DPI on a mouse means counts per inch: how many movement counts it reports for each inch it travels. Counts per inch, or CPI, is the accurate name.
- To measure it, move the mouse a known distance with the pointer locked and divide the counts by the inches.
- The browser must hand over raw movement, skipping the system's pointer acceleration, or the figure is the system's and not the mouse's.
- Do at least 3 passes and take the median. Passes more than 10% apart mean the distance or the line varied.
The short answer
Lay a ruler, or a bank card's long edge (85.60 mm by the ISO/IEC 7810 standard), on the desk. Lock the pointer in the test, move the mouse exactly that distance along a straight edge, and repeat 3 times. The median is your mouse's real counts per inch. Set it against the DPI you chose in the mouse's software.
The arithmetic
Counts divided by inches. A pass of 3,370 counts over a bank card's 85.60 mm, which is 3.37 inches, is about 1,000 counts per inch. The distance carries your own error: 1 mm off over 10 cm moves the figure by 1%, so a few percent from the setting can be the measurement as easily as the sensor.
Raw or accelerated movement
With the pointer locked, the browser hands the page each movement. Where it offers raw movement (the unadjustedMovement option), the counts skip the system's pointer acceleration and are the mouse's own. Where it does not, the counts have passed through acceleration and scaling and can differ from the mouse's. The test says which you got. Chrome and Edge offer raw movement on the systems they support it on.
If the result says accelerated, turn off the system's acceleration (Enhance pointer precision on Windows) and run again.
Getting passes that agree
- Move along a straight edge at an even speed, and start and stop exactly on the marks.
- Keep the mouse flat. A pass with a lift in it reads short.
- Check the mouse's software has not switched to a different DPI stage.
- No ruler? Enter the screen's diagonal and a ruler is drawn from it, then check it with a card held to the screen.
Terms used here
Questions
- Why does my mouse measure a different DPI from its setting?
- A sensor can deviate from its nominal setting, and the measured distance carries your own error. Measure several passes on a straight edge before reading a few percent as the sensor's.
- Does the test measure polling rate too?
- No. Polling rate is how often the mouse reports, which the mouse test measures. DPI is how far each count is; the two are separate settings.