Short answer: Divide CFM by 35.31 to get m³/min, then multiply by the specific power at your discharge pressure — typically about 5.3 kW per m³/min at 7 bar. A 300 CFM system at 7 bar therefore needs roughly 45 kW, which is 270,000 kWh and about 21,600 USD a year at 0.08 USD/kWh · 6,000 h/yr. Quote every flow at a pressure, or the number means nothing.
By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.
Most quotes we are asked to compare are not wrong — they are uncomparable. One supplier quotes flow in CFM at 100 psig, another in m³/min at 8 bar, a third “at standard conditions”, and the price difference turns out to be entirely in the units. In the field we have audited plants where the machine was sized correctly on flow and wrongly on pressure; the buyer paid for the extra compression ratio every hour of the year.
What does FAD actually mean, and why is it not displacement?
FAD is free air delivery: the volume of atmospheric air the compressor actually takes in per minute while discharging at its rated pressure. It is measured per ISO 1217:2009 Annex C at 1 bar absolute, 20°C and 0% relative humidity.
Displacement is a different number. Swept volume from the airend geometry ignores intake losses, leakage past the rotors and re-expansion, so a 300 CFM displacement figure can correspond to under 300 CFM of delivered air. When a catalogue says “air delivery”, ask which of the two it is.
How do you convert CFM to m³/min and back?
One m³/min equals 35.31 CFM, so the arithmetic is a single division in either direction. Keep the factors in your head and unit confusion disappears:
- CFM → m³/min: divide by 35.31.
- m³/min → CFM: multiply by 35.31.
- Quick check: 100 CFM is very close to 2.8 m³/min; 350 CFM is about 10 m³/min.
Then convert flow to power, remembering that the same flow quoted at 7 bar and at 10 bar is not the same duty.
How do you turn FAD into kW without a test bench?
Multiply FAD in m³/min by the specific power at your discharge pressure. Specific power is input kW divided by delivered flow — the figure our datasheets print as kW/m³, and the one number that lets two quotes be compared without a site visit.
| Discharge pressure | Typical specific power, oil-injected screw | What it means in practice |
|---|---|---|
| 4 bar | typically 4.2–4.8 kW per m³/min | low-pressure duty, least energy per cubic metre |
| 7 bar | typically 5.0–5.5 kW per m³/min | the common plant band for general factory air |
| 8 bar | typically 5.5–6.2 kW per m³/min | general purpose packages, most catalogue figures |
| 10 bar | typically 6.5–7.0 kW per m³/min | laser assist, PET pre-blow, higher-ratio duty |
| 13 bar | typically 7.5–8.2 kW per m³/min | special duty; two stages are worth checking above this |
Our planning benchmark across the range is a ~35% energy reduction against a fixed-speed machine sized for the same duty — a screening figure, not a guarantee. A VSD unit at part load beats the fixed-speed figure, which is why the load profile belongs in the calculation.
Why does delivered FAD fall as discharge pressure rises?
Because the airend sweeps the same volume but each pocket leaks and re-expands more at a higher compression ratio. A machine delivering 13 m³/min at 7 bar typically delivers about 11 m³/min at 10 bar on the same motor — roughly 15% less air for the same shaft power.
The reverse is the useful part: run the same 45 kW airend at 4 bar instead of 7 bar and delivered flow rises by a similar margin. That is the entire argument for low-pressure machines on 3–5 bar duty, and why a CFM requirement that omits a pressure cannot be sized at all.
What does the sizing look like for a 50–500 CFM system?
Here is the worksheet in one table, at 7 bar, 5.3 kW per m³/min, 6,000 hours a year and 0.08 USD/kWh:
| FAD, CFM | FAD, m³/min | Average input, kW | Energy, kWh/yr | Cost, USD/yr |
|---|---|---|---|---|
| 50 | 1.4 | 7.5 | 45,000 | 3,600 |
| 100 | 2.8 | 15 | 90,000 | 7,200 |
| 200 | 5.7 | 30 | 180,000 | 14,400 |
| 300 | 8.5 | 45 | 270,000 | 21,600 |
| 500 | 14.2 | 75 | 450,000 | 36,000 |
Work one row end to end. A plant logs 300 CFM of average demand at 7 bar:
- Convert the flow: 300 ÷ 35.31 = 8.5 m³/min.
- Apply specific power: 8.5 × 5.3 = 45 kW average input.
- Size on the peak, not the average: a logged peak of 380 CFM is absorbed by storage rather than by 15 kW of extra compressor running all year.
- Check the pressure: at 10 bar the same 8.5 m³/min would need 8.5 × 6.8 = 58 kW — 12.8 kW more, or 76,800 kWh and 6,144 USD a year at 0.08 USD/kWh · 6,000 h/yr.
- Add the derate: above 40°C ambient, check the manufacturer’s derating curve. Our packages are rated for full output at 50°C ambient, and the rating holds at 4 bar as well as at 8 bar.
What did the numbers look like in a real installation?
Our 75 kW PMV VSD installation case study documents an iron processing plant in Ho Chi Minh City — 15,000 m², more than 200 employees, 16 hours a day of operation, compressed air costing over 3,200 USD a month in electricity, and maintenance at 850 USD a month on the old fixed-speed unit. Pressure swung ±0.5 bar. The number that makes it a useful sizing case is the 850 USD a month: fixed-speed behaviour showing up in the maintenance budget as much as on the meter.
How do you size a low-pressure air compressor for the textile, ceramic, glass and cement industry in CFM?
A low-pressure air compressor for the textile / ceramic / glass / cement industry is sized with the same worksheet, with one substitution: the specific power comes from the 3–5 bar row, not the 7 bar row. Our low-pressure series spans 15–300 HP for 3–5 bar duty, and its published flow figures are quoted at the low-pressure band.
| Industry | Typical pressure band | What sets the CFM requirement |
|---|---|---|
| Textile | typically 3–5 bar | air-jet looms and relay nozzles running continuously |
| Ceramic | typically 3–5 bar | spray-dryer atomisation, plus press and conveying peaks |
| Glass | typically 3–5 bar | forming support, mould cooling and blow-off cycles |
| Cement | typically 3–5 bar | pneumatic conveying duty cycle and pulse-cleaning storage |
The lesson from the four: measure the flow at the pressure the process actually uses. The low-pressure compressor series and the 15–300 HP series page cover the air side.
How much does a 10% sizing error actually cost?
About 2,000 USD a year on a 300 CFM duty, and it is usually paid twice. Oversizing by 10% adds roughly 4.5 kW of input at 7 bar — 27,000 kWh and 2,160 USD a year at 0.08 USD/kWh · 6,000 h/yr — and the oversized machine also runs further down its part-load curve, where a fixed-speed unit still draws roughly 20–30% of full-load power while unloading.
Undersizing costs differently: the machine runs loaded continuously, pressure sags at peak, and the plant buys the second compressor it did not budget for. The cheap fix on both sides is storage — recalling the arithmetic from a 4-second peak: 40 m³/min × 4 s ÷ 60 = about 2.7 m³ of air, which a 2.7 m³ receiver at a 1 bar differential covers.
What should you ask a supplier for in writing?
Four numbers and one method, and a supplier who cannot produce them is guessing:
- FAD in m³/min at your discharge pressure, per ISO 1217:2009 Annex C — not at 8 bar if you run at 5.
- Specific power in kW per m³/min at that pressure, so two quotes compare directly.
- Derate curve above 40°C ambient, because tropical compressor rooms rarely see the test conditions.
- Motor efficiency class — IE4 or IE5 on the packages we build, which matters when the machine runs 6,000 hours a year.
- An acceptance test method, so a disputed flow figure is settled by measurement rather than by opinion.
Why does specific power compare two compressors more fairly than price?
Because it combines the two things you actually buy. A cheaper machine with worse specific power costs more by the end of the first year: at 0.3 kW per m³/min of difference on a 300 CFM duty, the gap is 2.6 kW — 15,600 kWh and 1,248 USD a year at 0.08 USD/kWh · 6,000 h/yr, every year for the life of the machine.
Our free compressed air kWh forecast and system review takes a logged duty point and returns the kW and the annual cost, which is the number a purchase decision should be made on.
Key takeaways
- CFM ÷ 35.31 = m³/min, and kW = m³/min × specific power — about 5.3 kW per m³/min at 7 bar for an oil-injected screw.
- 300 CFM at 7 bar is about 45 kW, or 270,000 kWh and 21,600 USD a year at 0.08 USD/kWh · 6,000 h/yr.
- The same flow at 10 bar costs about 12.8 kW more — 76,800 kWh, or 6,144 USD a year, so always quote flow with a pressure.
- A 10% oversize costs about 2,160 USD a year and leaves the machine running further down its part-load curve.
Frequently asked questions
Is 1 m³/min the same as 35.31 CFM exactly?
Yes, that is the exact conversion to two decimal places, and it is accurate enough for sizing. Dividing by 35 instead stays within 1%, well inside the uncertainty of a logged demand figure.
Can I use the kW on the motor nameplate instead of specific power?
No. The nameplate figure is the motor’s rated output, not its input, and it says nothing about how much air the package delivers. Two 45 kW packages can differ by 10–15% in FAD at the same pressure.
Why does my compressor deliver less air than the catalogue says?
Almost always because the catalogue figure was taken at a lower pressure or at 20°C intake. FAD falls by roughly 15% going from 7 bar to 10 bar, and high intake temperature reduces it further — check the derating curve before assuming the machine is faulty.
Does a VSD compressor change the CFM-to-kW calculation?
It changes the specific power, not the arithmetic. At full load a VSD package matches or slightly beats a fixed-speed unit; at part load its specific power improves, which is where the savings come from on a variable profile.
Sources and standards
- ISO 1217:2009 — Displacement compressors — Acceptance tests. Defines free air delivery and the Annex C test conditions used throughout the worksheet.
- ISO 11011:2013 — Compressed air — Energy efficiency — Assessment. Basis for the load-profile and specific-power method behind the kW figures.
- ISO 8573-1:2010 — Compressed air — Contaminants and purity classes. Relevant as soon as the air touches a product or an instrument.
- ISO 9001:2015 — quality management certification held by AirSpace Machinery Co., Ltd., with CE marking under Machinery Directive 2006/42/EC.
- Our published references — the low-pressure series and our energy-efficient compressor manufacturing page.
About the author
Johnny Wayne (Wei Zhuang) is General Manager of AirSpace Machinery Co., Ltd., the Shanghai manufacturer behind ChinaCompressor.org. The company builds oil-injected and Class 0 oil-free screw compressors from 2 to 540 HP (1.5–400 kW), including the low-pressure series at 15–300 HP for 3–5 bar duty, from a 4,000 m² production facility with 20 years of engineering history. Products carry CE marking and ISO 9001:2015 certification, and we sell through distributors rather than around them.
Send a logged duty point — flow, pressure, running hours, ambient — to sales@chinacompressor.org and we will return the kW and the annual energy, or start with our rotary screw versus piston comparison if you are still choosing the technology.
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