In the game you find the setting under Settings → General → Mouse → Sensitivity: Aim.
Conversion
Rainbow Six Siege to Valorant Sensitivity
In short: Rainbow Six Siege sens times 0.0819. 10 becomes 0.819. For your own value, just edit the sentence below.
How to set it in Valorant
What changes when you go from Rainbow Six Siege to Valorant
Valorant turns 12.22 times further per mouse count than Rainbow Six Siege. That is why the number gets smaller while your mouse distance stays the same.
Valorant shows 11.5° more field of view than Rainbow Six Siege by default (103° against 91.49°). The picture looks a little wider and movement on screen a little slower.
In Valorant 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 Valorant?
Multiply your Rainbow Six Siege sens by 0.0819. 10 in Rainbow Six Siege becomes 0.819 in Valorant.
Where do I set the sens in Valorant?
Settings → General → Mouse → Sensitivity: Aim.
Why does Valorant still feel a bit different?
Rainbow Six Siege: Slider from 60 to 90, vertical (default 60, about 91° horizontal at 16:9). Valorant: 103° horizontal. 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.