In the game you find the setting under Settings → Keyboard/Mouse → Mouse Sensitivity.
The quicker way is the console: sensitivity 2.604
Conversion
Rainbow Six Siege to CS2 Sensitivity
In short: Rainbow Six Siege sens times 0.2604. 10 becomes 2.604. For your own value, just edit the sentence below.
How to set it in CS2
What changes when you go from Rainbow Six Siege to CS2
CS2 turns 3.84 times further per mouse count than Rainbow Six Siege. That is why the number gets smaller while your mouse distance stays the same.
CS2 shows 14.8° more field of view than Rainbow Six Siege by default (106.26° against 91.49°). 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 Rainbow Six Siege sens to CS2?
Multiply your Rainbow Six Siege sens by 0.2604. 10 in Rainbow Six Siege becomes 2.604 in CS2.
Where do I set the sens in CS2?
Settings → Keyboard/Mouse → Mouse Sensitivity. Or in the console: sensitivity 2.604.
Why does CS2 still feel a bit different?
Rainbow Six Siege: Slider from 60 to 90, vertical (default 60, about 91° horizontal at 16:9). 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.