In the game you find the setting under Settings → Mouse & Keyboard → Sensitivity X and Y.
Set X and Y sensitivity to the same percentage.
Conversion
Rainbow Six Siege to Fortnite Sensitivity
In short: Rainbow Six Siege sens times 1.0313. 10 becomes 10.3%. For your own value, just edit the sentence below.
How to set it in Fortnite
What changes when you go from Rainbow Six Siege to Fortnite
Rainbow Six Siege turns 1.03 times further per mouse count than Fortnite. That is why the number gets bigger while your mouse distance stays the same.
Fortnite 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 Fortnite you can enter up to 1 decimal place. Use all of them to keep the conversion exact.
Common questions
How do I convert Rainbow Six Siege sens to Fortnite?
Multiply your Rainbow Six Siege sens by 1.0313. 10 in Rainbow Six Siege becomes 10.3% in Fortnite.
Where do I set the sens in Fortnite?
Settings → Mouse & Keyboard → Sensitivity X and Y. Set X and Y sensitivity to the same percentage.
Why does Fortnite still feel a bit different?
Rainbow Six Siege: Slider from 60 to 90, vertical (default 60, about 91° horizontal at 16:9). Fortnite: Default 80°, check in the game whether you changed it. 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.