Here’s another incredibly useful tool created by the Diablo 4 community! SSHH90 calculated the pattern for World Boss spawns since the game’s launch and extrapolated when they’ll be spawning in the future! There was a lot of data to draw from and the google sheet seems very accurate. The estimated times match the actual ones completely so far! Not only that, but you can also see which boss is spawning.
You’ll have to zoom out your browser a little to get a better overview. Also, the local timezone doesn’t seem to be accurate (at least here in CET-land), so you may have to calculate from ET to your own as well. You can use one of the many timezone converters (this is a simple but easy to use one) to get it to your local time. The next spawn as of this post is at 18:17 UTC/20:17 CET/11:17 PST.
Head on over to SSHH90‘s spreadsheet and always be informed of when a World Boss is spawning!
