The formula
eDPI = DPI × sensitivity · cm/360 = 360 ÷ (yaw × sensitivity) ÷ DPI × 2.54
Where it comes from
Neither DPI nor in-game sensitivity means anything on its own: 400 DPI at sensitivity 4 moves exactly like 1600 at sensitivity 1. What matters is the product, and out of it comes the figure you actually feel, which is the centimetres of mousepad a full turn takes.
How to work it out by hand
- Multiply the mouse DPI by the in-game sensitivity
- For centimetres per turn, divide 360 by the yaw times the sensitivity
- Divide by the DPI and multiply by 2.54 to go from inches to centimetres
What is worth knowing
Yaw is how many degrees the camera turns per mouse count, and it is 0.022 in most games built on the Source engine. On which sensitivity to pick: professional tactical shooter players usually sit between 30 and 50 cm per turn, which forces aiming from the arm and gives far more precision at long range. Above 20 cm you are aiming from the wrist, quicker to react but less precise. What matters is not the exact figure but not changing it: muscle memory takes weeks to settle.