There are several ways to calculate.

Everything you need to know is here:

http://en.wikipedia.org/wiki/Root_Mean_Square
I should clarify....Audition's VISUAL meters are peak meters...They measure the absolute peaks and vallies. RMS meters, common in analog equipment, measure average level or RMS. Audition can calculate RMS for a file or segment of a file, but Audition's display will only show peaks.

For a more true representation of percieved loudness, we measure RMS A-weighted (called the equal loudness contour), which essentially means mid frequencies have higher value than very low and high frequencies, so loudness is determained by power, time and frequency, whereas true RMS measures only power over time.

Emmett