By Byron Raal, CAS Founder-Editor · Last updated 12 July 2026 · About the author
How do I size an air compressor for my plant? Add up the flow demand of every pneumatic tool and process on your network, apply a simultaneous-use factor of 0.4 to 0.9 depending on your industry (most tools never run at once), then add 15 per cent for leaks and 10 to 20 per cent for planned growth. Convert the result to Free Air Delivery (FAD) under ISO 1217:2009 Amd 1:2016 reference conditions, then derate for your site ambient temperature and altitude: every 10°C above the 20°C reference cuts the air the machine actually delivers by 3 to 5 per cent, and every extra bar of required pressure cuts FAD by roughly 5 per cent. The output is the nameplate FAD specification a supplier can quote against. The calculator below runs that full five-step method on your own numbers, showing every factor so the result is auditable rather than a black box.
Page conventions: all pressures are bar gauge (bar g) unless otherwise noted. Flow is shown in L/s, CFM and m³/min. FAD figures follow ISO 1217:2009 Amd 1:2016 reference conditions (20°C, 1.0 bar absolute, 0 per cent relative humidity).
- Sizing by raw tool-demand totals, without a simultaneous-use factor, produces an oversized compressor that cycles short and wastes energy for the next 10 to 20 years.
- Every 10°C of ambient temperature above the 20°C ISO 1217 reference cuts the air the machine actually delivers by 3 to 5 per cent, counting both the fall in air density and the water vapour a humid intake carries; a 40°C Australian summer day strips 6 to 10 per cent off nameplate delivery.
- FAD (Free Air Delivery, ISO 1217:2009 Amd 1:2016) is the only performance number that compares fairly between compressor brands. “Maximum flow” or “swept volume” figures on a spec sheet are not equivalent.
- A 15 per cent oversizing error on a 45 kW compressor adds tens of thousands of dollars to a 10-year electricity bill; an undersizing error costs a production outage.
- This calculator runs the same five-step method as the CAS sizing guide: demand audit, simultaneous-use factor, leak and growth allowance, FAD conversion, then ambient and altitude derating.
Air Compressor Sizing Calculator
How this calculator works
What the derating factor is actually made of. A compressor is a fixed-displacement machine: it swallows the same volume of air every revolution, so the air it actually delivers tracks the density of the air at its intake. Three things cut that density on an Australian site. Temperature is the first: density falls with absolute temperature, so a 30°C intake gives 293 K ÷ 303 K = 0.967, a 3.3 per cent loss against the 20°C reference. Humidity is the second: the ISO 1217 reference condition is dry air at 0 per cent relative humidity, and a humid intake carries water vapour that displaces air the machine would otherwise draw in. Those two terms together are what produce the 3 to 5 per cent per 10°C band, the low end for a dry inland intake and the high end for a humid coastal one. Altitude is the third: lower barometric pressure means lower density again, roughly 1.2 per cent for every 100 m of elevation. This calculator computes all three from the physics and assumes 50 per cent relative humidity at intake, which reproduces the published guide bands.
Capacity derating is not the same thing as package derating, and the two get conflated often enough to be worth separating. Everything above is capacity: the air that physically enters the machine. Package derating is what the compressor does to protect itself once intake air passes roughly 35 to 40°C, when its own cooling limits bite and it steps output back or trips on a high-temperature alarm. That is a threshold effect rather than a smooth per 10°C figure, it is a compressor room design problem rather than a sizing coefficient, and this calculator does not model it.
This calculator applies the same five-step sizing method published on the CAS air compressor sizing guide: a tool-demand audit, a simultaneous-use (coincidence) factor, leak and growth allowances, conversion to Free Air Delivery under ISO 1217:2009 Amd 1:2016 reference conditions (20°C, 1.0 bar absolute, 0 per cent relative humidity), and derating for ambient temperature and altitude. Every constant and factor range used here is drawn directly from that guide, which is the canonical source.
FAD is the only defensible basis for comparing compressor nameplate capacity across manufacturers. “Swept volume” or “maximum flow” figures on a data sheet are not equivalent and should not be substituted, a point the sizing guide already makes and this tool enforces by only accepting and returning FAD-basis numbers.
When to use this tool versus the written guide
Open the written sizing guide first if you need to understand the method, see a fully worked example, or check the receiver-sizing formula that pairs with compressor sizing. Open this calculator once you have your own tool audit numbers and want the arithmetic run automatically, with the derating and pressure-adjustment steps applied for you. If you are still choosing between compressor types, start with the air compressors hub, or compare the two main technologies in the rotary screw vs piston compressor guide.
Common sizing mistakes this tool is built to prevent
- Sizing by motor kW instead of FAD. A 37 kW motor can drive very different actual air delivery depending on design and output pressure; the calculator only accepts and returns FAD.
- Skipping the simultaneous-use factor. Raw tool-demand totals overstate real load; the calculator forces a factor selection rather than defaulting to 1.0.
- Ignoring derating. The 40°C Australian summer day that strips 6 to 10 per cent off nameplate delivery happens every January; the calculator applies it automatically rather than leaving it to memory.
- Oversizing “for safety”. An oversized compressor cycles short, wastes energy on unloaded run time, and wears out its control valves early. Correct sizing is consistently cheaper over the asset life than a margin added “just in case”.
That number is your starting specification, not the finish line. Get this result as a short report, plus a free match to an independent Australian compressor supplier who can confirm it against your actual site and quote against the exact nameplate FAD, not a rounded-up guess.
Independent and vendor-neutral. No obligation, and no sales calls from us.
Frequently asked questions
What is Free Air Delivery (FAD) and why does this calculator use it instead of CFM or kW?
FAD is a performance measurement standard defined by ISO 1217 that specifies air flow under reference inlet conditions (20°C, 1.0 bar absolute, 0 per cent relative humidity), making it directly comparable between compressor brands. CFM and L/s are just units of flow and can describe FAD or a non-comparable “maximum flow” figure; kW describes motor power, not delivered air. This calculator only works in FAD terms so its output can be quoted directly against a supplier FAD-rated spec sheet.
Why does the calculator ask for my site ambient temperature and elevation?
Because a compressor nameplate FAD is rated at 20°C sea-level reference conditions, and real Australian sites are rarely at those conditions. Every 10°C of ambient temperature above 20°C reduces the air the machine actually delivers by 3 to 5 per cent. Two terms sit inside that band. Inlet air density falls with absolute temperature (293 K divided by 303 K is 0.967 at 30°C, a 3.3 per cent loss), and a humid intake loses more again because water vapour displaces air at the inlet, which is why the ISO 1217 reference condition is dry air at 0 per cent relative humidity. Elevation is a third term, since lower barometric pressure means lower density again. Skipping this step is one of the most common sizing errors and the reason many “correctly sized” compressors underperform in summer.
Can I use this calculator if I do not know my exact tool-by-tool demand yet?
Yes, using the built-in reference presets for common applications, but the result will be less precise than a full tool audit. For a capital decision above a few thousand dollars, complete the tool audit described on the sizing guide first, then use this calculator to run the downstream steps.
Does this calculator replace a supplier own sizing quote?
No. This tool gives you a defensible starting specification to quote against, so a supplier proposal can be checked rather than taken on faith. Final specification should be confirmed by a qualified engineer against your actual site conditions before purchase.
Is this calculator method accepted by Australian compressor suppliers?
The method (tool audit, simultaneous-use factor, leak and growth allowance, ISO 1217 FAD conversion, ambient and altitude derating) reflects standard Australian industrial sizing practice and is documented in full, with a worked example, on the CAS sizing guide. It is a defensible minimum method, not a proprietary or novel approach; any competent compressed air engineer should recognise and be able to validate each step.
General information disclaimer. This calculator and page are general in nature and provided for educational purposes only. They are not engineering, safety, or professional advice, and do 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.