Most compressor sizing mistakes aren't made at the compressor. They're made a step earlier, when someone adds up the CFM ratings printed on a few tool spec sheets, picks a compressor that covers the total, and moves on. That number is almost never the number that actually matters, and the gap between them is why plants end up either starving their tools of pressure or paying to run a compressor twice the size they needed.

Start with what SCFM and FAD actually mean

Tool manufacturers rate air consumption in SCFM, standard cubic feet per minute, a figure calculated back to a fixed reference condition, usually close to sea level at a standard temperature. Compressor manufacturers rate output in FAD, free air delivery, which under the ISO 1217 standard is measured at the compressor's actual intake conditions and referenced back to 20°C, 1 bar absolute pressure, and 0% relative humidity.

These aren't interchangeable, and the difference isn't academic. A compressor rated for 500 CFM of free air delivery, installed in a hot, humid location, can deliver meaningfully less actual air mass than the same rated CFM would suggest, because hot, humid intake air is less dense. For a plant in Mumbai running through peak summer heat, this isn't a rounding error. It's a real enough gap that sizing off the FAD-rated number without accounting for local conditions can leave a system short of what the tools on the floor actually need.

Step 1: add up your actual air consumers, not their nameplate maximums

List every pneumatic tool, actuator, and piece of equipment that will draw from the same compressed air line, along with its rated SCFM. The mistake here isn't the addition, it's assuming everything runs at once, at full rated draw, continuously. Most plants don't operate that way. A more realistic number accounts for the fraction of time each tool actually runs, known as the duty cycle, rather than summing every nameplate figure as if all of it fires simultaneously.

Step 2: convert to free air delivery at your operating pressure

Once you have a realistic total SCFM demand, it needs to be expressed as the FAD figure a compressor's datasheet actually quotes, at your required delivery pressure. As a working approximation at sea level: actual CFM at a given gauge pressure is roughly your SCFM figure multiplied by 14.7, divided by the sum of your gauge pressure in PSI and 14.7. A compressor rated to deliver 20 SCFM of free air, aiming for 100 PSIG output, actually delivers close to 2.5 CFM of compressed air at that pressure; the rest of the number represents the volume reduction from compression itself. This is the calculation most sizing mistakes skip entirely, comparing a tool's SCFM demand directly against a compressor's rated FAD at a completely different reference pressure.

Step 3: correct for altitude, temperature and humidity

A compressor rated for a given FAD at standard sea-level conditions won't deliver that same figure everywhere. As a rough guide, a compressor delivering 500 SCFM at sea level can drop to closer to 400 SCFM at around 2,000 metres of elevation, simply because the intake air is less dense to begin with. Most Indian industrial sites sit well below that altitude, so elevation itself is rarely the deciding factor here, but heat and humidity produce a related effect locally: hot, humid intake air is lower density air, and a compressor rated under ISO 1217's cooler, drier reference conditions will underperform that rating on a hot Mumbai afternoon unless the sizing already built in a margin for it.

Step 4: size the receiver tank, not just the compressor

A correctly sized compressor still needs somewhere to buffer short bursts of demand so it isn't cycling on and off constantly, which wears the machine down faster than steady running does. As a starting reference, intermittent-use systems are often sized around one gallon of receiver capacity per CFM of demand, while continuous-duty systems generally need three to four gallons per CFM to smooth out cycling properly. Convert to litres for local tank sizing, but treat these as a starting point to refine against your actual demand pattern, not a fixed rule.

A worked example

Take a small fabrication unit running a grinder rated at 8 SCFM, a spray gun rated at 12 SCFM, and an impact wrench rated at 10 SCFM, none of which run continuously or simultaneously at full draw. A realistic duty-cycle-adjusted demand might land closer to 18 to 20 SCFM combined rather than the full 30 SCFM nameplate total. Targeting 100 PSIG delivery, that converts to roughly 2.5 CFM of actual compressed air demand at the outlet, before adding the 10 to 15 percent margin for Mumbai's summer heat and humidity, and before accounting for planned growth in the shop over the next few years. This is the point where a compressor sized purely off nameplate addition, which would have suggested a machine roughly 50 percent larger than actually needed, gets corrected back down to something that matches real usage.

Where Atlas Copco's range fits these numbers

Atlas Copco's G series covers exactly this smaller end of industrial demand, with the G 2-5 suited to lighter workshop loads and the G 7-15 and G 15 stepping up for busier, multi-tool shops. For plants running closer to continuous, high-demand duty cycles, variable speed models like the GA 7-75 VSD adjust rotor speed to actual demand rather than running at fixed output regardless of load, which matters more once your calculated FAD requirement starts fluctuating meaningfully across a shift rather than sitting flat.

Sizing correctly the first time avoids two expensive outcomes: a compressor too small to hold pressure under real load, and a compressor large enough that it spends most of its life running well below its efficient operating range. If you'd rather work through your specific tool list and duty cycle with someone who does this calculation daily, get in touch with our team.

Frequently asked questions

What's the difference between SCFM and FAD?

SCFM is a standard reference figure used to rate tool air consumption. FAD, free air delivery, is how compressor manufacturers rate output under ISO 1217, measured at the compressor's actual intake conditions. They use different reference points, so comparing them directly without conversion leads to sizing errors.

Should I size a compressor for my current tools or for future growth?

Build in headroom for planned expansion rather than sizing exactly to today's tool list. Retrofitting a larger compressor later is more disruptive and often more expensive than sizing with reasonable growth margin from the start.

Does Mumbai's climate actually affect compressor sizing?

Yes. Compressors are rated under ISO 1217's cooler, drier reference conditions, and hot, humid intake air is less dense, which reduces actual delivered air mass compared to the rated figure. Building in a margin for local climate conditions avoids underperformance during peak summer months.

How do I know if my receiver tank is sized correctly?

If your compressor is cycling on and off frequently rather than running in longer, steady stretches, the receiver tank is likely undersized for your demand pattern relative to your compressor's output.

Is it better to oversize a compressor slightly, just to be safe?

Not without cost. An oversized compressor running well below its efficient range wastes energy and can suffer from wet-stacking or inefficient cycling. A margin of 10 to 15 percent above calculated demand is generally enough without moving to a significantly larger unit than needed.