- Author
- Byron Raal, CAS Founder-Editor About the author
- Date last checked
- 10 September 2026
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How do you size an air compressor? Establish the flow and pressure your operation needs over time, record air quality and site conditions, then ask suppliers to demonstrate how their proposed equipment will meet that profile. A demand calculation starts the specification; it cannot by itself select motor power, compressor controls or receiver volume.
This guide uses five practical steps to prepare that brief. It does not prescribe an industry-wide simultaneous-use factor, a standard leakage allowance or a universal temperature correction. Use the compressor sizing calculator to keep your demand arithmetic explicit, and the worksheet below to record what still needs verification. Pressures are bar gauge unless stated otherwise.
Working through a sizing job now? Get the worksheet.
Free two-page PDF. Record demand, overlapping uses, short peaks, pressure and site conditions. Separate measured facts from planning allowances and take the outstanding equipment checks to a supplier. Email only, and the download is on the next screen.
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Why the demand profile matters
Buying against the largest catalogue number can leave a machine poorly matched to normal operation. Buying against an average without checking bursts can leave the plant short of air. The useful brief includes normal, minimum and peak demand, how long each lasts, and the pressure needed at the point of use.
For an existing installation, collect representative flow, pressure and package-power records across production, breaks and start-up. Note the measurement location and operating conditions. The Australian Government compressed air guide explains how monitoring supports demand profiling, leak reduction and control improvements. Resolve avoidable demand before pricing a larger supply.
Step 1: Record each use and its flow basis
Start with the actual tool or process manufacturer data, or a suitable measurement. Record equipment identity, operating pressure, flow units, reference conditions and whether the stated consumption is continuous, average or per cycle. A cylinder volume at line pressure is not already a free-air flow figure. Ask the equipment supplier to resolve an unclear basis before combining it with other figures.
| Record | What belongs in the brief |
|---|---|
| Equipment and location | Actual model, quantity and where it is used |
| Consumption | Flow or air per cycle, pressure and source of the figure |
| Operating pattern | Cycle duration, frequency, running hours and overlapping uses |
| System measurements | Meter location, flow basis, sampling period and production state |
| Uncertainty | Missing data, assumptions and the person responsible for confirming them |
Keep an audit trail: a spreadsheet row should lead back to the relevant data sheet or measurement. A generic tool table can help identify missing information, but it does not verify the consumption of your equipment.
Step 2: Separate normal demand from short peaks
For a tool list, establish which uses actually overlap. Do not assume that every listed tool runs continuously, or that all possible simultaneous operation can be dismissed. Record short events separately with their duration, repetition and minimum acceptable pressure. If you start from measured whole-system flow, a second coincidence reduction may count the same effect twice.
The CAGI system design handbook, Chapter 4, distinguishes average consumption from demand events and considers storage with compressor controls. Use a supported operating schedule or measurement for your factor; the name of the industry does not establish it.
Step 3: State leakage and future demand explicitly
List known leakage and the repairs planned before the new system is commissioned. When measured system demand already includes leaks, do not add the same leakage again. For future demand, name the planned equipment or production change and when it is expected. A percentage may be useful for an early scenario, but label it as an assumption and avoid carrying overlapping allowances through several worksheets.
The calculator uses planning flow = audited demand × simultaneous-demand factor × (1 + leakage allowance) × (1 + growth allowance). Here each percentage is an addition to the preceding result. It is not a measured leak fraction of total compressor output, and the two allowances compound. Keep zero as an option when no additional allowance is justified.
Worked example: a planning demand of 66 L/s
These numbers are illustrative, not a survey of an actual plant or an equipment recommendation. Suppose the tool audit totals 100 L/s on a common reference basis. An observed schedule supports a 0.50 overlap factor. The planner records a 10 per cent additional leakage assumption and 20 per cent future demand assumption.
| Calculation | Result |
|---|---|
| 100 L/s × 0.50 | 50 L/s concurrent demand |
| 50 × 1.10 | 55 L/s including the stated leakage addition |
| 55 × 1.20 | 66 L/s planning demand |
| 66 × 0.06 | 3.96 m³/min on the same reference basis |
The brief also records 7 bar gauge at the required operating point, a 35°C compressor inlet design condition, 300 m elevation, and a separate 90 L/s demand event lasting 20 seconds. Those conditions do not turn 66 L/s into an automatic nameplate rating. The supplier must explain how the proposed capacity, storage and controls cover the full profile, including recovery before the next event.
Step 4: Compare delivered flow at the required pressure
CFM, L/s and m³/min are volume-flow units. Free Air Delivery (FAD) expresses delivered air as an equivalent flow at stated inlet or reference conditions. Changing units does not change that basis. A catalogue displacement figure cannot be treated as verified delivered flow, and a motor kW figure is not an airflow rating.
| Conversion | Factor |
|---|---|
| CFM to L/s | × 0.4719474432 |
| L/s to CFM | ÷ 0.4719474432 |
| L/s to m³/min | × 0.06 |
For example, 100 L/s is approximately 211.9 CFM. Keep the same reference basis on both sides of that conversion. Compare the proposed package at the required discharge pressure, taking account of the pressure needed at the end use and the assessed losses through treatment and distribution. There is no universal five-per-cent flow penalty per additional bar in this method.
ISO 1217 describes displacement-compressor acceptance testing; it does not approve this planning guide or prescribe the allowances above. CAGI performance data can support comparison at stated operating points. Ask for the exact offered model, delivered flow, test/reference conditions, operating pressure and total package power, including the limits of any performance verification.
Step 5: Have the offered equipment checked for the site
Record the expected compressor inlet temperature range, elevation or local inlet pressure, humidity, cooling conditions, ventilation and installation constraints. Ask the supplier to state available flow and package limits under those conditions. Atlas Copco explains that altitude assessment depends on the machine, drive and operating conditions. A simple air-density ratio is not a verified correction for every compressor design.
Confirm the air-quality requirement at each critical use with the process owner and competent adviser. Specify how the supplier will verify it, including the measurement point and treatment performance across the expected operating range. Choosing an oil-free compressor alone does not settle every air-quality requirement. The US Department of Energy air-quality guidance discusses selecting treatment to suit the application.
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Receiver storage and controls need a joint check
Ask the designer to show how storage covers the identified demand events, how far pressure may fall, where storage is located, and how it refills. Also ask how receiver volume interacts with the compressor control method and permitted cycling. A single litres-per-L/s multiplier cannot establish the right vessel for every installation.
The US Department of Energy storage guidance relates storage to demand patterns, air requirements and controls, including storage near intermittent uses. A storage calculation is also separate from pressure-vessel design, registration and inspection duties. Have those checked for the installation and jurisdiction; the receiver guide provides background.
Compare control options and outage arrangements
Request a comparison of the proposed control options against your actual load profile, including minimum flow, unloaded or standby power, permitted starts, transitions between machines and maintenance arrangements. Do not select VSD solely from a demand-ratio rule or assume a payback period. The US Department of Energy control guidance explains why supply, storage and changing demand need to be considered together.
State which production can stop and for how long. Ask what happens when a compressor, dryer, controller or electrical supply is unavailable, and whether the remaining system can maintain the required flow and air quality. N+1 means enough duty capacity remains when one required unit is unavailable; it need not mean one working compressor and one identical spare. Agree commissioning tests and maintenance rotation with the equipment supplier rather than imposing a universal weekly exercise rule.
Compare operating cost using package power and time
For each operating state, multiply measured or supported package input kW by hours and the applicable energy tariff, then add the states. For illustration only, a constant 52 kW input for 4,000 hours at A$0.30/kWh gives A$62,400. That is energy arithmetic, not the price of operating every 45 kW compressor, a current Australian tariff quote or a complete ownership-cost estimate.
Include treatment energy, unloaded operation, maintenance, installation, capacity charges where applicable, and any planned changes in use. Compare offers over the same scenario. The US Department of Energy cost method distinguishes operating levels rather than applying one motor rating to every hour. Ask each supplier to show the assumptions behind a savings or payback claim.
Before accepting a sizing proposal
- Can each demand figure be traced to actual equipment or a measurement?
- Are overlap, leakage and future demand counted once, with assumptions marked?
- Are short peaks, recovery time and minimum point-of-use pressure covered?
- Does the offer state the exact model, delivered flow basis, pressure and site limits?
- Are storage, controls, air treatment and failure arrangements explained?
- Do the cost comparison and commissioning checks use the same operating scenario?
Get the 2026 report
For background on leakage costs, request the 2026 report below. Treat its scenarios as background, not as measured leakage or a savings forecast for your installation.
Form not loading? Email us directly at byron@compressedairsolutions.com.au.
We email you the PDF and nothing else unless you ask.
Frequently asked questions
How do I calculate how much airflow (L/s) my air compressor needs?
Record actual tool or process consumption and its flow basis, establish overlapping uses, and state any additional leakage or future demand assumptions. Keep short peaks and their duration separate. The resulting planning flow must be checked with pressure, storage, air quality, controls and site conditions before equipment is selected.
What is the difference between cfm and FAD?
CFM is a volume-flow unit. FAD expresses delivered air as an equivalent volume flow at stated inlet or reference conditions. L/s and CFM can both describe FAD. Compare the reference basis, operating pressure and test conditions; changing units alone does not make different catalogue ratings equivalent.
What size air compressor do I need for a workshop?
There is no workshop-wide answer from headcount alone. Record the actual equipment, operating schedule, pressure and short peaks, then ask for a model-specific capacity and duty assessment. A hand-tool workshop and a continuously operating process can have very different requirements.
How do I convert L/s to cfm for compressor sizing?
Divide L/s by 0.4719474432 to get CFM, or multiply by approximately 2.119. Thus 100 L/s is about 211.9 CFM. Multiply CFM by 0.4719474432 for L/s. These are unit conversions on the same flow-reference basis.
Does ambient temperature really affect compressor sizing?
Inlet temperature and other site conditions can affect equipment performance and operating limits. The effect depends on the compressor and its installation. Ask the supplier to verify the offered model at the stated site conditions; do not apply a fixed per-degree density factor to every machine.
How big should my air receiver tank be?
Storage volume depends on the demand event, its duration and repetition, permitted pressure fall, refill capacity, location and compressor controls. Request a documented storage and cycling check. A single litres-per-flow rule does not establish a suitable receiver or its statutory obligations.
Should I oversize my compressor for future growth?
Include identified future demand explicitly, with its likely timing. Avoid adding the same growth in several places. Ask suppliers to compare current operation and the expansion scenario, including part-load performance and options for staged capacity. No universal percentage guarantees the best choice.
Can one formula size any air compressor?
No. Demand arithmetic is one part of a system specification. Actual equipment selection also needs pressure, air-quality, site, control, storage and reliability checks. This guide helps prepare the information for that review; it does not certify a compressor selection.
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