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Undertale Wiki

The fun value is a mechanic in Undertale whereby a random number, selected on Reset, between 1 and 100 (the "fun value") determines the occurrence of several rare events in the game. Many of these events link to W. D. Gaster, the royal scientist before Alphys.


Wrong Number Song

Wrong Number Song screenshot.png

This song plays in Snowdin if the fun value is within the range 2 to 39. The phone call begins asking for somebody whose name starts with the letter "G." The caller then recognizes that they dialed the wrong number, and proceeds to sing the Wrong Number Song. This phone call is speculated to be for Gerson, Glyde, Gyftrot, Grillby, or W. D. Gaster. Even in the Japanese localization, the initial greeting is in English so as to give no further hints to what character it could be.

There is a known glitch with the Wrong Number Song where the avatar of whomever the protagonist spoke to last shows up as the ID of the call. This glitch was patched in version 1.001.

Sans's Call

This occurs when the fun value is between 40 and 45. Sans calls the protagonist in Snowdin. He asks if their refrigerator is running, and his response varies depending on what answer is chosen.

Alphys's Call

This occurs when the fun value is between 46 and 50. Alphys calls the protagonist in Snowdin, ostensibly to order a pizza. She says that she texts the protagonist the toppings but instead ends up sending ASCII art of an anime cat girl (possibly Mew Mew). The protagonist's old phone cannot receive this, so it is read out character by character.

Nightmare Mode

Monster Kidz Word Search screenshot Nightmare.png

This occurs when the fun value is between 56 and 57. On the unsolvable word search in Snowdin, the puzzle additionally contains a bear-faced snowman called "Nightmare."

Gaster Followers

These monsters may appear near Hotland elevators if the fun value is between 61 and 63, though there is only a 50 or 20% chance of them appearing anyway. They talk about W. D. Gaster, the royal scientist before Alphys. They reset the fun value to 0 on appearance, so they do not reappear when re-entering.

Sound Test Room

By traveling upwards from the "Box Road" intersection in Snowdin Forest, if the fun value is set to 65 and Papyrus has been fought, there is an additional 50% chance for a hidden "sound test" room to appear. The menu itself contains three very short looping tracks and a longer track that, once picked, cannot be deselected. None of these tracks are used anywhere else in the game.

Happy Town

A short, cheery jingle with the filename "mus_st_happytown".

Meat Factory

A short robotic track with the filename "mus_st_meatfactory".

Trouble Dingle

A short series of seemingly random robotic noises with the filename "mus_st_troubledingle".

Gaster's Theme

Upon selecting this track, no other songs can be selected. After listening to Gaster's Theme for a period, the room ceases playing music and displays a screen that says "Thanks for your feedback! Be seeing you soon!". The protagonist is then returned to the room with the fishing pole. The file is titled "mus_st_him".

Fake Hallway

Mysteryman overworld 2.png

The fake hallway is a hallway in Waterfall that appears if the fun value is 66. Once this hallway is exited by the protagonist, it disappears.

The Mystery Man appears behind a grey door (with an additional 10% chance) in this hallway. Once interacted with, he fades away to snd_mysterygo. The room is called room_mysteryman.

Clam Girl

Clam Girl overworld.png

Clam Girl appears if the fun value is between 80 and 89 and the protagonist is not on the Genocide Route. She talks about the protagonist meeting "Suzy". If the fun value is exactly 81 on the Nintendo Switch version, she becomes a Goner NPC with additional dialogue.

Goner Kid

Goner Kid overworld.png

A gray version of Monster Kid (spr_mkid_goner) can appear in Room 91 (room_water7) on a dock right before the room where Undyne first throws spears at the protagonist if the fun value is 90 or greater.


The fun value can be modified using a text editor on line 36 of file0 and the "[General]" header of the undertale.ini file. These files are stored at the following locations by default:

  • Windows:
    • Windows XP: %SystemDrive%/Documents and Settings/%USERNAME%/Local/Application Data/UNDERTALE
    • Windows Vista/7/8/10: %LOCALAPPDATA%/UNDERTALE
  • Linux:
    • DRM-free: ~/.config/UNDERTALE_linux
    • Steam: ~/.config/UNDERTALE_linux_steamver
    • Steam (later versions): ~/.config/UNDERTALE
  • Mac: ~/Library/Application Support/com.tobyfox.undertale/

The presence of fun events is dictated primarily by line 36 of file0, internally referred to as global.flag[5]. Several of the events additionally require the named value in undertale.ini to match that of global.flag[5].

Fun Value Chart

Fun Value Chart.png


  • Several fun events are triggered regardless of the fun value if debug mode is enabled through game modification.
  • Before v1.001, the fun value was capitalized as "Fun value" in several code checks and thus made those events (Nightmare Mode, Gaster Followers, the Sound Test room, the fake hallway, and Goner Kid) inaccessible.