How to Test Keyboard Rollover and NKRO
Rollover is the number of keys a keyboard can report simultaneously, and ghosting is when a combination causes one to be silently dropped. N-key rollover means any combination registers, with no practical limit.
Why keyboards drop keys
Most keyboards do not wire every key separately. They arrange keys in a grid and scan it, which keeps cost and complexity down but creates ambiguity: certain combinations of three or more keys produce an electrical pattern indistinguishable from a different combination. Rather than report a key you did not press, the controller suppresses one you did.
Keyboards that avoid this give each key its own electrical path, usually with a diode, so no combination is ambiguous. That is what N-key rollover means, and it is why NKRO keyboards cost more to build.
Finding your real limit
Hold two keys, then add a third and a fourth without releasing any. Watch the count. The point where it stops rising as you add keys is your practical rollover limit for that combination — and it genuinely is per-combination, because the ambiguity depends on where the keys sit in the matrix.
Test the combinations you actually use rather than arbitrary ones. For games that usually means movement keys plus a modifier and a spacebar. For typing it rarely matters, because typing seldom holds more than two keys at once.
The six-key result is normal
A great many keyboards report a maximum of six keys plus modifiers. This is not a defect or a cost-cutting measure: it is the USB keyboard boot protocol, the mode that guarantees a keyboard works in a BIOS screen before any driver has loaded. Keyboards that support more usually switch to a different reporting mode once the operating system takes over.
If your keyboard advertises NKRO but tests at six, it may be sitting in that compatibility mode. Many boards need a key combination or a setting in the manufacturer's software to switch. It is worth checking the manual before concluding the feature does not work.
When the browser is the limit
Some keys never reach a web page at all. Browsers and operating systems reserve certain combinations, and anything involving the system key is usually captured before any page sees it. If a specific combination seems to vanish, try it in a plain text editor to see whether the keyboard or the software is responsible.
Connection type matters too. The same keyboard often reports fewer simultaneous keys over Bluetooth than over USB, because the wireless protocol carries less. If rollover matters to you, test in the mode you actually use.
Test it yourself: Keyboard Ghosting Test
Frequently Asked Questions
How many keys should my keyboard register at once?
Six plus modifiers is the common standard and is sufficient for almost all typing and most gaming. Keyboards with N-key rollover handle any number. Two or three is typical of inexpensive membrane keyboards and can drop inputs in games.
Is ghosting the same as a rollover limit?
They are closely related but not identical. A rollover limit is how many keys can be reported at once. Ghosting is the specific behaviour where a particular combination causes a key to be dropped because the matrix cannot distinguish it. Both come from the same wiring design.
Can I fix ghosting with software?
No. It is determined by the keyboard's physical wiring. Software can remap a binding to avoid a problematic combination, which solves the practical problem, but it cannot make the hardware distinguish a combination it electrically cannot.
Does NKRO matter for typing?
Rarely. Typing almost never holds more than two or three keys simultaneously, which is well within any keyboard's capability. NKRO matters for gaming and for specialised input like stenography.
Why does my keyboard test differently over Bluetooth?
Wireless protocols generally report fewer simultaneous keys than USB. This is a limitation of the connection rather than of the keyboard, and it is why some players use a cable even with a wireless-capable board.
Sources: MDN — KeyboardEvent.code · MDN — KeyboardEvent.key
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