Turn the wheel over the list below. Every step is counted, the size of one notch is measured, and any step that goes backwards in the middle of a flick — the signature of a dying wheel — is caught and shown.
Scroll here — the page stays put
Last steps — backward ones in red
Waiting for the wheel
Turn the wheel over the list — about twenty steps up and down are enough for a verdict.
If your wheel tips left and right, the tilt track moves with it. Hardly anything else in a browser can prove that tilt still works, so it is worth a few taps each way. No tilt on your mouse? Hold Shift and turn the wheel: that scrolls sideways on any mouse, and it gets the second track to itself so the two are never confused.
A scroll wheel is a mechanical part with a short life, and its failures are subtle enough that people blame their browser for months. Here is what the numbers on this page mean.
Almost every scroll wheel turns a small rotary encoder. As the wheel rotates, the encoder produces two signals slightly out of step with each other, and their order tells the mouse which way you turned. When the encoder wears out or fills with dust, that order occasionally comes out wrong and the mouse reports one step in the opposite direction. On screen the page jumps back a line in the middle of a scroll. This test counts those reversed steps, which is something no operating system will show you.
Each detent of a notched wheel sends the browser a fixed delta, so the same number keeps repeating. This page finds the value that repeats most and shows it as the notch size, along with the unit the browser used — pixels, lines or pages. A free-spinning or high-resolution wheel sends many different values instead, so no single notch size exists and the page says so rather than inventing one.
Some mice let the wheel tip left and right for horizontal scrolling. The browser reports it as horizontal wheel delta, which is exactly what a trackpad's two-finger swipe produces. Most mice have no tilt at all, so the usual way to scroll sideways is to hold Shift and turn the wheel — that works on any wheel, which is also why it proves nothing about yours. Each gets its own track here: the tilt track moves only without Shift, the Shift track only with it. If the tilt track moves, your wheel really does tilt.
The top speed is the fastest the wheel was spun, measured in steps per second over the last second of scrolling. It is not a pass or fail number: it depends far more on how hard you flick the wheel than on the mouse. It is useful for comparing a free-spinning wheel with a notched one, and for spotting a wheel whose steps stop arriving when it is turned quickly.
Before you buy a new mouse, try cleaning the wheel. The encoder sits in a sealed-looking housing but is not sealed at all, and dust settling on its optical slots or contacts is the single most common cause of backward steps. A few short bursts of compressed air through the wheel slot, with the wheel turning, fixes a surprising number of mice. Some people go further and open the mouse to clean the encoder itself, which is a job for someone comfortable with a screwdriver and a warranty already expired.