Scroll Wheel Test

Scroll up and down inside the box at your normal pace. Each notch shows its direction and size; wrong-way notches are logged in red.

Notches

0/s

Per second

Skips

Scroll hereUp, then down, at your normal pace

The fault this catches

The wheel turns an encoder, a little disc with contacts. When the contacts wear or get dirty, the encoder sometimes reads a notch backwards. On screen the page scrolls down, down, down, then jumps up a line, then continues. People blame the browser for months before they suspect the mouse.

The test logs any notch that arrives in the opposite direction less than 120 ms after the previous one. Nobody reverses a scroll that fast on purpose, so those are the encoder's mistakes. Scroll steadily one way for a while, then the other. A healthy wheel logs nothing.

The other numbers

Notches per second is how fast you scroll. Free-spinning wheels will show large numbers here; that's fine.
The big label shows the size of the last step in pixels. Mice usually report 100 or so per notch; touchpads report small varying values because they scroll smoothly rather than in notches.
deltaMode 0 means the browser is reporting pixels, 1 means lines. It's a detail for the curious and doesn't change the verdict.

If it skips

Compressed air into the wheel slot fixes a surprising number of them. If not, the encoder is a cheap part but a fiddly repair; on a mouse older than three or four years it's usually the sign to replace it.

Other tests

CPS testMouse or spacebar, 1 to 60 seconds Spacebar clickerThe idle game: press, buy, prestige Spacebar counterTimed runs, 100-press race, tally Reaction time testFive rounds, median in milliseconds Keyboard testerEvery key, ghosting limit, chatter Click counterA big button that counts, with undo Tap counterTwo pads for two thumbs, left/right balance Jitter click testChecks whether the run looks like jitter Butterfly click testScores the two-finger alternation Drag click testCounts the burst and the sub-30 ms gaps Kohi click testThe 10 second Minecraft standard Right click testSame engine, other button Double click testFinds a switch that fires twice Polling rate test125, 500 or 1000 Hz, measured Scroll wheel testCatches a wheel that skips backwards