In a hurry? These are the things you need to know:
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1 bar = 14.5 PSI = 100 kPa. These are the three conversions most frequently needed when working with compressed air equipment and hose specifications.
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Flow rate is measured in CFM (cubic feet per minute) or l/min (litres per minute). Flow rate, not just pressure, determines whether a compressor and hose can adequately supply the tools being run.
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Free Air Delivery (FAD) is the volume of air a compressor produces at atmospheric conditions and is the most useful way to compare compressor output. It is different from displacement, which is the theoretical volume swept by the pistons.
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Always cross-reference the pressure and flow requirements of your tools against the compressor's rated output. Running tools below their required pressure causes poor performance; running them above it risks tool damage.
How compressed air pressure is measured
A compressor bought from a UK supplier may be rated in bar. The air tool it feeds may have a minimum PSI printed on the casing. The hose between them is listed in both. You need to know how they relate.
Pounds per square inch (PSI)
PSI measures the force that compressed air exerts on a given area. When air is compressed, it resists expanding back to atmospheric pressure; that resistance is what PSI quantifies. Most air tools specify a minimum operating pressure in PSI: impact wrenches typically need 90 PSI; spray guns vary between 40 and 90 PSI depending on the application.
Bar
Bar is the unit you’ll see on the majority of UK and European compressor data sheets. One bar is approximately equal to atmospheric pressure at sea level, so a system running at 7 bar is operating at seven times atmospheric pressure. Workshop compressors typically run between 6 and 10 bar. Industrial and hydraulic systems run considerably higher.
Kilopascal (kPa) and megapascal (MPa)
The pascal (Pa) is the SI base unit for pressure, but it’s too small for practical use. 1 bar equals 100,000 Pa, so the numbers become unworkable. Kilopascals and megapascals are the scaled versions you’ll actually see:
In compressed air work, kPa appears on some continental European equipment and in scientific contexts. MPa is more common in hydraulics, where working pressures are high enough that bar becomes cumbersome, a system running at 350 bar is more usefully described as 35 MPa.
Key conversions:
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1 bar = 100 kPa
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1 PSI = 6.895 kPa
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1 MPa = 10 bar = 145 PSI
Bar to PSI conversion table
1 bar = 14.504 PSI. The table below covers the range most commonly encountered in workshop and light industrial compressed air work.
|
Bar
|
PSI
|
|
1
|
14.5
|
|
2
|
29.0
|
|
3
|
43.5
|
|
4
|
58.0
|
|
5
|
72.5
|
|
6
|
87.0
|
|
7
|
101.5
|
|
8
|
116.0
|
|
9
|
130.5
|
|
10
|
145.0
|
|
12
|
174.0
|
|
14
|
203.1
|
|
16
|
232.1
|
|
20
|
290.1
|
For hydraulic systems at higher pressures, multiply the bar figure by 14.504.
PSI to bar conversion table
1 PSI = 0.0689 bar. The table below covers the PSI values most commonly encountered across air tools, workshop compressors, and tyre inflation equipment.
|
PSI
|
Bar
|
|
30
|
2.07
|
|
32
|
2.21
|
|
35
|
2.41
|
|
36
|
2.48
|
|
38
|
2.62
|
|
40
|
2.76
|
|
60
|
4.14
|
|
80
|
5.52
|
|
90
|
6.21
|
|
100
|
6.89
|
|
120
|
8.27
|
|
150
|
10.34
|
|
200
|
13.79
|
|
300
|
20.68
|
The 30–40 PSI range also covers standard car and van tyre pressures, which are typically specified in bar on European vehicles and PSI on older UK and US-spec equipment.
Gauge pressure vs absolute pressure
Worth knowing before you read a data sheet for the first time.
Gauge pressure (bar g or PSI g) is measured relative to atmospheric pressure. It reads zero at sea level with no additional pressure applied. This is the figure shown on a standard workshop pressure gauge, and the figure quoted for compressor output, tool requirements, and hose ratings.
Absolute pressure (bar a or PSI a) is measured relative to a perfect vacuum. It equals gauge pressure plus atmospheric pressure, at sea level, 0 bar g = 1.01325 bar a.
For compressed air work, you’re almost always dealing with gauge pressure. Absolute pressure appears in thermodynamic calculations and some scientific applications. If a spec sheet doesn’t state which it is, it’s almost certainly gauge.
Flow rate: CFM and l/min
Pressure tells you how hard the air is pushing. Flow rate tells you how much of it there is. Both matter. A compressor that delivers high pressure but low flow will starve a tool that needs volume, regardless of what the pressure gauge reads.
Flow rate is measured in:
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Litres per minute (l/min) - the standard metric unit for compressed air
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Cubic feet per minute (CFM) - the imperial equivalent, common on US-origin equipment
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Cubic metres per hour (m³/h) - used for larger industrial installations
1 CFM = 28.3 l/min.
Most air tools have a minimum CFM requirement. An impact wrench running at 90 PSI typically needs 4–5 CFM for a 1/2" drive. A conventional gravity-feed spray gun used for general painting needs 4–9 CFM at around 40–50 PSI. An HVLP gun, which trades higher pressure for higher volume to reduce overspray, typically needs 9–13 CFM at a lower inlet pressure of 25–30 PSI at the gun. If you’re running multiple tools from one compressor, add the CFM figures together and check the total against the compressor’s rated output before you commit to a setup. The bore of your compressed air hose also affects how much flow reaches the tool, a long run of narrow-bore hose will drop pressure and restrict volume even if the compressor is adequately sized.
Free Air Delivery (FAD)
FAD is the volume of air a compressor actually delivers at atmospheric conditions. Not what it draws in, not what the piston theoretically displaces per stroke. It’s the figure that tells you what the machine can genuinely sustain under load.
Displacement (sometimes labelled ‘swept volume’) describes the theoretical capacity of the pump mechanism. FAD is always lower. Bearing friction, valve leakage, and heat losses all reduce what reaches the outlet. A compressor rated at 200 l/min displacement will typically deliver 130–170 l/min as measured FAD. The exact figure depends on compression ratio, valve condition, and operating temperature, a well-maintained reciprocating piston compressor generally runs at 70–85% volumetric efficiency; a worn or budget unit can fall to 55–70%.
When comparing compressors, use FAD. When reading spec sheets, check which figure is actually quoted, the two are frequently conflated in product listings, and the gap matters if you’re sizing a system for a specific tool load.
FAD also drops at higher altitude and in hot environments. For most UK workshop use that’s not a concern. For anyone running plant in non-standard conditions, it’s worth factoring in. Once you’ve confirmed your compressor’s FAD meets the tool demand, an air preparation unit (filter, regulator, lubricator) lets you set the precise working pressure downstream and protect tools from contaminated air.
Frequently asked questions
How do I convert bar to PSI?
Multiply the bar figure by 14.504. So 6 bar = 87 PSI, 10 bar = 145 PSI. For a quick approximation, multiplying by 14.5 is close enough for most practical purposes. Use our bar to PSI converter above.
How do I convert PSI to bar?
Multiply the PSI figure by 0.0689. So 90 PSI = 6.21 bar, 120 PSI = 8.27 bar. Use our bar to PSI converter above.
What is the difference between bar and PSI?
Both measure pressure. Bar is metric and is the standard unit on UK and European compressed air equipment. PSI (pounds per square inch) is imperial and remains common on US-origin tools and some older UK equipment. 1 bar = 14.504 PSI.
What is the difference between bar and MPa?
MPa (megapascal) is a larger unit. 1 MPa = 10 bar. MPa is used where pressures are high enough that bar becomes unwieldy, hydraulic systems in particular. For workshop compressed air work, bar is the more practical unit.
What is gauge pressure?
Gauge pressure is pressure measured relative to atmospheric pressure. It reads zero at sea level with no additional pressure applied. This is the figure shown on a workshop pressure gauge and the figure quoted in compressor and tool specifications. It’s written as bar g or PSI g, as opposed to absolute pressure (bar a or PSI a), which is measured from a perfect vacuum.
What is Free Air Delivery (FAD)?
FAD is the volume of air a compressor delivers at atmospheric conditions, the actual usable output, as opposed to the theoretical displacement of the pump. It’s the most accurate figure for comparing compressors.
Measure air pressure accurately with The Hosemaster
Now you do indeed know your PSI from your kPa – and everything else in between – it’s time to make sure you’re kitted out with the best quality compressed air measurement tools. Investing in top-quality products will ensure that the readings are accurate and in some cases, will also help reduce energy inefficiencies and loss of energy.
Here at The Hosemaster, we stock a wide range of pressure gauges designed to suit a whole host of different applications and industries.
Hopefully you’re able to find everything you need on the website but if you’ve got any questions, please don’t hesitate to get in touch for some further guidance – our 5-star-rated customer service will have you covered.
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