What the converter is for
The Compressed Air Unit Converter turns a flow or pressure figure written in one unit into its equivalent in another, so a litres-per-minute figure from a UK compressor listing can be checked against a cubic-feet-per-minute figure on an older tool manual, or a bar rating checked against a psi spec sheet. It does the arithmetic. It does not tell you whether the figure you are converting was free air delivery (FAD) (the volume a compressor actually delivers per minute, measured at atmospheric pressure and temperature, at the stated working pressure) or displacement (the volume swept by the piston or screw element before efficiency losses, always the higher of the two figures), and it does not know whether a pressure figure was gauge or absolute. Getting a useful answer out depends on knowing what went in.
Why compressed air figures come in so many units
Flow: litres a minute, litres a second, cubic metres a minute, cubic feet a minute
Most UK compressor listings state flow in litres a minute (l/min), the volume of air passing a point in one minute. Larger industrial machines are sometimes rated in cubic metres a minute (m³/min), a thousand times bigger: 1 m³/min equals 1,000 l/min. Litres a second (l/s) turns up on some spray equipment and dryer specifications; multiply by 60 to reach litres a minute. Cubic feet a minute (cfm) is the imperial unit still printed on a lot of tool manuals and older compressor plates, and it converts at roughly 1 cfm to 28.3 l/min, so a figure of 100 l/min sits close to 3.5 cfm.
Pressure: bar, bar gauge and psi, and the gap between them
A compressor's gauge reads in bar gauge (bar g), the pressure above the surrounding atmosphere, because that is the pressure doing the work at the tool. Absolute pressure (bar a) adds atmospheric pressure back in, roughly 1 bar, and shows up in some technical calculations. Pounds per square inch (psi) is the imperial pressure unit still printed on a lot of tool specifications, and roughly 14.5 psi equal 1 bar. A spec sheet that just says "bar" without stating gauge or absolute is almost always giving gauge pressure, but where a calculation depends on the difference, it is worth checking against the source.
Common mistakes when converting compressed air figures
- Converting the unit without checking the basis. A flow figure in cfm converts to l/min correctly by arithmetic, but if the original number was displacement, the converted figure is still displacement, just in different units. See Displacement and Free Air Delivery for how the two are worked out from a maker's published figures.
- Mixing gauge and absolute pressure in the same calculation. A working pressure of 6.2 bar gauge is not the same number as 6.2 bar absolute, and a formula built for one that gets the other returns a wrong result even though every unit conversion inside it was correct.
- Rounding early and carrying the error forward. Converting a figure, rounding it to a whole number, then feeding that rounded figure into a further sizing calculation compounds the rounding error. Keep the decimal places until the last step.
- Assuming a cfm figure from one source was measured the same way as a cfm figure from another. Cfm is cfm arithmetically, but the pressure a figure was measured at is not always stated the same way across different manuals, so two cfm numbers can describe different working conditions even after conversion.
Worked examples
Converting a displacement figure from a listing
Suppose a compressor listing states a displacement of 356 l/min. Converting to cubic feet a minute: 356 ÷ 28.3 ≈ 12.6 cfm. That figure is the swept volume. The free air delivery at the compressor's stated working pressure will be lower, and Displacement and Free Air Delivery sets out how much lower to expect and why.
Converting a tool's pressure requirement
Suppose a spray gun's data sheet asks for 90 psi at the gun. Converting to bar: 90 ÷ 14.5 ≈ 6.2 bar. That is the pressure needed at the gun itself, so any drop along the hose and fittings between compressor and gun still needs adding back before the regulator is set.
Where unit conversion fits into sizing a compressor
Converting a figure from one unit to another is a step inside a bigger sizing calculation. Working out how much free air a workshop needs means adding up tool demand at the pressure each tool runs at, allowing for how many run together, and comparing the total against a compressor's free air delivery. The full arithmetic for that comparison is set out in How Much Air Your Tools Need; the converter is most useful once those figures are gathered and need putting on a common footing before the comparison is made.
Limits worth knowing
A unit converter applies a fixed arithmetic factor; it cannot tell whether the figure entered was free air delivery or displacement, gauge or absolute pressure, or measured at the working pressure the label claims. A wrong figure converted correctly is still a wrong figure, so the number that goes in is worth checking against its source before it goes anywhere near a sizing decision. Where a term here needs a fuller definition, the Compressed Air Glossary sets out how each one is measured and what it is used for.
Where to go next
For the sizing arithmetic that uses converted figures, see How Much Air Your Tools Need. For the difference between the displacement and free air delivery figures printed on a listing, see Displacement and Free Air Delivery. Use the Compressed Air Unit Converter itself once the figures needing conversion are to hand.