Page-in & page-out

Using Activity Monitor to read System Memory and determine how much RAM is being used

The only thing you need to track is page outs, which are an indication that you're maxing out the RAM you have installed. To determine if you can benefit from more RAM, launch Activity Monitor and click the System Memory tab at the bottom to check your page outs. Page outs are cumulative since your last restart, so the best way to check is to restart your computer and track page outs under your normal workload (the apps, browser pages and documents you normally would have open). If your page outs are significant (say 1GB or more) under normal use, you may benefit from more RAM. If your page outs are zero or very low during normal use, you probably won't see any performance improvement from adding RAM.

GGJstudios said:

Using Activity Monitor to read System Memory and determine how much RAM is being used

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I would have posted that too, but it seemed too obvious somehow, since it shows up in the top of search results. Hmm, should learn to search less, Google already thinks I am a spambot.