618 is the recurring arc number in Gravity Falls. The number is based on the date June 18, (the day before June 19 from Don't hug me I'm scared) the birthday of series creator, Alex Hirsch, and his twin sister, Ariel. Also, using A1Z26 cypher, the creator's initials spell out: 1 = A; 18 = R; 8 = H (A.R.H.).
The following are instances of 618 used in the show:
- The show's main production code is G618.
- The Mystery Shack's address is 618 Gopher Road.
- The Mystery Shack's cash register is always seen displaying $6.18. With the exception of the occurrence in "Dungeons, Dungeons, and More Dungeons."
- In the attic of 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.
- The introduction page of Journal 3 has the date June 18th, which would be 6-18.
- The number 816, which is 618 backwards, appears multiple times on Bill Cipher's entry page in Journal 2.
- When Stan was in a Colombian prison, his prison ID number was S618.
- The number 618 appears on a post-it note with the reminder, "TK 618 Return 7/3 GF Library."
- Ford's quantum destabilizer is Experiment #618.
- 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.
- A paper on the desk in the underground laboratory is seen with the Greek letter phi, a representation for the golden number: 1.618.
- On Dipper's "Big Mysteries" board the ripped up half of a dollar bill has 618 written under the Eye of Providence.
- When Bill turns his eye into a clock while making a deal with Dipper, the clock appears to display the time 6:18.
- 618 is the episode number of Believe in Yourself that Mabel was watching.
- Old Man McGucket got his hat from a scarecrow on day 618, after he started using his memory erasing device.
- In "Blendin's Game," when Dipper, Mabel, and Blendin fight the Cyclocks, its eye shows a clock that reads 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.
- When Ford is chasing the Cycloptopus in the shack, the cash register displays the word "bill" instead of its usual "618."
- In "The Last Mabelcorn," both 618 and 918 are written on Ford's chalkboard.
- In "Roadside Attraction," one of the phone numbers written on Dipper's arms ends with "0618."
- In "Dipper and Mabel vs. the Future," 816 is written on Ford's Weirdmageddon board.
- In "Weirdmageddon Part 1," 618 is seen on the back and sides of the tanks that Ghost-Eyes and Gideon use to chase after Wendy and Dipper. The number is also seen on the top of Ford's quantum destabilizer case labeled as "Experiment 618."
- During the credits of "Weirdmageddon 3: Take Back The Falls," at the part where it credits Rough Draft Korea Co., Ltd., "618" can be seen.
- On the double page spread with the phrase "My muse was a monster" there are a set of symbols under the largest eye. When converted to the English alphabet they read "Six Eighteen"
- On the double page spread with the blueprints for the portal, a code on the upper left page, when decoded, reads "Calibrate the oscillator to 618."
- The Oregon Parks Department stamped the words "Item #618" onto the last page of the journal.
- In the page of the events of Not What He Seems, Agent Trigger marks the journal "Case #212618".
- During "Old Man" McGucket's Conspiracy Corner Marathon, 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."