618

618 is the recurring arc number in Gravity Falls. The number is based on the date June 18th, the birthday of series creator, Alex Hirsch, and his twin sister, Ariel.

Instances
The following are instances 618 is used in the show:
 * The show's main production code is G618.
 * E.g., Dipper and Mabel's rough model sheets are labeled "618G99.01."
 * E.g., An episode production code starts with 618, such as "618G-107" for "Tourist Trapped."
 * The Mystery Shack's address is 618 Gopher Road.
 * Every cash register in Gravity Falls is always seen as reading $6.18
 * In the attic in the Mystery Shack, an old game has the score 8190 which, when flipped, reads 0618.
 * In Tumbleweed Terror, the score on the barrel is 618.
 * Gideon's school ID number is 618.
 * The Whack a Customer game in Dipper and Mabel's room has the number 819 on it, which, when turned upside down, is 618.
 * One of the blocks a baby chews on in the television show, Baby Fights, has the numbers 6, 1, and 8 on it.
 * The introduction page of book Journal 3 has the date June 18th, which would be 6-18.
 * When Stan was in a Colombian prison, his prison ID number was S618.
 * The number 816, which is 618 backwards, appears multiple times on Bill Cipher's page in Journal 2.
 * When Bill becomes angry, his eye flashes six symbols. The third, fourth, and fifth in the sequence are Thai numerals that spell out the numbers 6 1 8.
 * The number 618 appears on a post-it note with the reminder, "TK 618 Return 7/3 GF Library."
 * A paper on Stan's desk in his underground laboratory is seen with the Greek letter phi, a representation for the golden number: 1.618.
 * On Dipper's board of "Big Mysteries" the ripped up half of a dollar bill has 618 written under the Eye of Providence.
 * When Bill Cipher turns his eye a clock while trying to make a deal with Dipper, the clock appears to display the time 6:18.
 * 618 is the episode number of the show Mabel watches.
 * McGucket got his hat from a scarecrow on day 618 after he started using his memory erasing device.
 * In the episode "Blendin's Game," when Dipper, Mabel and Blendin fight the Cyclocks, the Cyclocks' eye shows a clock that says 6:18.
 * Stan's prisoner number in "Not What He Seems" is 061800.
 * Stan's faked car crash occurred in a ditch next to Highway 618.

Trivia

 * During Old Man McGucket's Conspiracy Corner, McGucket mentions the recurring number in the series, saying that it's been haunting his dreams, and even pointing out some of the times it was cited. He comes to the conclusion that "it's a clue from a creature from another dimension that [they are] all just drawings on a page."