Why a unit alone does not explain a number
Two compressor offers can both say 500 cfm and still describe different amounts of air. One may quote free air delivery (FAD) to ISO 1217 at the inlet, another a displacement figure or a flow at different reference conditions. Two pressure gauges can both read 7 and still disagree by a full bar if one shows gauge pressure and the other absolute. Most expensive misunderstandings in compressed air start with numbers that looked alike but were not.
A complete value carries five things: the quantity, its unit, its reference basis, the point where it applies and the operating state. “500 cfm FAD at 7 bar(g) discharge, inlet 1 bar(a) and 20 °C, at full load” can be compared. “500 cfm” cannot.
| Element | Example | What goes wrong if it is missing |
|---|---|---|
| Quantity | Free air delivery | Displacement is mistaken for delivered air |
| Unit | m³/min or cfm | Conversion errors of a factor of about 35 |
| Reference basis | Inlet conditions, normal or standard | Flows differ by several percent or more |
| Point of measurement | Compressor discharge, after dryer, at user | Pressure drop is mistaken for a compressor fault |
| Operating state | Full load, part load, unloaded | Power and flow are compared at different conditions |
Pressure terms and units
| Term | Symbol or unit | Meaning |
|---|---|---|
| Gauge pressure | bar(g), psig | Pressure above local atmospheric pressure; what most plant gauges show |
| Absolute pressure | bar(a), psia | Pressure above a perfect vacuum; gauge plus atmospheric |
| Atmospheric pressure | Patm | About 1.013 bar(a) at sea level, lower at altitude |
| Differential pressure | Δp, mbar or bar | Pressure difference across a filter, dryer, pipe or valve |
| Working pressure | bar(g) | The pressure the compressor is set to deliver in service |
| Maximum working pressure | bar(g) | The highest pressure the package is rated for |
Key conversions: 1 bar = 100 kPa = 14.5 psi. 7 bar(g) is 101.5 psig, 10 bar(g), 145 psig, 14 bar(g), 203.1 psig and 24 bar(g), 348.1 psig. All gas-law calculations need absolute pressure. Use the pressure unit and gauge-absolute conversion tool when in doubt.
Airflow terms and units
| Term | Unit | Meaning |
|---|---|---|
| Free air delivery (FAD) | m³/min, l/s, cfm | Compressed air delivered, expressed at the compressor’s inlet conditions; tested to ISO 1217 by makers |
| Displacement | m³/min, cfm | Swept volume of the compressing element; always higher than FAD, mainly quoted for piston compressors |
| Normal flow | Nm³/min, Nm³/h | Flow converted to 0 °C and 1.013 bar(a), dry |
| Standard flow | scfm, Sm³/h | Flow converted to a stated standard; the standard varies, so it must be named |
| Actual flow | acfm, m³/min at line conditions | Volume flow at the real pressure and temperature in the pipe |
| Mass flow | kg/s, kg/h | Mass of air per unit time; the same everywhere in a leak-free system |
Key conversions: 1 m³/min ≈ 35.3 cfm, 1 l/s ≈ 2.12 cfm, 100 cfm ≈ 2.83 m³/min. At 7 bar(g) actual flow is roughly one-eighth of FAD. The difference between FAD, normal and standard flow is explained on our page on airflow definitions, FAD and reference conditions.
Common traps with flow figures
- Comparing a piston compressor’s displacement with a screw compressor’s FAD
- Reading “scfm” without asking which standard temperature and pressure it uses
- Using FAD to size pipes, where actual flow is needed
- Treating a flow-meter reading in Nm³/h as if it were FAD at site inlet conditions
- Assuming a portable compressor delivers rated FAD at 45 °C ambient without checking the maker’s derating
Temperature and moisture terms
| Term | Unit | Meaning |
|---|---|---|
| Ambient temperature | °C | Air temperature around the compressor; affects cooling and FAD |
| Discharge temperature | °C | Temperature at the airend outlet; about 75–95 °C is normal for an oil-injected screw |
| Absolute temperature | K | °C + 273.15; used in all gas-law calculations |
| Pressure dew point (PDP) | °C | Temperature at which water condenses at line pressure |
| Atmospheric dew point | °C | Dew point of the same air expanded to atmospheric pressure; lower than PDP |
| Relative humidity | % | Water vapour present as a share of the maximum at that temperature |
Typical PDP values are about +3 °C for refrigerated dryers and −20 °C or −40 °C for desiccant dryers. Always say whether a dew point is pressure or atmospheric, because they can differ by many degrees. Air-quality classes are written in the ISO 8573-1 form [particles:water:oil], for example Class 1.4.1.
Power, energy and performance terms
| Term | Unit | Meaning |
|---|---|---|
| Motor rated power | kW, hp | Nameplate output of the motor; not the power drawn |
| Shaft power | kW | Power delivered to the airend shaft |
| Package input power | kW | Total electrical input including motor losses, fans and integrated dryer |
| Energy | kWh | Power multiplied by time |
| Specific power | kW per m³/min | Package input divided by FAD at a stated pressure; lower is better |
| Specific energy | kWh per m³ | Energy divided by delivered air over a period; includes part-load effects |
| Power factor | PF, no unit | Ratio of real to apparent electrical power |
1 hp ≈ 0.746 kW. Motor rated power is often confused with input; at full load the input is higher than the rated output because of motor losses. When comparing specific power, use the same pressure, reference condition and power boundary for every machine.
Common symbols used in our calculations
| Symbol | Quantity | Usual unit |
|---|---|---|
| P | Pressure (state gauge or absolute) | bar, Pa, psi |
| Q | Volume flow (state basis) | m³/min, m³/s, cfm |
| ṁ | Mass flow | kg/s |
| T | Temperature | K or °C |
| V | Volume (receiver or system) | m³, litres |
| ρ | Density | kg/m³ |
| v | Velocity | m/s |
| D | Internal diameter | m, mm |
| r | Pressure ratio | none |
| k | Ratio of specific heats, about 1.4 for air | none |
| R | Gas constant for dry air, about 287 | J/(kg·K) |
| cp | Specific heat at constant pressure | kJ/(kg·K) |
The equations that use these symbols, with their limits, are set out in the compressed air engineering formula register.
Two quick examples
Comparing two offers
Offer A gives 10 m³/min FAD at 7 bar(g). Offer B gives 360 cfm at 100 psig. Converting, 360 cfm is about 10.2 m³/min, and 100 psig is 6.9 bar(g), so the figures look similar. But if Offer B’s figure is displacement rather than FAD, its delivered air could be noticeably lower. Ask both suppliers for FAD to ISO 1217 at the same pressure and for total package input power.
Reading a service report
A report shows 6.2 at the far end of the plant and 7.0 at the compressor. If both are bar(g) at the same time, the system loses 0.8 bar between them, which is a piping or treatment problem, not a compressor problem. If one was absolute, the real gap is about 1.8 bar or about 0.2 bar in reverse, which changes the diagnosis entirely. Recording the basis avoids that confusion.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka in Secunderabad, has served industry since 2002 as an independent multi-brand compressed-air specialist. We sell new and used or refurbished compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, rent from our own fleet of 50–2,000 cfm at 2–30 bar, provide service and AMC, carry out air audits and supply spare parts. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.
When you compare offers, we can put each one onto the same basis so that flow, pressure and power figures are genuinely comparable. Our service reports state the basis of every reading, so a later visit or a second engineer can compare like with like and see a real trend rather than a change of units. Our teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India.
What to send us
- The offers, data sheets or reports you want compared or explained
- Nameplate photos of your compressors and dryers
- The pressure your users need and where it is measured
- Any logged data with a note of units and timestamps
- Site ambient temperature range and altitude
Frequently asked questions
What is the difference between bar(g) and bar(a)?
Bar(g) is gauge pressure, measured above local atmospheric pressure, which is what most plant gauges show. Bar(a) is absolute pressure, measured above a perfect vacuum. At sea level, absolute pressure is gauge pressure plus about 1.013 bar. Gas-law calculations such as receiver sizing and flow conversion must always use absolute pressure, or the results will be wrong.
How do I convert cfm to m³/min?
Divide cfm by about 35.3 to get m³/min, or multiply m³/min by 35.3 to get cfm. For example, 500 cfm is about 14.2 m³/min. The conversion only changes the unit; it does not change the reference basis. Make sure both figures are free air delivery, or both normal or standard flow, before comparing them.
What is the difference between FAD and displacement?
Displacement is the swept volume of the compressing element, calculated from its geometry and speed. Free air delivery is the air actually delivered, measured and expressed at inlet conditions. FAD is always lower because of internal leakage, heating and clearance effects. Piston compressors are often sold on displacement, so check which figure an offer uses before comparing it with a screw compressor.
What does Nm³/h mean in compressed air?
Nm³/h means normal cubic metres per hour: the flow converted to 0 °C and 1.013 bar(a), usually dry. Flow meters and process specifications often use it. It is not the same as FAD at your site’s inlet conditions, which are warmer, so a given Nm³/h figure corresponds to a somewhat larger FAD figure in a hot Indian plant.
What is the difference between pressure dew point and atmospheric dew point?
Pressure dew point is the temperature at which water condenses in the compressed air at line pressure. Atmospheric dew point is the dew point of the same air after expanding to atmospheric pressure, which is much lower. Dryer performance should be stated as pressure dew point, for example about +3 °C for refrigerated dryers, so always check which one a figure refers to.
Can you help us compare compressor quotations?
Yes. Send us the quotations or data sheets and we will put flow, pressure and power onto the same basis, point out missing information and explain the real differences. Because we work with many brands and are not an authorised dealer of any, our comparison is neutral. We can also check delivered performance on site in Telangana and Andhra Pradesh.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Pressure unit and gauge/absolute conversion
Convert between bar, psi, kPa and MPa and between gauge and absolute pressure. A mix-up between gauge and absolute is a classic cause of wrong sizing.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Pressure: 7
- Unit: bar g
Result
- bar gauge: 7
- bar absolute: 8.01
- psi gauge: 101.5
- psi absolute: 116.2
- kPa gauge: 700
- MPa gauge: 0.7
Method. Absolute pressure = gauge pressure + atmospheric (1.01325 bar at sea level). 1 bar = 14.504 psi = 100 kPa = 0.1 MPa. Hyderabad is about 540 m above sea level; atmospheric pressure there is roughly 0.95 bar, so enter the local value in the flow tool if it matters.
What to take from it. A 7 bar gauge system is 8.01 bar absolute. Ratio and flow-conversion work uses absolute pressure.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Flow and reference-condition conversion
Convert a flow quoted at one set of conditions to another, and between units. Different suppliers quote at different reference conditions, so always compare like with like.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Flow as quoted: 10
- Unit: m³/min
- Quoted at pressure (bar a): 1.013
- Quoted at temperature (°C): 20
- Convert to pressure (bar a): 1.013
- Convert to temperature (°C): 0
Result
- Converted flow: 9.32 m³/min
- 155.3 L/s
- 559.1 m³/h
- 329.1 cfm
Method. Q₂ = Q₁ × (P₁ / P₂) × (T₂ / T₁), with absolute pressure and absolute temperature. Dry-air basis; humidity is ignored. Unit factors: 1 m³/min = 16.667 L/s = 60 m³/h = 35.31 cfm.
What to take from it. A flow quoted at 20 °C and the same air quoted at 0 °C differ by about 7%. Check the reference conditions before comparing two machines.