Compressed Air

What is the starting point?

Don't be silly, it's obviously just the same as volume of whatever the container holding it.
Del

Good point.

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As Del said.

But if some type of change is occurring, use the Ideal Gas Law:

http://en.wikipedia.org/wiki/Ideal_gas_law

compressor cylinder displacement x rpm will give you the volume@ zero pressure.

compressed volume is best described in units atmospheric pressure @sea level. 14.7 pounds per square inch ? compressing two atmospheric volumes of air will double the air pressure. that relates to displacement x rpm. if you need to know the volume/ pressure while using the compressor, you'll need to know the output volume needed. intake volume minus output volume should give you the compressor size you need.