Mouse DPI (CPI)

The sensor reports movement in counts, and DPI on a mouse box means counts per inch. CPI is the accurate name, since no dots are involved. A higher setting moves the pointer further for the same hand movement; it is a sensitivity choice, not a quality measure, and the operating system's pointer speed and acceleration then scale the counts again before the pointer moves.

Measuring the real figure is division: counts over inches. Move the mouse a known distance with the pointer locked and divide. The mouse DPI test offers a bank card's long edge, 85.60 mm by ISO/IEC 7810, or a ruler, and takes the median of at least 3 passes; passes more than 10% apart mean the distance or the line varied. Your own distance carries error: 1 mm off over 10 cm moves the figure by 1%.

The counts are the mouse's own only if the browser hands over raw movement. Pointer lock with the unadjustedMovement option, which MDN documents as disabling operating-system mouse acceleration, gives raw input where the browser supports it; otherwise the counts have passed through acceleration and the test says so. DPI is separate from polling rate, which is how often the mouse reports.