Key rollover and ghosting: test how many keys register at once
CheckbenchKeyboard, mouse and mic testsOpen
Rollover is how many keys a keyboard can report while they are all held down. You do not need a spec sheet to find out. Hold keys in the keyboard tester and read “Most keys held at once”: that number is what your keyboard, cable and browser actually delivered.
How the tester counts
Every key that is down at the same moment counts, including Shift, Ctrl and the other modifiers. The tester remembers the largest number it has seen, and lists the keys in that group under Details and settings. A key you let go before pressing the next one does not count. Caps Lock and the Korean Han/Yeong and Hanja keys are the exceptions: they count as presses but never as held, because some systems send only half of their signal.
| What you do | Held at that moment | Most keys held at once |
|---|---|---|
| Hold A, S, D, F one after another | A S D F | 4 |
| Let go of A, then also hold G | S D F G | still 4 |
| Hold A and S, release A, then hold D, F, J | S D F J | 4 |
Test the combinations you actually use
- Press Start over so old presses do not count.
- Hold the keys you use together, for example W, A, Shift and Space for a game, or a chord from your editor.
- Check that every held key is drawn dark on the diagram and that “Held down now” lists them all.
- Add one more key at a time until a new key no longer shows. The number before that is your limit for that group of keys.
Try several groups. A keyboard can handle one combination and not another, so the result for one group of keys does not tell you about the next.
What ghosting and blocking look like
- A key you pressed does not light up while others are held. That combination did not arrive, so for now treat it as one your keyboard cannot report.
- A key you did not press lights up. People call this ghosting. Let go of everything and try the same combination again to be sure it was not a stray finger.
Either way the tester is only the messenger: it shows what arrived. It cannot add keys the keyboard never sent.
Where a limit of six keys comes from
If your keyboard stops at six ordinary keys while Shift, Ctrl or Alt still add on top, that number has a likely source. The USB specification for keyboards defines a basic report, the one a computer’s BIOS reads at start-up: one byte for the eight modifier keys (left and right Ctrl, Shift, Alt and GUI) and room for six other keys. The specification puts the limit at six non-modifier keys when a keyboard uses that report layout, and has the keyboard report a rollover error when more are held. It says this about that layout, not about every USB keyboard.
Source: USB-IF, Device Class Definition for Human Interface Devices (HID) 1.11, Appendix B and Appendix C (PDF, checked 2026-10-10).
Limits of testing in a browser
Some combinations belong to the system or the browser and may never reach a web page, for example shortcuts that switch windows. If a combination fails here, try a different group of keys before you conclude the keyboard is the limit. While the tester is capturing keys it holds on to Tab and most shortcuts so the page does not jump around. Press Esc twice to release them.
Found a mistake, or a rule that has changed? Email contact@lumenlab.page. We check it, fix it and put the new date on this guide.