In the game you find the setting under Settings → Keyboard/Mouse → Mouse Sensitivity.
The quicker way is the console: sensitivity 1.515
Conversion
Fortnite to CS2 Sensitivity
In short: Fortnite sens times 0.2525. 6% becomes 1.515. For your own value, just edit the sentence below.
How to set it in CS2
What changes when you go from Fortnite to CS2
CS2 turns 3.96 times further per mouse count than Fortnite. That is why the number gets smaller while your mouse distance stays the same.
CS2 shows 26.3° more field of view than Fortnite by default (106.26° against 80°). The picture looks a little wider and movement on screen a little slower.
In CS2 you can enter up to 3 decimal places. Use all of them to keep the conversion exact.
Common questions
How do I convert Fortnite sens to CS2?
Multiply your Fortnite sens by 0.2525. 6% in Fortnite becomes 1.515 in CS2.
Where do I set the sens in CS2?
Settings → Keyboard/Mouse → Mouse Sensitivity. Or in the console: sensitivity 1.515.
Why does CS2 still feel a bit different?
Fortnite: Default 80°, check in the game whether you changed it. CS2: 90° at 4:3, about 106° at 16:9. Match the field of view as closely as you can; then the converted sens feels most natural.
Does this also apply to scopes?
No. The values are for hipfire. Zoom and scope sensitivities are set separately in the game.
Should I change my DPI?
No. Keep your DPI and only change the sens in the game. That keeps your feel the same everywhere.