How the test works
Press Start and hold the pointer over the dark panel. Every wheel event the browser delivers moves the tape one step, up or down, and lights the arrow for that direction. The chip counts the steps, the readout shows steps per second and the reversals caught so far. The page itself does not scroll while the pointer is over the panel, so you can spin the wheel as hard as you like.
Browsers report wheel movement in different units. Chrome and Edge report about a hundred pixels per notch on Windows, Firefox reports three lines per notch, and macOS mice report accelerated pixel values that grow with speed. The test treats one event as one step and only splits an event into several steps when it is clearly a few notches delivered at once. Trackpads and free-spinning wheels send many tiny events; when most events are that small the result is labelled smooth scrolling and the steps are samples rather than notches.
A reversal is counted when the direction flips less than 120 milliseconds after the previous step. A person turning the wheel around needs longer than that; a worn encoder does not. The time of the busiest second is kept too, which is how the test measures how fast the wheel can be spun.
What the result means
The result is the number of steps counted. The context line adds the reversals, the busiest second, how many steps went each way, the average size of a step in pixels where the browser reported pixels, and whether scrolling looked smooth.
- 0 reversals after slow and fast rolls. The encoder is clean. A page that still jumps has a software cause: a scrolling utility, a browser extension or smooth-scroll animation.
- 1 or 2 reversals in a long fast run. Borderline. Repeat the run; a consistent count on every run is the wheel, a single one is probably your finger.
- Reversals on every fast scroll. The optical or mechanical encoder is worn or dirty. Cleaning may help for a while; replacement is the fix.
- Far fewer steps than notches you felt. Skipped notches, the same fault from the other side.
- Steps only in one direction. One of the two encoder channels has failed.
What to do about a jumping or skipping wheel
Blow compressed air into the wheel slot from both sides and roll the wheel while doing so; dust and hair on the encoder wheel are the most common cause. If the mouse has a high-resolution or free-spin mode in the vendor software, turn it off and test again, because that mode looks like smooth scrolling and hides notches. Remove scrolling utilities and browser extensions that alter scrolling, then retest. For a mechanical encoder, a drop of contact cleaner reaches the contacts through the gap next to the wheel; for an optical encoder, cleaning the slot is all that can be done from outside.
When to return or claim the warranty
A new mouse whose wheel reverses or skips is defective and should be exchanged within the return window. For a warranty claim, screenshot the panel after a fast roll so the reversal count is visible, and describe the symptom as the page jumping back while scrolling; it is a well-known failure on several popular models and service centres recognise it. Encoder wear under normal use inside the warranty is covered; a wheel that stopped working after liquid got into the mouse is not.
Frequently asked questions
My page jumps up while I scroll down. Is that what the test catches?
Yes. That jump is the wheel encoder sending an up step in the middle of a down motion. The test counts a reversal when the direction changes less than 120 milliseconds after the previous step, which is quicker than a hand can turn the wheel around. One reversal in a long run can be a slip of the finger; reversals on every fast scroll mean the encoder is worn or dirty.
Why does the test say smooth scrolling for my mouse?
The browser reported many small steps rather than whole notches. That is how trackpads, free-spinning wheels and some mice with high-resolution scrolling behave. Steps then mean samples, not clicks of the wheel, and the reversal count is what still matters. A notched wheel that shows as smooth usually has its high-resolution mode enabled in the vendor software.
What is a normal number of steps per second?
A relaxed flick gives five to ten steps per second, a fast spin twenty to thirty, and a free-spinning wheel much more. The number itself is not a grade; compare it between two mice or between a slow and a fast roll on the same mouse. A wheel that produces far fewer steps than the notches you feel is skipping.
Can I test a trackpad or a touch screen?
A two-finger swipe on a trackpad works and shows as smooth scrolling. Finger scrolling on a touch screen does not send wheel events to the page and is not measured here; use the touch screen test for that.
The page scrolled instead of the tape. What went wrong?
The pointer was outside the panel, or the test was not started. Wheel events are only captured inside the dark panel while the test runs; everywhere else the page scrolls as usual, so you can still read the article.