Key rollover test
Hold down four keys and a cheap keyboard starts ignoring some of them. It is why a movement key sometimes stops working mid-game while two others are held, and it is a property of how the keyboard is wired rather than a fault.
Hold down as many keys at once as you can and watch the count.
While this test is focused, keys are captured and will not act on the page. Click outside to release.
- Held now0
- Most at once0
Keys registered —
How to run it
Hold down as many ordinary keys as you can at once and watch the count. Add them one at a time rather than mashing, so you can see exactly where it stops rising.
Then try it again with a different group of keys — particularly ones close together. Rollover limits are not uniform: a keyboard may handle six keys in one part of the board and three in another, because the limit comes from the wiring rather than from a number.
Reading the result
| Most at once | What it means |
|---|---|
| 2–3 | Basic membrane wiring; common on cheap and older keyboards |
| 4–6 | Typical of better membrane and most laptops |
| 6 | The standard USB reporting limit, and a good result |
| More than 6 | The keyboard is using an extended reporting mode |
Modifiers are reported on a separate channel and do not use up the budget, so try holding shift and control alongside your group — the count should keep rising.
Ghosting, which is the other fault
Rollover is the keyboard dropping a key you did press. Ghosting is the opposite: reporting a key you did not. On a matrix wired without a diode per switch, three keys held at the corners of a rectangle in the wiring let current reach the fourth corner, and the controller sees a key that nobody touched. In a game that is a character walking the wrong way; in a document it is a letter that appears from nowhere.
The list under the count shows every key the keyboard is currently reporting. That is where a ghost shows up: a key in the list with no finger on it. Hold three keys close together and watch the list, not the number — the letters under one hand, or W, A and S with a fourth, are where it is most often found, because those are the combinations games use.
Most keyboards made in the last decade block the fourth key rather than ghost it, which is what “anti-ghosting” on the box means: the count stops rising, honestly, and nothing false is reported. That is the same wiring limit as rollover seen from the other side.
What this is not
It is not a fault. A keyboard that stops at three keys is working exactly as it was built to, and no setting or driver will change it: the limit is in how the switches are wired into a grid. The only fix is a keyboard with independent switch wiring, which is what N-key rollover means when it appears on a specification.
It is also not affected by the operating system, which is why there are no Windows or macOS sections here. The keyboard decides what it reports before the computer ever sees it.
Common questions
What is keyboard ghosting, and does this test find it?
Ghosting is the keyboard reporting a key you did not press. It happens on a matrix without diodes when three held keys sit on the corners of a rectangle in the wiring: current finds a path through them to the fourth corner, and the controller sees that fourth key as down. This test shows every key it currently believes is held, by name, so a ghost appears as a key in the list that is not under any finger. Hold three keys close together — the letter keys around one hand are the usual place — and read the list rather than the count.
The box said anti-ghosting. Is that the same as N-key rollover?
No, and the difference is the whole story. Anti-ghosting means the controller refuses to report a fourth key when three could produce a ghost — it blocks instead of guessing, which is honest and is why the count on this test stops rising. N-key rollover means every switch has its own diode and any number of keys can be reported correctly. Nearly every keyboard sold in the last decade is anti-ghosting; far fewer are N-key. Marketing uses the first phrase to imply the second.
What is key rollover?
How many keys the keyboard can report at the same time. Most membrane keyboards manage between two and six before they start dropping presses, because their keys share electrical rows and columns and certain combinations become ambiguous. N-key rollover means every key is independent and any number can be held at once.
What is a normal result?
Six is the practical ceiling for anything connected as a standard USB keyboard, because the basic protocol reports six keys plus modifiers. Many keyboards that genuinely support more use a second reporting mode to get past it. Under six, you are seeing the keyboard matrix rather than the protocol.
Why do modifiers not count?
Shift, control, alt and the meta keys are reported separately from ordinary keys, so they do not consume one of the six slots. That is why shift plus a fistful of letters works when the same number of plain letters does not.
Some specific pairs block a third key. Why those?
Because of where they sit in the matrix. Keys share rows and columns, and three keys forming a rectangle in that grid produce an ambiguous reading — the controller cannot tell your combination from a different one, so it drops a key. Which combinations do this depends entirely on the wiring, which is why it feels arbitrary.
Does it matter outside games?
Rarely. Ordinary typing almost never holds more than two or three keys at once, and shortcuts are mostly modifiers plus one key, which is the case rollover handles best. It matters for games, and for anyone using chorded input.