Litres, air flow (L/min) and bar: how to read a compressor's data sheet
Quick answer: on a compressor's data sheet, the litres indicate the size of the tank (a reserve of air), the air flow in L/min indicates how much air the compressor produces and determines which tools you can run, and the bar is the maximum pressure it can work at. To choose well, look first at the air flow, then at the bar and lastly at the litres.
What each figure on the data sheet means
- Litres (air tank capacity): volume of the tank where the compressed air is stored. It is a reserve: the larger it is, the longer you can work before the motor has to start again, and the fewer start-ups it will make.
- Air flow (L/min): amount of air the compressor delivers per minute. This is the figure you should compare with the consumption of your pneumatic tool.
- Maximum pressure (bar): the highest pressure to which the compressor fills the tank. Your tool needs a specific working pressure; the compressor must reach it with some margin.
- Power (HP): motor power. It gives an idea of the capacity to produce air, but it does not replace air flow.
The typical mistake: “more litres = more powerful”
A large tank does not produce more air: it only stores it. Two compressors with the same motor and the same pump head deliver the same air flow even if one has 24 litres and the other 100. The difference is autonomy: the 100-litre one starts less often and works more comfortably, but a tool that consumes more air than the compressor produces will end up emptying the tank, however many litres it has.
The opposite also happens: a compressor with few litres but good air flow can run tools used intermittently, such as a nailer, but it will start up more frequently. As an example, the data sheet of the 6-litre, 180 L/min portable compressor gives that air flow with just a 6-litre tank.
Delivered air flow and displacement: a nuance to bear in mind
A compressor's air flow can be expressed in two ways: as air intake or displacement (what the pump head moves) or as effective air flow at the outlet, sometimes called FAD (“free air delivery”), which is usually lower because it deducts losses. In addition, the air flow drops as the working pressure rises. The data sheets of our compressors give the air flow in L/min, and on some models the pressure at which it is measured (for example, 8 bar), but they do not always specify the criterion used to measure it. For this reason, when comparing models, it is advisable to always compare the same figure and leave some margin.
The relationship between HP, air flow and duty cycle
For the same type of compressor, more power usually translates into more air flow, but the relationship is not exact: it depends on the pump head, the number of cylinders and the revolutions. That is why two units of the same power can give different air flows. The duty cycle (how long it can run continuously without resting) also matters: an occasional-use compressor is not designed to supply a continuous-consumption tool every day. For intensive use, belt-driven, continuous-duty or three-phase models are designed for it; if you would like to go into more detail, there is the guide to belt-driven and three-phase compressors.
How to estimate the compressor you need
An indicative calculation in four steps:
- Note the air consumption (L/min) and the working pressure (bar) of each tool you are going to use; they are given on its data sheet.
- Add up only the tools that will run at the same time.
- Add a safety margin (as a guide, 25-30 %) for losses in the hose and couplings and so that the compressor is not always working at its limit.
- Choose a compressor whose air flow exceeds that total and whose maximum pressure is clearly higher than the one the tool requires.
If the tool is used intermittently (nailing, inflating, blowing), you can cut it finer; if it consumes continuously (painting, sanding, spraying), it is advisable to have plenty of air flow to spare. For details by tool, see the guide to pneumatic tools and air flow.
Real examples of Cevik compressor data sheets
Data taken from the product data sheets; see how litres, power and air flow do not always go hand in hand.
| Model | Tank (L) | Power | Max. pressure (bar) | Air flow (L/min) |
|---|---|---|---|---|
| Portable oil-free 6 L | 6 | 1.5 HP | 8 | 180 |
| Continuous-duty 4 HP | 20 | 4 HP | 10 | 590 |
| Oil-free 50 L, 9 bar | 50 | 2.5 HP | 9 | 246 |
| Quiet 50 L, twin head | 50 | 2.8 HP | 8 | 300 |
| Belt-driven 100 L | 100 | 3 HP | 10 | 320 |
| Three-phase 500 L, 10 HP | 500 | 10 HP | 11 | 1,074 |
Note that the 20-litre model delivers more air flow than the 100-litre one, and that the 6-litre model comes close to the 50-litre 2.5 HP one: litres do not determine air flow. If you would like to see the whole range, see the guide to Cevik compressors: range and how to choose.
Quick guide: what to look at according to use
- Inflating tyres, blowing and light DIY: a small unit is enough; prioritise one that is easy to carry. See the portable compressors.
- Indoors, small workshop or low-noise carpentry: look for a balance between air flow and noise level. See the quiet compressors and the guide to quiet oil-free compressors.
- Spray-gun painting or textured wall finishes: consumption is continuous, so prioritise air flow. Here is the guide to compressors for painting, textured wall finishes and airbrushing.
- Daily professional use and several tools: look at the belt-driven compressors and the guide to belt-driven and three-phase models.
- Outdoors or building sites without electricity: see the petrol compressors and their guide.
For an overall view, start with the guide to choosing an air compressor, and for looking after your equipment, the guide to compressor maintenance. You can also see the whole Cevik range or the compressors and machinery section.
Frequently asked questions
What is more important, litres or air flow?
To know whether a compressor can run your tool, the air flow in L/min is more important. The litres only indicate how much air is stored and, therefore, how often the motor starts; they do not increase the amount of air that is produced.
How many bar do I need?
As many as are needed for your tool to reach its working pressure, with some margin. Many pneumatic tools work at around 6-8 bar, but the pressure drops because of losses in the hose and couplings, so it is advisable to have a compressor with a maximum pressure higher than the one the tool requires. Always check the tool's data sheet.
What does L/min mean on a compressor?
It is the litres of air the compressor delivers each minute. It is compared with the tool's consumption in L/min: if the compressor produces less air than the tool consumes, the pressure drops while you work.
How many litres do I need for inflating tyres?
Very few. Inflating tyres requires little pressure and is intermittent use, so a small or portable compressor is enough; more litres would only give more autonomy between start-ups. Check your vehicle's recommended pressure.
Why does my compressor not keep up its pressure with the tool?
The most common cause is that the tool consumes more air (L/min) than the compressor produces, so the tank empties faster than it fills. Leaks, a hose that is too long or too thin, or a badly adjusted regulator can also play a part. Compare the tool's consumption with the compressor's air flow and check the installation.
