As Atichity explains there are two ways to get onto Rotten Tomatoes and into the Tomatometer -- to write something for an approved website (like, say, Time Out New York) or to write enough for approved websites that you become an approved critic. As a writer, I've been on both sides of this. Way back when I was an intern for The Village Voice in the mid-2000s, my occasional capsule reviews would get published on Rotten Tomatoes. I didn't have to submit them to the site either; RT automatically added every film review published in The Voice to the Tomatometer. But once I left The Voice, I didn't have enough juice on my own to get my reviews on the site, and my Rotten Tomatoes page lay dormant for a long time. Eventually, after a few years of writing for IFC and assorted other sites, I got approved as a critic. Now I can log in to Rotten Tomatoes and upload my reviews as I write them.
Campbell links to a "Tomatometer Criteria" page that features even more info on just how they make the widgets. For example, online-only outlets looking to join Rotten Tomatoes' network "must achieve and maintain a minimum 500,000 unique monthly visitors." Perhaps even more problematic is this requirement:
"Publications must also show a consistent standard of professionalism, writing quality, and editorial integrity across all reviews and articles."
Ouch, that's tough. Looks like my blog of reviews written as dirty limericks ain't getting added anytime soon (the man from Nantucket is going to be so bummed, too).
Why am I sharing all this ever-so-dry information? Because this is the stuff we don't think about when we haphazardly throw out a movie's "Rotten Tomatoes score" like it's the gospel truth. I think Rotten Tomatoes is a great site and a very useful resource (especially if you're writing a blog about film criticism), but it is important to remember a Tomatometer rating is not necessarily the whole story about the critical consensus surrounding a film. If you started another Tomatometer comprised entirely of outlets that aren't approved by Rotten Tomatoes -- you'd have to call it the Tomatometer, but pronounce it "Toe-MAH-Toe Meter" -- the resultant scores could be very different than the one that gets attached to movies. The outlets you include make a huge difference. If you poll horror sites, for example, that might affect the scores of horror movies (whether more positively or negatively, I'm honestly not sure). So keep this in mind when you're calling movies fresh or rotten. Always be mindful of just who is hurling those tomatoes.