In the game you find the setting under Settings → Mouse/Controller → Mouse Sensitivity.
Conversion
Fortnite to Apex Legends Sensitivity
In short: Fortnite sens times 0.2525. 6% becomes 1.52. For your own value, just edit the sentence below.
How to set it in Apex Legends
What changes when you go from Fortnite to Apex Legends
Apex Legends turns 3.96 times further per mouse count than Fortnite. That is why the number gets smaller while your mouse distance stays the same.
Apex Legends shows 25.5° more field of view than Fortnite by default (105.5° against 80°). The picture looks a little wider and movement on screen a little slower. For Apex Legends the field of view is only a rough guide, check it in the game.
In Apex Legends 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 Apex Legends?
Multiply your Fortnite sens by 0.2525. 6% in Fortnite becomes 1.52 in Apex Legends.
Where do I set the sens in Apex Legends?
Settings → Mouse/Controller → Mouse Sensitivity.
Why does Apex Legends still feel a bit different?
Fortnite: Default 80°, check in the game whether you changed it. Apex Legends: Slider from 70 to 110 (about 105 to 106° horizontal at 90). 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.