The formula
Take the DPI set in your mouse software and multiply it by the sensitivity slider in the game. 800 DPI at 0.35 in Valorant is 280 eDPI. 400 DPI at 2.0 in CS2 is 800 eDPI.
The point of the number is that different splits collapse into the same result. 1600 DPI at 0.5 and 400 DPI at 2.0 are both 800 eDPI, and in the same game they turn you exactly the same distance per centimeter of mouse travel.
Why players use it
Pro settings lists quote eDPI because it lets you compare two players inside one game without caring how they set up their mouse. A 400 DPI player and a 1600 DPI player sit on the same scale.
The ranges are narrow enough to be useful. Valorant pros mostly sit between 200 and 360 eDPI. CS2 pros mostly sit between 640 and 1040 eDPI. If your number is far outside those bands you are not wrong, but you are making a choice most players at the top did not make.
Where eDPI breaks
eDPI does not carry between games. Each game turns a different number of degrees per mouse count, so the same eDPI feels completely different from one title to the next. 280 eDPI is ordinary in Valorant and unplayably slow in Overwatch 2.
It also assumes nothing else is touching your input. Mouse acceleration, a Windows pointer speed away from the default notch, or a game with its own acceleration curve all make the number lie.
When you need a sensitivity that means the same thing everywhere, use cm/360, which measures the physical distance your hand moves. That is what the sensitivity converter uses under the hood.
How to change it
Raise DPI in the mouse software or raise the sens in the game, and eDPI goes up. Move both in opposite directions and it stays put. Most players keep DPI fixed at 400, 800, or 1600 and adjust the in-game slider, so a change is one number.
The eDPI calculator does the multiplication, shows your cm/360, and tells you what the same turn is in six other games.