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
Fortnite to Call of Duty Sensitivity
In short: Fortnite sens times 0.8418. 6% becomes 5.05. For your own value, just edit the sentence below.
How to set it in Call of Duty
What changes when you go from Fortnite to Call of Duty
Call of Duty turns 1.19 times further per mouse count than Fortnite. That is why the number gets smaller while your mouse distance stays the same.
The default field of view is almost the same: Fortnite 80°, Call of Duty 80° horizontal. The picture feels alike.
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 Fortnite sens to Call of Duty?
Multiply your Fortnite sens by 0.8418. 6% in Fortnite becomes 5.05 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?
Fortnite: Default 80°, check in the game whether you changed it. 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.