The number a compressor actually has to deliver
Every tool on a line takes air at a rate, and that rate is what a compressor has to keep up with. The figure that matters is free air delivery (FAD): the volume of air a compressor puts out per minute, measured back at atmospheric pressure and temperature, at whatever working pressure the tools need. A compressor sized against FAD in litres per minute (l/min), with the imperial equivalent in cubic feet per minute (cfm) alongside it, will keep a tool running at the pressure it was designed for. One sized against a smaller or differently measured figure will not, no matter how big the tank looks.
The calculator's job is to turn a list of tools, how many run together and the pressure they need into that single delivered-air figure, with the arithmetic shown at every step so it can be checked and adjusted for a different set of tools. It can only work from what it is given, so a tool consumption figure copied from the wrong page, or a pressure guessed rather than read off the tool's own data plate, produces a wrong answer with the same confidence as a right one. Treat the result as a starting point for a quote.
What the calculator asks for, and why each answer changes the result
Three inputs do the work: the tools in use, how many run at the same time, and the working pressure. Each one moves the answer in a different way, and it helps to know which.
- Tool consumption at a stated pressure. A tool's air use is only meaningful next to the pressure it was measured at, because the same tool draws more air at a higher pressure. A figure quoted without its pressure is not comparable to anything.
- Simultaneous users, not tools owned. A workshop with six air tools on hooks around the walls might never run more than two at once. Sizing for six would mean paying for capacity that sits idle; sizing for two, when three sometimes overlap, means a tool starves the moment a second one starts.
- Working pressure. Raising the pressure a tool runs at increases the power a compressor has to put into producing the same volume of air, so the pressure figure entered is not a formality. It is worth reading off the tool itself or its data sheet.
The calculator returns the required free air delivery in litres per minute and cubic feet per minute, the allowance it has applied for overlap, and the displacement figure a compressor would need to show on its listing to meet that demand once efficiency losses are accounted for. That last distinction is explained fully on Displacement and Free Air Delivery, because it is the single most common place a sizing goes wrong after the demand figure has been worked out correctly.
Why the tools you run together decide the size
Two tools on a line rarely draw at their full stated rate at the same instant, because a sander is lifted off the panel while the operator repositions it, or a grinder pauses while a cut is checked. That gap is what a usage factor is trying to capture: a multiplier applied to the sum of simultaneous consumption to account for tools that overlap but do not both run flat out for the whole period. Published guidance on this multiplier is not consistent. Some manufacturer sizing tables apply a factor of around 1.2 to 1.25 for tools expected to run near-continuously alongside each other, on the reasoning that overlap is the normal case. Other guidance applies a factor closer to 4× for tools used briefly and unpredictably, such as a blow gun or an impact wrench that fires for a few seconds at a time, on the reasoning that peak demand during that burst matters more than the average over the hour. Neither number is wrong; they answer different questions, about tools that run together for most of a shift and tools that spike briefly and separately, and the right one depends on which pattern actually describes the workshop being sized.
This is also where duty cycle (the proportion of time a compressor runs under load, expressed as running time per hour or as a percentage) comes back into the calculation. A tool with a low duty cycle can be under-counted by a simple sum of nameplate figures, and over-counted by a flat usage factor applied without thought to how the shop actually works. The worked examples below show both patterns.
A bodyshop panel bay, worked through
Suppose a panel bay runs two DA sanders and a spray gun, with the sanders used more or less continuously through a respray and the gun used in shorter, separate bursts once the panel is prepped. If each DA sander is rated at, say, 250 l/min at 6.2 bar and the spray gun at 350 l/min at 3.5 bar, a simple sum of all three running flat out would overstate demand, because the gun and the sanders are rarely both open at once. Sizing for the two sanders running together, with an allowance for the occasional overlap with the gun, gives a working demand well below the sum of all three nameplates. That is the number the calculator carries through to a displacement figure and, from there, to a compressor size worth quoting on.
A fabrication bay, worked through
Say a fabrication bay runs an angle grinder and a die grinder, with the die grinder drawing a lower rate but running for longer stretches during fine work, and the angle grinder drawing more but only in short cuts. If the angle grinder is rated at, say, 450 l/min at 6.2 bar and the die grinder at 170 l/min at 6.2 bar, and the two are genuinely used one at a time rather than together, sizing for the higher of the two plus a margin for the moment they briefly overlap gives a more accurate demand than adding both nameplates. Adding them regardless of overlap is the mistake that leads a workshop to buy a larger compressor than the work ever calls for.
Where a tool list sizing goes wrong
- Quoting displacement as if it were delivered air. A compressor's headline cfm figure is very often displacement, the volume swept by the piston or screw element before losses, which is always higher than what actually reaches the tool. Sizing against displacement rather than free air delivery is the single most common way a workshop ends up short of air on the shop floor despite a compressor that looked big enough on paper.
- Reading a tool's consumption at the wrong pressure. The same tool uses more air at a higher pressure, so a figure lifted from a spec sheet without checking the pressure it was measured at can understate demand by a wide margin.
- Sizing for every tool owned. This is the mistake the usage factor exists to correct, and skipping it produces an answer with no relationship to how the workshop actually operates.
- Ignoring how the demand figure feeds the rest of the system. The free air delivery figure also sets the minimum size of the receiver and shapes whether a written scheme of examination applies, covered in full in Sizing the Air Receiver, and the 250 Bar-Litre Line.
Turning the litres-per-minute figure into a compressor
A free air delivery figure in l/min is a specification. The next step is matching it to a compressor type suited to the duty, whether that is a piston unit for intermittent bodyshop work or a rotary screw for continuous fabrication line use, set out in Piston, Rotary Screw, Scroll or Vane. Once a candidate compressor is in view, its own listing needs reading the same way the calculator reads a tool: check whether the headline figure is displacement or free air delivery, and at what pressure it was measured, before comparing it against the demand figure worked out here.
A sizing figure worked out this way is also what a distributor needs to quote against. Get Compressor Quotes sets out what to send and what to ask once the figures are in hand.
Where to go next
The trade terms used here, free air delivery, displacement, duty cycle and the rest, are collected with short definitions in the Compressed Air Glossary, useful for checking a term against a manufacturer's own listing. Deciding whether a specific system meets its legal duties is a matter for a competent person's assessment of that system.