Back in 2011, to gather that original data, I wrote some JavaScript to measure both the screen size and the browser window size and save them both to a database as one record. We found that less than 1% of people had a browser window that was exactly as big as their monitor. But that’s a little unfair as it doesn’t account for browser UI and such, so with generous padding allowed (200px), we found that 61% of sessions has a browser window that was pretty close to their monitor size.
That’s still 39% of sessions where the browser window is smaller, though, which is interesting.
via Google Analytics Can Show You Screen Resolution ≠ Browser Window — CSS-Tricks
b19y 