Air Compressor Sizing Guide: The Australian Specification Method

By Byron Raal, CAS Founder-Editor · Last updated 22 June 2026 · About the author

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Compressor sizing is the most expensive decision in a compressed air project. Get it right and the system runs efficiently for 20 years. Get it wrong and the site pays a hidden electricity tax every month for the life of the asset, or loses production to pressure drops that were baked into the specification from day one.

Air compressor sizing in Australia uses a five-step method: tool audit (L/s or CFM per device), simultaneous-use factor (0.4 to 0.9), leak and growth allowances (~15% each), Free Air Delivery conversion under ISO 1217:2009, and ambient temperature plus altitude derating. The output is the nameplate FAD specification a supplier can quote against your duty profile. Audit first, size second: an independent energy audit tells you what your real demand profile is before you commit to a kW rating.

This guide walks through a defensible five-step sizing method used by compressed air engineers on Australian industrial sites. It covers the inputs that matter, the simultaneous-use and derating factors most supplier worksheets skip, a worked example against a real production profile, and the link between compressor sizing and receiver tank sizing (which is where most specifications quietly fail).

It is written for plant engineers, procurement teams, facilities managers and maintenance planners who need a repeatable method rather than a single rule of thumb. The method is based on ISO 1217:2009 Amd 1:2016 (Displacement compressors: acceptance tests), which is the only compressor performance metric that survives scrutiny.

If you are still choosing between compressor types rather than sizing a specific one, start with the air compressors hub and then return here.

For a detailed comparison of the two main compressor technologies used in Australian industry, see our rotary screw vs piston compressor guide, which includes duty cycle analysis, noise comparison, and a ten-year total cost of ownership worked example.

Why sizing is the most expensive decision in a compressed air system

A 45 kW rotary screw compressor (drawing approximately 52 kW of actual electrical package power) operating for 4,000 hours a year at an Australian industrial tariff of $0.30 per kWh will cost approximately $62,400 annually in electricity. Over a 10-year life that is approximately $624,000 against a capex of around $55,000. Capex is around 8% of total ownership cost. Electricity is more than 90%.

That ratio is why sizing matters. An oversized compressor is not “safe”. It is a machine that cycles short, runs inefficiently, wears out its seal rings prematurely, and costs more every week than the compressor it replaced. An undersized compressor is worse: pressure drops at the point of use interrupt production, force downstream tools to draw more air to compensate, and push operators to crank the setpoint up, which wastes more electricity.

Correct sizing is not just about specifying the right machine. It is about locking in the operating cost curve for the next decade. Every 1% error in sizing compounds into 1% of $624,000 over ten years. A 15% oversizing error costs approximately $93,600. A 15% undersizing error costs a production outage and then another compressor.

The five inputs to an honest sizing calculation

Every compressor sizing calculation depends on five inputs. Miss any one and the answer is a guess dressed up as a specification.

1. Air flow demand. The total volume of compressed air consumed by every tool, machine and process on the network, expressed in L/s, CFM or m³/min at the relevant pressure. This is the primary number.

2. Operating pressure. The highest pressure any tool or process on the network requires at its inlet, plus the pressure drop across the distribution system from the compressor house to the furthest point of use, plus the filtration and drying pressure drop.

3. Duty cycle. The fraction of the working day the compressor is actually under load. This drives the choice of compressor type (continuous-duty rotary screw vs intermittent-duty piston) and determines how tightly the spec can be drawn.

4. Air quality. The required ISO 8573-1:2010 air quality classification for particulate, water content and oil content. Air quality drives the filtration and drying specification, which in turn adds pressure drop that must be compensated for in the compressor size.

5. Future expansion. The planned growth in air demand over the asset’s operating life. Specifying against today’s load creates a compressor that is obsolete on day one.

Every step in the method below processes one or more of these inputs. The output is a specification that can be defended against a purchase audit or a production outage investigation.

Step 1: List every air-consuming device

The first step is unglamorous and unavoidable: walk the site and list every pneumatic tool, machine, actuator and process that will draw from the compressed air network. For each device, record:

• Equipment identification: description and location.
• Flow demand: in L/s or CFM at the required operating pressure.
• Operating pressure: in bar or psi.
• Usage profile: daily hours and rough duty cycle.

Start with the manufacturer data sheets. If a data sheet is missing or the tool is generic, published tool tables from reputable Australian industrial suppliers or from the pneumatic tool manufacturer’s engineering data are usable substitutes. Once you have a total, sanity-check it against the existing compressor’s output (if one is in place); that catches the obvious mistakes before they propagate.

Example fragment (partial tool audit):

Equipment	Location	Demand: L/s (CFM)	Pressure (bar)	Daily use (hrs)
Impact wrench ¾”	Assembly line 1	4 (8)	7	4
Paint spray gun HVLP	Finishing booth	7 (14)	3.5	3
Die grinder	Fabrication bay	3 (6)	7	2
CNC machining centre 1	Machine shop	12 (25)	7	8
Blow-off gun	Packing line	2 (4)	7	1
Air actuators (× 12)	Packaging line	8 (18)	6	8

The complete audit for a medium factory runs to 40 to 80 line items. The effort is recovered by the quality of every downstream calculation.

Step 2: Apply a simultaneous-use (coincidence) factor

Add up every tool on the site at maximum flow and you get a theoretical peak that never actually occurs. Operators take breaks, tools cycle, automated stations pause for loading, and process equipment staggers demand across the working day. The simultaneous-use factor (sometimes called the coincidence factor or load factor) is the fraction of that theoretical peak the network will actually see, and applying it is how you stop yourself buying a machine for a load that never arrives.

Typical simultaneous-use factors by industry:

Industry / application	Typical factor
Heavy manufacturing, automated lines	0.70 to 0.85
Automotive assembly	0.55 to 0.70
Fabrication and welding shop	0.40 to 0.60
Food packaging, continuous lines	0.80 to 0.90
Construction site, mixed tools	0.30 to 0.50
Workshop, light fabrication	0.25 to 0.40
Process plant, fixed actuators	0.85 to 0.95

Select the factor conservatively, and if you’re unsure use the upper end of the range. Multiply it through your total tool-list demand to get the expected simultaneous peak.

Example: a fabrication shop tool audit totals 161 L/s (340 CFM) of instantaneous peak. Applying a 0.50 coincidence factor: expected simultaneous demand = 340 × 0.50 = 161 L/s × 0.50 = 80 L/s (170 CFM).

Step 3: Add leak and growth allowances

Every compressed air system leaks, and yours is no exception. Based on ISO 11011 audit guidelines and US DOE benchmarks, typical leak rates are: a new, well-installed network leaks 5 to 10% of total flow; a typical operating network leaks 15 to 25%; a neglected system can leak 40%. Build an explicit leak allowance into the calculation or your compressor will run harder than its specification under normal operating conditions.

Leak allowance guidance:

• New install (10%): professionally installed system.
• Well-maintained (15%): existing maintained system.
• Older system (20%): existing system, unknown condition.
• Audit-first: legacy system with known leaks; audit and repair before sizing.

On top of leaks, add a growth allowance for expansion. A typical growth allowance is 10 to 20% above current demand, and you should pick the figure against your actual business growth plan rather than pluck it from thin air.

Example (continuing the fabrication shop): 80 L/s (170 CFM) simultaneous demand, 15% leak allowance, 15% growth allowance:

80 × 1.15 (leak) × 1.15 (growth) = 106 L/s (225 CFM) adjusted demand

Step 4: Convert to compressor Free Air Delivery

Compressor specifications are published against ISO 1217:2009 Amd 1:2016 (Displacement compressors: acceptance tests), which is the volume of ambient air moved through the compression stage per minute at reference inlet conditions (20°C, 1.0 bar absolute, 0% relative humidity). FAD is the only honest number for comparing compressors, so this is the number you size against. Swept volume, displacement or “maximum” flow figures are not equivalent and should not be substituted.

Unit conversions

Australian compressor specifications come in L/s (SI), CFM (Imperial) or m³/min, often on the same data sheet, so you will need to convert. The useful conversions are:

From	To	Factor
cfm	L/s	× 0.4719
L/s	cfm	× 2.119
cfm	m³/min	× 0.02832
m³/min	cfm	× 35.31
L/s	m³/min	× 0.06

Example: 106 L/s (225 CFM) = 6.37 m³/min.

Pressure adjustment

Compressor FAD is quoted at a specific output pressure (commonly 7 bar g). If your application needs a higher pressure (say 10 bar g), the same physical machine delivers less flow, because compressing to a higher pressure takes more work per unit volume. According to industry benchmarks, including those outlined by the US DOE Compressed Air Sourcebook, every 1 bar increase in output pressure reduces FAD by approximately 5% for standard rotary screw airends. Supplier data sheets usually publish FAD curves at several pressures, so read the curve for your actual required pressure rather than the headline figure.

Step 5: Apply ambient and altitude derating

Published FAD assumes the ISO reference conditions. Your real Australian site almost never matches them, and the compressor delivers less than its rated FAD in proportion, so you have to derate.

Heat derating

Every 10°C increase in inlet air temperature above the 20°C reference reduces volumetric efficiency by 3 to 5%. On a 40°C ambient day (20°C above reference), a rotary screw compressor delivers 6 to 10% less than nameplate. Your plantroom ventilation design feeds straight into this: a poorly ventilated compressor room can sit 10°C above outdoor ambient, compounding the derating. Plantroom layout and ventilation requirements are addressed by Safe Work Australia’s plant and workplace guidance.

Altitude derating

Higher altitude means lower inlet air density, which means less mass flow per unit volume. The derating is small at low elevations but becomes significant above 500 m, so check your site elevation before you finalise the spec.

Combined derating factors (approximate):

Site condition	Derating factor
Coastal, 20°C ambient, sea level	1.00 (reference)
Coastal, 30°C ambient, sea level	0.95
Inland, 35°C ambient, 300 m	0.91
Inland, 40°C ambient, 400 m	0.87
Remote inland, 42°C ambient, 500 m	0.85

Divide your demand calculation by the derating factor to find the required nameplate FAD.

Example (continuing the fabrication shop, located inland NSW at 35°C design ambient, 300 m elevation):

Required nameplate FAD = 106 L/s (225 CFM adjusted demand) ÷ 0.91 (derating) = 117 L/s (247 CFM)

Round up to the next commercially available size: approximately 123 to 132 L/s (260 to 280 CFM) FAD at 7 bar, which corresponds to a 45 kW rotary screw class machine.

Worked example: 45 kW food packaging line

The method above applied end-to-end to a single realistic site.

Site profile: food packaging facility, inland NSW, 2-shift operation (16 hours/day, 5 days/week), 50,000 operating hours target, design ambient 35°C, elevation 300 m.

Step 1. Tool audit (summary):

Equipment group	Count	Peak demand: L/s (CFM) each	Total peak: L/s (CFM)
Packaging line pneumatic actuators	24	0.9 (2.0)	23 (48)
Labeller and date-coder	4	1.7 (3.5)	7 (14)
Case erector	2	3.8 (8.0)	8 (16)
Case sealer	2	2.8 (6.0)	6 (12)
Palletiser	1	12 (25)	12 (25)
Blow-off stations	8	1.9 (4.0)	15 (32)
Workshop tools (maintenance)	mixed	pooled	7 (15)
CIP actuators	12	0.7 (1.5)	8 (18)
Total instantaneous peak			85 L/s (180 CFM)

The 85 L/s total is taken from the 180 CFM column sum (180 CFM is 85 L/s); the individual L/s values are each rounded from CFM and add to 86 in the column.

Step 2. Simultaneous-use factor: food packaging line, well-automated, continuous production. Factor 0.85. Expected simultaneous demand = 85 L/s × 0.85 = 72 L/s (153 CFM).

Step 3. Leak and growth allowances: existing well-maintained system, 15% leak allowance. Planned 20% production growth over asset life, 20% growth allowance. Adjusted demand = 72 × 1.15 × 1.20 = 100 L/s (211 CFM).

Step 4. Pressure conversion: line operates at 7 bar g. No pressure adjustment needed. Adjusted demand in FAD terms = 100 L/s (211 CFM) at 7 bar g.

Step 5. Derating: 35°C ambient, 300 m elevation, combined derating factor 0.91. Required nameplate FAD = 100 ÷ 0.91 = 110 L/s (232 CFM) at 7 bar g.

Specification: 45 kW rotary screw compressor rated approximately 123 L/s (260 CFM) at 7 bar g, giving a 13 L/s (28 CFM) headroom (12%) above the required FAD. To achieve ISO 8573-1:2010 Class 1.2.1 air quality in a food contact application, an oil-free rotary screw compressor or an oil-lubricated rotary screw compressor with Class 1 filtration is typically used; desiccant drying is required to reach Class 2 water content. For the air quality discussion see the compressed air dryers and air quality guide.

Receiver tank sizing tied to the compressor specification

Don’t treat compressor sizing and receiver tank sizing as separate exercises. Install a compressor without matching receiver capacity and it cycles hard, wears out early, and drops pressure at the point of use. Specify a receiver without reference to the compressor and all you have is a steel tank on a concrete slab, not an engineered buffer. Pressure vessels used as receivers must also comply with AS 1210:2010 (Pressure vessels).

The standard receiver sizing formula for smoothing short-duration peaks is:

V = (Q × t × Pa) ÷ ΔP

where V = receiver volume in litres, Q = peak flow demand above compressor capacity in L/s, t = duration of the peak in seconds, Pa = atmospheric pressure in bar absolute (1.013), ΔP = permitted pressure drop in bar.

Example (continuing the food packaging line): compressor rated 123 L/s (260 CFM). Assume a transient peak of 85 L/s (180 CFM) lasting 45 seconds on case-erector startup. Excess demand Q = 85 - 123 = negative (compressor covers it), so no transient buffer needed from the receiver for steady operation.

However, the same plant experiences a morning startup spike: all 24 packaging actuators and the palletiser cycle simultaneously for a brief period. Peak demand during that spike reaches approximately 94 L/s (200 CFM) for 30 seconds. Excess over compressor capacity: Q = 94 - 123 = still covered.

For food packaging with this profile you size the receiver primarily for cycle smoothing rather than peak buffering. The rule of thumb you reach for depends on compressor control: 3 to 5 L of receiver per L/s of FAD for VSD rotary-screw (which smoothly tracks demand), 10 to 15 L per L/s for load-unload rotary-screw with steady demand, and up to 20 L per L/s for reciprocating or highly intermittent duty where storage buffers large transient draws. For a 123 L/s load-unload rotary-screw: 123 × 12 (mid-range) ≈ 1,500 L, rounded up to a standard 1,500 L vessel. A VSD equivalent needs only 123 × 4 ≈ 500 L. Intermittent duty with frequent case-erector spikes typically runs 2,000 to 2,500 L.

For the complete receiver sizing methodology including wet-vs-dry placement, compliance and worked examples see the air receiver tanks Australia guide.

Matching the sized demand to compressor type

Once the required FAD and pressure are known, the compressor type must match the duty profile.

Duty profile	Compressor type
Continuous operation, >6 hr/day	Rotary screw fixed-speed or VSD
Variable demand, >2:1 load swing	Rotary screw VSD
Intermittent, <30% duty cycle	Piston two-stage
Food, pharma, electronics	Oil-free compression typically selected to avoid hydrocarbon carryover (see oil-free air compressors)
Remote or mobile	Portable diesel or electric
Critical instrument air loop	Oil-free rotary screw plus N+1 redundancy

Duty cycle is the most frequent source of type-selection error. A piston compressor sized correctly on peak demand but asked to run continuously will cook inside two years. A rotary screw oversized to cover short peaks will cycle short and waste energy. The correct answer is almost always to match the machine type to the duty cycle and buffer transients with receiver capacity.

When to specify redundancy (N+1 logic)

Any compressed air network supporting a continuous process that cannot tolerate interruption should be specified with N+1 redundancy: one operating unit plus an identical standby that can carry the full load. For critical process-air loops, sites commonly specify N+1 or N+2 redundancy; specific topology depends on the site’s risk register and business-continuity tolerance.

N+1 sizing is not a theoretical exercise. The standby must be exercised on a regular rotation, usually weekly, so that seal rings and bearings do not set. Automatic load-sharing controllers can run both units at partial load and alternate primary status, but every week the standby must take full duty at least briefly.

For non-continuous processes where a short interruption is tolerable, simple redundancy (a second unit sized smaller than the primary) is a cheaper option that still avoids a total outage.

Common sizing mistakes that cost money

1. Sizing by motor horsepower instead of FAD. Motor power is a convenience number, not a performance number. A 37 kW motor might drive a rotary screw delivering anywhere from 85 to 109 L/s (180 to 230 CFM) depending on design and output pressure.
2. Ignoring the simultaneous-use factor. Adding raw tool demand produces a theoretical peak that never occurs. The compressor sized on raw totals is oversized for the real load.
3. Skipping leak and growth allowances. A sized-for-today compressor is obsolete on day one and under-performs against real leaks within six months.
4. Forgetting derating. The 40°C ambient day that strips 10% off the compressor’s nameplate delivery is not optional. It happens every January.
5. Oversizing for “safety”. An oversized compressor cycles short, wastes energy on unloaded run time, and wears out its control valves earlier. Correct sizing is always cheaper.
6. No receiver capacity calculation. The compressor without matching storage cycles hard and drops pressure under transient peaks.
7. Specification written against supplier quote. The supplier’s quote reflects what they want to sell, not what the site needs. The two are rarely identical.
8. Treating pressure like flow. Increasing the pressure setpoint to cover a pressure drop wastes electricity and accelerates wear. Fix the pressure drop at its source (filtration, piping, leaks).

Quick-check rules of thumb (when they work, when they fail)

Engineers often fall back on rules of thumb when time is short. They are useful for sanity-checking a formal calculation but dangerous as primary sizing tools.

• 2 to 2.4 L/s per kW (4 to 5 CFM per kW) of connected pneumatic tool capacity. Works for mixed workshop duty. Fails for automated lines where actuators are much more efficient and for process air where the relationship breaks down entirely.
• 9 to 12 L/s (20 to 25 CFM) per skilled tradesperson using air tools. Works for fabrication and mechanical workshops. Fails for food packaging where the tradesperson count is irrelevant.
• 3 to 5 L per L/s (1.5 to 2.5 L per CFM) for VSD rotary-screw; 10 to 15 L per L/s (5 to 7 L per CFM) for load-unload rotary-screw; up to 20 L per L/s for reciprocating or highly intermittent duty. Must be verified against transient load profile.
• VSD pays back in 18 to 36 months if demand swings more than 2:1. Correct for most Australian industrial tariffs. Does not hold for constant-load applications where fixed-speed matches VSD efficiency.

Use rules of thumb to sanity-check the formal calculation. Never use them as the calculation itself.

Frequently asked questions

Related resources

• Air Compressors Hub: selecting the right compressor type
• Rotary Screw Air Compressors: continuous-duty detail
• Piston Air Compressors: workshop and intermittent duty
• Oil-Free Air Compressors: food, pharma, electronics
• Portable Air Compressors Australia: CFM sizing for site and mobile applications where duty cycle and altitude derate apply
• Industrial Air Compressors Australia: centralised plant
• Air Receiver Tanks Australia: storage sizing
• Air Compressor Installation Australia: installation planning
• Compressed Air for Manufacturing: system-level sizing including VSD trim, base load split, and demand audit methodology
• Compressed Air Dryers and Air Quality: drying and filtration
• Pharmaceutical Compressed Air: sizing for GMP validation and redundancy.
• Packaging and Bottling Compressed Air: 24/7 high-volume sizing with blow moulding demands.
• System Design Guide: full system architecture from demand profiling to distribution layout

General information disclaimer. The information on this page is general in nature and provided for educational purposes only. It is not engineering, safety, or professional advice, and it does not account for the specifics of your site, equipment, or duty. Compressed air system design, pressure equipment selection, and regulatory compliance must be confirmed with a qualified engineer and the relevant work health and safety regulator before you act. Compressed Air Solutions is a publisher and referral service, not a licensed engineering practice, and accepts no liability for decisions made on the basis of this content. Verify all figures, standards references, and regulatory requirements against current primary sources.

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