Running the test
In the tester above, hold keys down one at a time without releasing any, and read the keys-at-once figure. It is a high-water mark, so it keeps the largest number reached during the session rather than the number currently held.
Use keys spread across the board rather than a cluster. Six keys in one corner test the matrix in the least favourable way and can hit a blocking limit that has nothing to do with rollover.
Reading the number
Modifier keys are usually carried separately from ordinary keys, so Shift, Control and Alt often do not count against a six-key limit. A board that reports six letters plus three modifiers is behaving exactly as expected.
| Result | What it indicates |
|---|---|
| 2 to 3 | A membrane keyboard with no diodes. Normal for an office or laptop keyboard. |
| 6, and never more | The standard USB keyboard report format, which carries six keys plus modifiers. |
| More than 6 | Full N-key rollover. The keyboard uses a report format built for it. |
Why six is such a common ceiling
A USB keyboard that wants to work in a computer BIOS, in a boot menu and on any machine it is plugged into uses a simple, long-standing report format that carries six keys at once. That format is the reason so many keyboards stop dead at six regardless of how good the hardware underneath is.
Keyboards with full rollover send a different, larger report, and many of them can switch between the two. Six keys in one mode and twenty in another is a firmware setting, not a hardware difference.
Older PS/2 keyboards avoided the issue because that interface sends key changes as they happen rather than as a fixed-size snapshot, which is why PS/2 is still occasionally recommended for full rollover on ancient hardware.
Does it matter?
For typing, no. Very few people hold more than three keys deliberately while writing, and the modifiers are carried separately anyway.
For games it can matter a great deal, and specifically for games where movement, sprint, crouch and an action key are held at the same time. Rhythm games and some flight simulators push further still. If a combination you rely on drops out mid-game, this test will show it in a few seconds.