it shouldn’t be hard to find one to cater for your needs. 

Of course, larger tools being used for prolonged periods will need higher operating pressures. This is where the larger industrial compressors need to be considered, that can work over 150 PSI (10 Bar).

The Airflow – Air Pressure Equation

As we have said above, all compressors have certain CFM and PSI ratings which you must align with your chosen tools. What is possibly not mentioned enough by people is that these are the compressors maximum ratings. So if you see that a compressor has a FAD of 8 CFM, this will only be at a very low pressure (approx 14.5 psi/1 Bar). In reality, when using a tool at 90 psi, (6 Bar) this compressor may only have a CFM (FAD) of 6.

Here is an example of this from one of our Swan Compressors:

SWAN DRS-215-50 AIR COMPRESSOR 230V – image

CFM (FAD) delivered @ 1 Bar = 5.15

CFM (FAD) delivered @ 7 Bar = 4.15

Now, this may not seem like a big drop, but if you have worked out your tools need a CFM of at least 5 before buying, you will find your tools won’t perform the way you need them to. With that in mind, we would recommend that you always ask for this information before purchasing.

Receiver Size (Tank size)

Very much tied in with how long you will be using your air tools. If you are using tools continuously, then you will need a larger receiver as it will be depleted less often. For intermittent work, a smaller one should suffice. We would always recommend that you buy as big a receiver as you can afford/fit into the workshop.

Here we take a look at two Swan compressors and what jobs they would be suited to doing;

Swan DRS-207-22 – 22 Litre compressor

Great for DIY, craft and some trade uses such as tyre inflation, nailing and stapling.

Swan DRS-207-22 - 22 Litre compressor

Swan SVP-203 3HP 106 litre compressor

Designed to run all day every day, these compressors are best suited to industrial and commercial applications.

Swan SVP-203 3HP 106 litre compressor

Space and Portability

Does your compressor need to be out on the road with you or can it just be kept in a corner of the workshop? Do you have a large area to store it or does it have to be under a workbench? These are questions that need to be answered but of course is also directly affected by the size of the receiver you need.

Oiled vs Oil-less (Oil-free)

A few years ago, this would have been a no-brainer but in recent times oil-less compressors are becoming more and more popular, especially for the home or DIY user.

In all compressors, the pistons need lubricating in order to work reliably for long periods. Oil types as the name suggest use oil to do this. In oil-less versions, the pistons are permanently lubricated with a material such as Teflon.

So what does this mean for you? This chart will show you the key differences between the two.

Oiled Oil-less
Maintenance The oil needs to be changed regularly to ensure the compressor works consistently. No oil means no maintenance. This is why these are often referred to as maintenance free compressors.
Usage As long as you keep the oil topped up, these compressors are much more durable. This is why oil versions are used in industrial applications. The Teflon can wear away over time, drying out the pistons meaning performance will suffer. For this reason these are mainly used for home or DIY use.
Noise Liquid lubrication means these compressors are much quieter than the oil-less alternatives. Generally speaking, these are much louder than oil compressors.
Contaminates There is a risk of contaminating your airline using oil compressors, which if you’re planning on spraying, could ruin your work. No contamination as there’s no oil. Great for spray finishers or clinical practices such as dentists.
Weight In most cases, oil compressors will weigh more than oil-less models that are the same size. These have fewer parts and therefore weigh less than oil compressors. This also means they tend to cost less.
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