I keep on trying to fight Sans, except every time I do the Genocide Route, he doesn't ask me if I want a bad time.

How do I fight Sans?

7 Answers 7


Kill 20 monsters in the Ruins before killing Toriel, kill 16 monsters before killing Papyrus, kill 18 monsters before killing Undyne, and then kill 40 monsters before killing Mettaton. You will know you have killed all the monsters when you get an encounter that says "But nobody came." This link might help some more if you are still confused: https://www.eurogamer.net/articles/2018-09-22-undertale-genocide-run-4873

  • This answer adds nothing not already covered by the other answers. Duplicate answers are not helpful and may be deleted. Commented Jul 18, 2019 at 9:17
  • 4
    Personally, I thought that putting the exact numbers would be useful
    – Andy Yao
    Commented Jul 21, 2019 at 21:49

Its easy, here is the answer, you must START from the begining killing everyone before kill toriel. Then repeat in the next areas.

After waking up in the Ruins, progress with the game as usual until you reach the room with a Save Point in the leaves. This area allows the player to have random encounters with monsters.

Stay in this room and walk around to grind random encounters. Whenever you trigger a random encounter, kill all monsters. As you continue to grind, the random encounter rate will gradually drop until you're only getting around one encounter every minute.

And my personal tip: Alt+Tabbing out of Undertale while moving makes you run in place while tabbed out, letting you do other stuff while the main character hunts for random encounters.

Hope it helps :)


You can't fight sans outside a genocide run.


You may have to walk around a lot in a certain area for a while. After you kill monsters for a while, the game will assume it's a genocide route. The save points will say Number left. Once all monsters are killed, the save points will say "Determination." In red text. Hope this helped.


Make sure to kill everything in an area until it says "But nobody came" or until saving just says "Determination" before fighting the boss of that area.

  • 1
    This doesn't quite complete a genocide run. Forgot to mention killing all bosses and attack the monster kid
    – bmarkham
    Commented Aug 26, 2016 at 2:34

Sorry I'm late, but the answer is grind, grind, grind. Kill until you literally aren't allowed to kill in the Ruins, kill Toriel, and repeat this for everywhere else. Note you have to kill all bosses, and mini bosses. It takes forever, so be patient. Or hack. Your choice.


Reset in all areas with area bosses such as ruins, hotland, waterfall, where no NPCs outside of battle are there. Hold up/down or left/right at the same time to find monsters quick and KILL EM ALL! Then keep saving until you see "determenation" instead of the amount of monsters you must kill. Then go the area boss.

The way you can tell if you are on the right track is if Undyne does... something.
(I'll let you find out yourself.)

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