Method, sources and limits
Physical constants, published tool consumption figures, and arithmetic on the two. No dataset, nothing fetched, nothing that can go stale.
The calculation
- Sustained demand per tool. Nameplate CFM times duty cycle for an intermittent tool; the full nameplate figure for a continuous one, because there is no idle time for the tank to recover in.
- Combine. Sum the sustained demand of the tools running together.
- Add margin, 30 percent by default, for hose and fitting losses, leaks, and optimistic published ratings.
- Select the smallest machine that covers it, in delivered SCFM at 90 PSI.
- Tank runtime where the compressor falls short: free air stored is tank volume times the pressure band divided by atmospheric pressure, and it drains at the rate demand exceeds supply.
Constants and sources
- 14.696 PSI standard atmospheric pressure at sea level.
- 0.133681 cubic feet per US gallon, which is 231 cubic inches divided by 1728.
- Pressure scaling. Delivered air scales with absolute pressure, so SCFM at a new pressure is the original times (P1 + atmospheric) over (P2 + atmospheric).
- Volumetric efficiency. 70 percent single stage, 80 percent two stage, the conventional figures relating displacement to delivered air.
- Tool consumption. Published averages for each class of tool at its rated pressure, with duty cycles reflecting normal use.
What the method cannot see
- Your specific tools. Class averages hide real variation. The plate on your own tool is the number that counts.
- Hose and fitting losses. Covered crudely by the margin. A long run of narrow hose, or undersized quick-connects, can cost a surprising amount of pressure at the tool.
- Altitude and temperature. Both affect real output, and neither is modelled. A compressor at 5,000 feet delivers noticeably less than at sea level.
- Compressor duty cycle. Many consumer machines are not rated for continuous running, and sizing one to run flat out is how they fail. The margin helps; it is not a substitute for checking the machine's own duty rating.
- Moisture. Compressed air carries water, which matters enormously for spraying and for tank life, and is a separate subject from sizing.
Why there is nothing to refresh
Atmospheric pressure and the volume of a gallon do not change. Tool consumption figures and compressor size ranges drift on a timescale of many years, and are worth a look every couple of years rather than on a schedule. This site is intended to be as correct in five years as it is today without anyone touching it.
Frequently asked questions
How accurate are these air compressor calculations?
The arithmetic is exact. The uncertainty is in the inputs: published tool consumption is a class average, and real tools vary. Use your own tool ratings where you have them, and keep the safety margin rather than sizing to the exact calculated figure.
Why add a safety margin to compressor sizing?
Hoses and fittings lose pressure, systems leak, and published tool figures are measured favourably. A compressor running permanently at its limit also runs hot and never rests, which shortens its life. 30 percent is the conventional allowance.
Does altitude affect air compressor output?
Yes, and it is not modelled here. Air is less dense at altitude, so a compressor moves less mass per stroke and delivers less. At a few thousand feet the reduction is meaningful enough to matter when a machine is already marginal.