In the game you find the setting under Settings → Mouse & Keyboard → Aiming → Mouse Sensitivity.
Applies to Modern Warfare (2019 and later), Warzone and Black Ops 6. Menu names can differ between titles.
Conversion
Rainbow Six Siege to Call of Duty Sensitivity
In short: Rainbow Six Siege sens times 0.8681. 10 becomes 8.68. For your own value, just edit the sentence below.
How to set it in Call of Duty
What changes when you go from Rainbow Six Siege to Call of Duty
Call of Duty turns 1.15 times further per mouse count than Rainbow Six Siege. That is why the number gets smaller while your mouse distance stays the same.
Call of Duty shows 11.5° less field of view than Rainbow Six Siege by default (80° against 91.49°). The picture looks a little tighter and movement on screen a little faster.
In Call of Duty you can enter up to 2 decimal places. Use all of them to keep the conversion exact.
Common questions
How do I convert Rainbow Six Siege sens to Call of Duty?
Multiply your Rainbow Six Siege sens by 0.8681. 10 in Rainbow Six Siege becomes 8.68 in Call of Duty.
Where do I set the sens in Call of Duty?
Settings → Mouse & Keyboard → Aiming → Mouse Sensitivity. Applies to Modern Warfare (2019 and later), Warzone and Black Ops 6. Menu names can differ between titles.
Why does Call of Duty still feel a bit different?
Rainbow Six Siege: Slider from 60 to 90, vertical (default 60, about 91° horizontal at 16:9). Call of Duty: Slider from 60 to 120 (horizontal, default 80). 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.