What free air delivery means, and why your compressor is sized by it
Free air delivery (FAD) is the volume of air a compressor actually delivers per minute, measured at atmospheric pressure and temperature, at a stated working pressure. It is the figure that tells you what reaches a tool, because it already accounts for the air lost to compression inefficiency, valve leakage and the volume the piston or screw element does not fully sweep.
Why displacement reads higher than the air you get
Displacement is the volume swept by the piston or screw element per minute, before any of those losses are taken out, so it is always higher than the free air delivery of the same machine. A compressor advertised on its displacement figure looks bigger than it will perform, because the number printed on the box and the air that reaches the tool are two different measurements of the same machine. Sizing against displacement rather than free air delivery is a common way a workshop ends up short on the day two tools run at once, because the gap between the two figures does not show up until the compressor is asked to supply load it was never built to deliver at that pressure. The displacement and free air delivery guide works through how much of the advertised figure on a typical listing actually reaches the tool.
Matching free air delivery to your tools
Each tool states its own air consumption at a given pressure, and adding those figures up, at the same working pressure and with an allowance for how much of the time each tool is actually drawing air, gives you the free air delivery your compressor needs to supply. The Air Demand Calculator runs that arithmetic on the tools and figures you enter, in your own browser, and shows the total in litres per minute and cubic feet per minute with every step worked out. It can only be as good as the figures put into it, and a distributor or the compressor maker is who confirms the final size against your own system.
How different air tools state their consumption
A consumption figure on its own tells you little. Read it alongside the pressure it was measured at and how continuously the tool actually draws air, both flagged below.
| Tool type | Typical use | What the figure assumes |
|---|---|---|
| Angle grinder | Continuous cutting or grinding work | Full consumption applies for as long as the trigger is held, since the tool draws air continuously while cutting. |
| DA sander (dual-action) | Bodywork finishing, panel prep | The figure is usually stated at one specific pressure; run the tool below that pressure and both its air use and its cutting performance drop together. |
| HVLP spray gun | Paint application, primer, topcoat | Consumption shifts with fan and fluid settings, and most guns are rated at the pressure measured at the gun inlet. |
| Impact wrench | Wheel and fastener work | Catalogue figures are usually an average over intermittent trigger pulls, so the usage factor you apply matters more here than for a tool run continuously. |
| Die grinder | Fine detail grinding, deburring | Smaller bore and lower displacement than an angle grinder, but often held on for long continuous stretches on fabrication work. |
| Blow gun | Cleaning down, drying, clearing swarf | Depends heavily on nozzle size and how long the trigger is held, so a single catalogue figure can understate what actually gets used on the shop floor. |
This sets out what to look for on a tool's own datasheet, not measurements taken by AirCompressors.uk. Once you have the real figures for your own tools, the Air Demand Calculator works out the free air delivery you need from them, including an allowance for leaks and growth you set yourself.
The usage factor, simultaneous use and the growth allowance
Three separate multipliers get applied when a compressor is sized, and each one answers a different question. Mixing them up is how a shop ends up with a compressor that is either too small for the busiest ten minutes of the day or twice the size the work ever needs.
What each multiplier is actually asking
The usage factor asks how much of the time a single tool's trigger is actually down, out of the time someone is holding it. A grinder finishing a weld is close to its full rated free air delivery for as long as the trigger stays pulled. A blow gun looks trivial on paper but, left open to clear swarf rather than used in short bursts, can end up drawing air for a larger share of the shift than a tool with a much higher headline figure.
Simultaneous use asks how many tools are likely to be drawing air at the same moment. A bodyshop with four sanders rarely runs all four together, so sizing for four running at once builds in capacity nobody uses.
The growth allowance answers a third question again, unconnected to either tool: how much spare capacity to leave for leaks that develop over time and for tools bought after the compressor is installed. It is added once, to the total, after the usage factor and simultaneous use have already been worked through tool by tool.
Why folding all three into one number goes wrong
Some rule-of-thumb guides bundle usage, simultaneous use and growth into a single multiplier applied to a shop's total tool count. That can work for a rough estimate, but it goes wrong the moment it is applied on top of tool figures that already carry their own usage factor: the same allowance gets counted twice, and the resulting compressor is oversized for no reason anyone can point to. Keeping the three questions separate, tool by tool, is what stops that from happening.
The Air Demand Calculator keeps the three apart deliberately. Each tool row takes its own usage factor as a percentage of the working hour, the total is built from every tool running at the same time, and the growth allowance is a single editable field applied once at the end, with a note that published sizing guides add between 20% and 25% and a way to set your own figure instead. The result is arithmetic on the numbers entered, and a distributor or the compressor maker is who confirms the final size against the system you actually run.
Worked example: sizing a bodyshop running two DA sanders and a spray gun
A bodyshop running two DA sanders on the same panel and a spray gun on the top coat is a common case, and it sets the compressor size at the moment all three are drawing air together. A single DA sander running alone rarely challenges a compressor sized for a small workshop. Two sanders and a gun running at the same time is a different question.
Finding the moment that sets the size
The sequence in a typical repair has one sander smoothing filler while a second follows behind on a different panel, and the spray gun goes on only once both sanders have stopped. Sizing for that sequence would size the compressor too small, because the demand that matters is the worst case a shift is likely to produce: both sanders held down together while a second job is prepped, or a sander running while the gun is already charged and waiting on the trigger. The compressor has to cover whichever combination actually happens on the floor.
Reading each tool's own figure, at its own pressure
Each DA sander and the spray gun states its own air consumption in the maker's manual or on a data plate, and each states it at a particular working pressure, which is not always the same pressure across all three tools. A sander's figure assumes the tool is actually cutting, and a spray gun's figure depends on the nozzle fitted and the pattern being sprayed. None of these figures can be added together until they are all read at the same pressure, because a tool's air consumption does not move in a straight line as pressure changes.
Usage factor and the allowance, then the total
Neither sander runs flat out for the whole time it is in someone's hand, and the usage factor accounts for that: a sander held on a panel for the length of a shift is still only drawing air for a share of each working hour. The same applies to the spray gun, which is only drawing air while the trigger is pulled. Once each tool's figure, pressure and usage factor are set, the demand for the moment the sanders and the gun run together is the sum of the three, with a leak and growth allowance added on top for the compressor to still cope a few years on. The Air Demand Calculator takes a row for each tool, its consumption, pressure, quantity and usage factor, and returns the total in litres per minute and cubic feet per minute with the allowance shown separately, working from whatever figures are entered. It cannot check that those figures match what the tools actually use, so the result is only as good as the numbers put into it, and a distributor or the tool maker's own data is still the way to confirm them for a particular bodyshop.
What the total is compared against
The figure that comes out the other end is a free air delivery requirement, and it has to be checked against a compressor's free air delivery at that same working pressure.
Worked example: a fabrication shop with angle grinders, a plasma cutter and a blow gun
A fabrication shop with two angle grinders, a plasma cutter and a blow gun on one line rarely needs sizing for what each tool draws on its own. The moment that decides the compressor size is the one where several of them run together: a grinder finishing a weld while the plasma cutter is set up on the next job, and someone reaches for the blow gun to clear swarf off the bench.
Reading each tool's own figure first
The angle grinder, the plasma cutter and the blow gun each state their own free air delivery (the volume of air a tool uses per minute, measured at atmospheric pressure) on a data plate or in the tool's manual, and each states it at a working pressure that may not match the others. A grinder held on a weld draws close to its full rated figure for as long as the trigger is down. A plasma cutter's duty cycle (the proportion of running time it spends actually drawing air under load) is usually lower, because the arc itself cycles on and off during a cut. A blow gun looks trivial next to either, but left open to clear swarf for a minute or two rather than in short bursts, it can draw air for longer over a shift than either of the other two.
Bringing the figures to one pressure before adding them
Before the three figures can be added together they need to be stated at the same working pressure, because a tool's free air delivery at one pressure does not scale in a straight line to another. The Compressed Air Unit Converter handles that conversion. Once every figure sits at the same pressure, the tools running at the same moment can be summed and carried through the usage and growth allowance set out above. Entering the same three tools as rows in the Air Demand Calculator, with each tool's own consumption, its stated pressure, how many are in use and the share of time each one runs, does that sum for a fabrication shop's own numbers.
Why the plasma cutter is worth checking twice
A plasma cutter starved of air mid-cut does not stall the way a grinder does. It produces a rougher edge and a slower cut, a fault that often gets blamed on the consumable or the material. Sizing for the moment the grinder and the plasma cutter run together is what keeps that from happening.
Put your own tools into the sizing arithmetic
Enter your tools, quantities and usage, and see the total demand in litres per minute before you check it against a listing.