Scroll Wheel Test — Up, Down and Middle Click
Scroll wheels fail in a specific way: the wheel is rotated by an optical or mechanical encoder, and when it gets dirty or worn you start seeing missed notches, backwards notches, or a wheel that only registers in one direction. This test counts each direction separately and shows the gap between notches.
Quick answer: Roll the wheel one notch at a time. Each notch should add exactly one to the matching counter; missed notches, jumps of two, or a direction that never counts all point to the encoder — often fixable by blowing out the wheel or replacing the encoder.
Press any button or roll the wheel to begin.
| Counts separately | Wheel up, wheel down and middle (wheel) click |
|---|---|
| Also shows | Notch interval in milliseconds, plus jumps larger than one notch |
| Common fault | Encoder wear: skipping in one direction, or a single notch registering as two |
| Related test | The mouse test page covers the remaining buttons |
Why wheels skip, and what the counters show
Inside most wheels sits a slotted wheel that interrupts an infrared beam. Dust, hair and dried lubricant either block the beam or cause the two internal sensors to fall out of sync, which reads as a skipped or reversed notch. Because the test counts up and down separately, an asymmetry — say 40 up and 12 down from the same physical motion — is visible immediately.
When to use this test
- Scrolling up jumps back down, or a page overshoots when you scroll slowly
- Web pages and spreadsheets drift in the wrong direction during a slow scroll
- You want to check a used mouse's wheel before buying it
- You suspect a game or app is scrolling on its own when the wheel is untouched