How to test your scroll wheel
- Move your pointer into the test area. The page stops scrolling while it's there.
- Scroll down slowly, one notch at a time, for 20 notches. Steps down should go up by 20 (more with smooth scrolling, see the FAQ) and reversals should stay at zero.
- Do the same scrolling up.
- Now scroll fast in each direction. A worn wheel often misreads at speed, so watch the reversals count and the step log.
How the test works
Each time the wheel moves, your browser fires a scroll event with a deltaY value: negative for up, positive for down. The test counts each event with a vertical movement as one step, and steps per second is the number of steps in the last second.
A reversal is a single step against the direction you're scrolling, followed straight away by a step back in your direction. For example: down, down, up, down. The test only counts it when at least two steps went one way first and each step came within 300 ms of the one before. That rule ignores normal changes of direction, because when you mean to scroll back, the steps keep going the new way. Reversed steps are highlighted in the step log.
The tool reads the same events a web page uses to scroll, so it can't see a notch that your mouse never sent. If you feel a click but the count doesn't change, the wheel is skipping. Nothing is uploaded.
Scroll event values
| deltaMode | Unit of deltaY | When you'll see it |
|---|---|---|
| 0 | Pixels | Most browsers on most systems |
| 1 | Lines | Some browser and system combinations |
| 2 | Pages | Rare, mostly when scrolling is set to one screen at a time |
The size of one notch depends on your browser, your zoom level and your scroll settings, such as the number of lines per notch in Windows. That's why the test counts events rather than adding up deltaY.
What the results mean
| What you see | Likely cause |
|---|---|
| Reversals while scrolling one way | A worn or dirty wheel encoder |
| Fewer steps than notches you felt | The wheel is skipping notches |
| Many steps with small deltaY for each notch | Smooth or high-resolution scrolling, which is normal |
| Steps in one direction only | A faulty encoder or a wheel setting in the mouse software |
More mouse tests
- Mouse test: check every button and catch double clicks.
- Mouse polling rate test: measure your mouse's report rate in Hz.
- Mouse DPI test: measure your real DPI.