Compressor Units Explained: CFM, LPM, m³/min, Bar, kg/cm² and PSI
A compressor quote has two numbers that matter: how much air (CFM, LPM or m³/min) and at what pressure (bar, kg/cm² or PSI). Indian catalogues mix all six, sometimes in one table: Karan Micro Industries' 5 HP KC 744 is listed at 20.00 CFM, or 566 LPM, at 80 PSIG, or 5.52 bar. 1 CFM is 28.32 litres per minute, and 1 bar is 1.02 kg/cm² or 14.5 PSI.
Air flow: CFM, LPM, m³/min and m³/h
Flow says how much air the compressor delivers each minute. Screw makers use CFM (cubic feet per minute) and m³/min, many Indian piston catalogues use LPM (litres per minute), and dryers are often rated in m³/h. They are the same quantity in different units: 1 CFM is 28.32 LPM.
| Unit | CFM | LPM | m³/min | m³/h |
|---|---|---|---|---|
| 1 CFM | 1 | 28.32 | 0.0283 | 1.699 |
| 1 LPM | 0.0353 | 1 | 0.001 | 0.06 |
| 1 m³/min | 35.31 | 1,000 | 1 | 60 |
| 1 m³/h | 0.589 | 16.67 | 0.0167 | 1 |
| Karan machine | As listed | Converted |
|---|---|---|
| KC 744 | 20.00 CFM at 80 PSIG | 566 LPM at 5.52 bar |
| KCS-05 | 19 CFM at 8 bar | 0.54 m³/min at 116 PSIG |
| KAD 010 | 100 CFM | 170 m³/h |
Pressure: bar, kg/cm² and PSI
Bar is the unit of the 2026 Karan catalogue and most screw makers; kg/cm² is still printed on many Indian gauges and piston tables; PSI comes from US specifications. At compressor pressures, bar and kg/cm² differ by only about 2%, so 7 kg/cm² is about 6.9 bar.
Quotes give gauge pressure (barg, PSIG): the pressure above the atmosphere, which is what a gauge reads. Absolute pressure (bara) adds about 1 bar. When a spec does not say, it is almost always gauge.
| Where you see it | bar | kg/cm² | PSI |
|---|---|---|---|
| Typical factory air | 7 | 7.14 | 102 |
| Karan KCS screw, standard rating | 8 | 8.16 | 116 |
| Karan KC 7S1 single-stage piston | 8.62 | 8.79 | 125 |
| Karan two-stage piston (12.3 kg/cm²) | 12.06 | 12.3 | 175 |
| Fibre laser cutting, Karan KCS-20 / KCS-30 | 16 | 16.32 | 232 |
| Karan KBC booster, 35 bar dryers | 35 | 35.69 | 508 |
| Karan KC 715T2 high-pressure (PET) | 70 | 71.38 | 1,015 |
HP does not tell you how much air you get
The same motor gives less air at higher pressure. Karan's 20 HP KCS-20 delivers 80 CFM at 8 bar, 65 at 10 bar, 59 at 12 bar and 50 at 16 bar. As a rule of thumb, Karan's KCS screws give about 3.8–4.8 CFM per HP at 8 bar, more on the bigger models. Size on CFM at your working pressure; the size calculator does the sum.
Searches such as “1 CFM to bar” or “1 CFM to kW” have no answer: CFM is flow, bar is pressure and kW is power. A compressor needs all three, which is why a quote should state the CFM at a stated pressure from a stated motor, as in “KCS-20: 80 CFM at 8 bar, 15 kW”.
CFM, FAD and piston displacement
Free air delivery (FAD) is the air a compressor actually delivers, stated at inlet conditions. Piston displacement (PD) is the volume the cylinders sweep, and is always higher, because some air is lost to clearance and leakage. Karan's two-stage piston table lists both: the KC 742 sweeps 17.32 CFM and delivers 13.83 CFM FAD. Its single-stage piston table states displacement. Compare quotations on FAD at the same pressure.
SCFM and Nm³/min restate flow at a fixed reference temperature and pressure, so they differ slightly from FAD stated at inlet conditions. Compare like with like.
Check your compressor's FAD on site: the BEE pump-up test
India's Bureau of Energy Efficiency gives a shop-floor test in its guide to compressor performance. It is less accurate than the nozzle method, but needs only a gauge and a stopwatch:
- Isolate the compressor and its receiver from the plant air system and close the receiver outlet.
- Drain the receiver and pipeline, then close the drain.
- Start the compressor and time how long it takes to fill from a low pressure P1 to its normal pressure P2.
- Work out the delivery with the formula below.
In the Bureau of Energy Efficiency's worked example, an 8.322 m³ system filled from 0.5 to 7.03 kg/cm² in 4.021 minutes, which works out at 13.17 m³/min against a rated 14.75 m³/min: the compressor was delivering about 11% less than its rating.
यही गाइड हिंदी में: Compressor CFM क्या होता है?
Frequently Asked Questions
Straight answers from our engineers, last reviewed November 2026. Click a question to see the answer.
Ask our engineers011 CFM is equal to how many LPM?
1 CFM (cubic foot per minute) is 28.32 litres per minute, 0.0283 m³/min or 1.70 m³/h. Karan Micro Industries' 5 HP KC 744 piston compressor, for example, is listed at 20.00 CFM, which is 566 LPM. To go back, divide LPM by 28.32.
02How do I convert kg/cm² to bar?
Multiply kg/cm² by 0.981 to get bar, or bar by 1.02 to get kg/cm²; 1 kg/cm² is also 14.22 PSI. The two differ by only about 2%, so Karan Micro Industries' two-stage piston rating of 12.3 kg/cm² is 12.06 bar, or 175 PSIG.
03Compressor CFM kya hota hai?
CFM (cubic feet per minute) is the volume of air a compressor delivers each minute; 1 CFM is about 28.3 litres per minute. It falls as pressure rises: Karan Micro Industries' 20 HP KCS-20 gives 80 CFM at 8 bar and 50 CFM at 16 bar. The same guide in Hindi: CFM क्या होता है.
04What is the FAD of a compressor, and how is it calculated?
Free air delivery (FAD) is the air a compressor actually delivers; piston displacement is always higher. Karan Micro Industries' KC 742 sweeps 17.32 CFM and delivers 13.83 CFM FAD. On site, the Bureau of Energy Efficiency's pump-up test gives FAD as (P2 − P1) ÷ P0 × V ÷ T from the receiver volume and filling time.
05Can CFM be converted to bar or kW?
No: CFM measures air flow, bar measures pressure and kW measures power, so none converts into another. A compressor is described by all three: Karan Micro Industries' 20 HP KCS-20 has a 15 kW motor and delivers 80 CFM at 8 bar. Size on CFM at your working pressure.