Why oversizing costs more than the machine
An oversized compressor short-cycles: it reaches pressure quickly, unloads, and starts again. That wastes energy, increases wear, and on screw machines shortens the life of the airend. An undersized one runs continuously, cannot recover pressure, and starves the tools at the end of the line. Sizing from measured demand and a stated duty cycle keeps quotations comparable, because two suppliers proposing different CFM are not proposing the same machine.
Compressed air is also the most expensive utility in most plants per unit of useful work, so the headroom figure is a commercial decision as much as a technical one. Over-provisioning for growth you can predict is sensible; over-provisioning for growth you cannot is paid for every hour the machine runs.
Frequently asked
Should I add up the CFM printed on every tool?
No. That total assumes every tool runs at once, continuously. Multiply each tool by how many run together and by its realistic duty cycle, then add headroom.
What duty cycle should I use?
Use measured figures where you have them. As a starting point, intermittent tools such as impact wrenches sit low, while grinders, sanders, and continuous blowing sit much higher.
How much headroom is sensible?
25-30% is the common range. It covers leakage, which grows over time in every system, plus the tools that get added after commissioning.
Does pressure change the requirement?
Yes. Delivering the same work at a higher pressure costs more power, and every unnecessary increase in system pressure raises both consumption and leakage.
Use the figure to shortlist machines in the right class, then request a quotation with your tool list, duty cycles, and working pressure attached.