Scroll Wheel Test: Check Scrolling, Skips and Reversed Ticks
This scroll wheel test logs the direction, size and timing of wheel events received by your browser. Roll slowly in one direction first, then reverse. Look for unexpected direction changes you can reproduce. A browser may combine or transform wheel input, so the log is a record of received events, not a count of physical notches or proof that the encoder is faulty.
How to run the test
Put the pointer inside the dotted area and spin — the scroll wheel test only counts events that land there. The page underneath will not move, because the test swallows the wheel event so that only the counters react.
- Scroll slowly downwards for about twenty notches, one at a time, and watch the
SCROLL EVENTSlist. - Do the same upwards.
- Then spin fast in both directions and compare.
- Hit the reset button in the toolbar to clear the counters between runs.
Consecutive notches in the same direction within 600 ms are grouped into one row with a count, and a new row starts only when the direction changes. That grouping is the point: a healthy downward spin is one long down row, while a failing wheel breaks it into down 7 · up 1 · down 5 · up 1. Every stray one-tick row is a fault you just caught.
What the numbers mean
The big figure is the total distance the wheel has asked for, in the pixel units the browser reports, and it counts up whichever way you scroll. Underneath it, SCROLLED UP and SCROLLED DOWN split that total by direction, and the toolbar shows how many separate wheel events arrived.
The quickest health check is the up/down split: scroll thirty notches down and nothing else, and the up figure should stay at zero. If it creeps up, the wheel reported motion you never made.
Speed and peak speed
The speed readout is the size of the last tick divided by the gap since the previous one, and it only computes while ticks arrive less than 300 ms apart; after 600 ms of stillness it drops to zero. Treat it as a relative smoothness indicator, not a calibrated measurement — useful for comparing a slow spin with a fast one on the same machine, not your mouse with someone else’s.
Free-spin wheels look different, and that is normal
A notched wheel sends one chunky event per detent, so twenty notches give roughly twenty ticks. A free-spin or hyper-scroll wheel disengages the detent and sends a stream of small deltas instead, so one flick can log hundreds of tiny ticks. Neither pattern is a fault; what matters is that the direction stays consistent while your finger moves one way.
When the wheel jumps the opposite way for one notch
This is the classic scroll wheel failure and it is almost always a dirty or worn encoder, not your PC. The encoder, which the wheel axle turns, reads rotation through two offset channels, and the order in which those channels change tells the driver which way you turned. Block one channel with a speck of dust, hair or skin oil, or wear its contacts down over a few years, and the decoder reads that step out of order and reports a notch in the opposite direction.
That is why the fault looks so odd: scrolling still works, it just spits out an occasional backwards step, and it gets worse the faster you spin. Count the reversals per hundred notches before and after cleaning — that number is your only honest before-and-after measurement.
Middle click firing while you scroll
The wheel does two jobs: it turns an encoder and it presses a switch. When that switch has gone soft, or the wheel wobbles on a worn axle, pushing down while spinning can register a middle click you never intended — and the opposite failure, a middle click that registers one press in five, has the same root cause.
This page listens to rotation only, so run the button side on the full mouse tester, which lights a middle-button indicator and keeps a per-button click count. If the middle button drops out halfway through an autoscroll or a pan, the mouse drag test counts those unwanted releases directly.
Rule out the browser, the OS and the touchpad first
Before you open the mouse, spend two minutes proving the hardware is at fault.
- Smooth scrolling. Browsers animate each notch, and a dropped frame makes a good tick feel like an overshoot. Clean, evenly sized ticks in the log mean the wheel is fine and you are watching a rendering effect.
- Lines per notch. Windows sends three lines per notch by default and vendor software can override it. A wheel that feels far too sensitive is usually this setting — the deltas in the log will be large but consistent.
- The touchpad. Two-finger scrolling fires exactly the same wheel events, so keep your hands off it on a laptop or you are measuring both devices at once.
- The computer, not the mouse. Repeat the test on a different machine. If the reversals follow the mouse it is the encoder; if they stay with the PC, look at drivers, a USB hub or vendor software.
Cleaning versus replacing the encoder
Try cleaning first, because it costs nothing and works more often than it has any right to. Hold the mouse close to your face and blow hard into the gaps on both sides of the wheel, or aim short bursts of compressed air into the same slots while turning it. Then re-run the test and count the reversals again — plenty of people report this alone clearing the fault.
If cleaning does not hold, the encoder is worn, and it is a replaceable part. Generic encoders from TTC, Kailh and Alps cost a couple of dollars, and swapping one means opening the shell and desoldering three pins: easy as mouse repairs go, but it is soldering. On an expensive wireless mouse the repair is clearly worth it; on a cheap office mouse, replacement usually wins.
What this test cannot tell you
The scroll wheel test reads what the browser reports, one step removed from the hardware, so there are real limits.
- Horizontal tilt is not measured here. This page tracks vertical rotation only. If your wheel tilts sideways, use the main mouse tester, which has a thumb-wheel indicator that lights on horizontal wheel events.
- Delta sizes are not comparable across machines. How many pixels one notch reports depends on the browser, the OS and your scroll settings, so compare runs on one machine rather than against a number quoted online.
- It cannot see inside the encoder. A reversed tick proves the wrong direction reached the browser, not whether dust, wear or a driver put it there.
- A clean run is not a clean bill of health. Intermittent faults hide, so if your wheel only misbehaves after an hour of use, scroll here for a couple of minutes.
Frequently asked questions
Why does my mouse wheel jump in the opposite direction every few scrolls?
Dust or wear in the wheel's encoder. The encoder reads rotation through two offset channels, and when debris blocks one of them the driver decodes a step backwards. Blowing compressed air into the gaps on either side of the wheel is the single most reported fix for exactly this symptom. Clean it, then re-run the test and count reversals per hundred notches — if the count drops to zero, that was it.
I've replaced my mouse and it still jumps back up and down. Is this a software problem?
Probably yes, if brand-new mice fail on that PC but work on another. Test the same mouse on a second computer: when the fault stays with the machine rather than the mouse, the encoder is not to blame. Look at vendor software, a USB hub, or a scroll-enhancement utility intercepting wheel events. Scrolling slowly making it better is a strong hint that something in software is batching the events.
My scroll wheel jumps up and down chaotically in browsers and games. Does blowing into it actually fix it?
Often, yes — it is worth trying before anything else. Get your mouth as close to the wheel as possible and blow hard into every gap around it, then repeat the test. Users across several mouse models report this clearing the fault outright, usually because a hair or flake of dust had settled on the encoder. If it returns within days, the encoder is worn rather than dirty.
Middle mouse click not working but scrolling does. What's broken?
The middle switch, not the encoder. Rotation and the click use two separate components that happen to share one wheel, so one can fail while the other is perfect. Confirm it by plugging the mouse into another computer: if the middle click stays dead there, it is the hardware. If it works there, reinstall the mouse driver and check whether vendor software has remapped the button.
My scroll wheel is too sensitive — barely touching it scrolls one or two notches. How do I fix it?
Change how many lines each notch sends before assuming the mouse is broken. On Windows that is Settings, Mouse, then the lines-per-notch slider, which defaults to three. Run this test first: if the delta values in the log are large but perfectly consistent, the wheel is healthy and the setting is doing it. Games often ignore the OS value entirely and need their own in-game sensitivity changed.
Why doesn't tilting my scroll wheel scroll sideways in some programs?
Because horizontal scrolling has to be implemented by each application; it is not handled automatically the way vertical scrolling is. A tilt wheel that works in your browser and Word but does nothing in Excel, Explorer or a design tool is almost always the application ignoring the event, not a broken mouse or missing driver. Vendor software can work around it by mapping the tilt to a key the app does understand.
Do you replace the mouse when the scroll wheel stops working, or can you repair it?
The encoder is a replaceable part, so repair is realistic. Generic TTC, Kailh or Alps encoders cost a couple of dollars and swapping one means opening the shell and desoldering three pins — straightforward as mouse repairs go, but it is soldering. On an expensive wireless mouse that is clearly worth the hour. On a cheap office mouse, a replacement usually costs less than your time and the parts.