Press and hold a few keys at once. On most keyboards there is a number where one of them stops registering — you are holding six, the computer sees five. That is not a broken keyboard, and it is not a driver problem. It is how keyboard circuits work, and it has a name: ghosting.
This guide explains where it comes from, shows you how to measure your keyboard's actual limit in about twenty seconds, and covers what NKRO does and does not buy you.
Why ghosting happens: the keyboard matrix
A keyboard with 104 keys does not have 104 wires running to its controller. Instead, keys sit at the intersections of a grid — rows and columns — and the controller scans the grid to work out which intersections are closed.
The problem appears when three keys form a rectangle in that grid. Current can flow through an unused corner of the rectangle and look exactly like a fourth key being pressed — a key you never touched. That phantom is the "ghost".
Keyboards prevent this by adding a diode to each key, which blocks current from travelling the wrong way through the grid. A keyboard with diodes on every key can register any combination you can physically press — that is NKRO, n-key rollover. A keyboard with diodes only on some keys can handle limited combinations — 6KRO or similar is the usual spec. This is why ghosting in a specific key combination is normal on a cheap keyboard and impossible on a good one.
Measuring your keyboard's rollover
The online keyboard tester reports a simultaneous hold peak — the maximum number of keys the system has ever registered as held at the same time. To use it:
- Click the page once so the browser window has focus.
- Press and hold keys one at a time, spreading across the keyboard:
A,S,D,F, thenJ,K,L,;. Keep them all down. - Watch the peak counter. It stops climbing when the keyboard or the operating system stops reporting new keys.
Two readings matter. If the peak matches the number of fingers you are holding, your keyboard is handling everything you throw at it. If the peak stalls well below the number of keys you are physically holding, you have found the limit — and it will almost always be lower when the held keys form a rectangle in the matrix.
Run it a second time with the keys chosen differently, for example the number row plus the arrow keys. A keyboard with real anti-ghosting gives the same answer every time; a keyboard with partial diode coverage gives different answers depending on the combination.
Why NKRO on USB is still not unlimited
Even a keyboard that claims full NKRO hits two ceilings that have nothing to do with the matrix:
- The USB HID boot protocol. The default USB keyboard report is six bytes of key data plus modifier bits. That is where the familiar 6KRO figure comes from. Keyboards get around it by using the NKRO report format over USB, which many, but not all, operating systems and BIOS environments handle correctly.
- The operating system and application. Some systems cap how many keys a focused window can receive, and some applications only track a few modifiers plus one key. A keyboard can be perfect and the game still ignores your seventh key.
Practical consequence: if your peak stalls at six while holding eight keys, check with a different combination before blaming the keyboard. If it always stalls at exactly six regardless of combination, you are seeing the boot-protocol limit, not a fault.
What is not a ghosting problem
- A key that never responds. That is a dead key, not ghosting. A dead key test is the right tool.
- A key that fires twice from one press. That is chatter, not ghosting — the circuit is registering too much, not too little.
- Not being able to hold Ctrl+Alt+Delete. That combination is deliberately intercepted by the operating system and never reaches an application, by design.
- Windows key and other reserved shortcuts. Intercepted at the OS level. A web tester cannot see them, and their absence from a test result is not a fault.
Who actually needs NKRO
- Gamers — genuinely. Multi-key combinations while moving, strafing and using abilities are exactly the pattern that trips a diode-less matrix.
- Fast typists — mostly not. Typing rarely involves more than three keys at once, and ghosting combinations need specific geometric patterns to appear.
- Anyone buying a keyboard — worth a twenty-second check. It is one of the few performance claims you can verify yourself rather than take on faith from a spec sheet.
Frequently asked questions
My peak stops at 6 no matter what. Is my keyboard broken?
Almost certainly not. Six is the standard USB boot-protocol report size. Consistently stalling at exactly six, regardless of which keys you choose, indicates the limit is in the connection protocol rather than the keyboard hardware.
Why do different key combinations give different limits?
Because ghosting depends on geometry. Three keys forming a rectangle in the keyboard matrix create a ghost, while three keys that do not create one are handled normally. Varying results across combinations is the signature of a keyboard without diodes on every key.
Does a laptop keyboard support NKRO?
Rarely. Built-in laptop keyboards typically have limited rollover. If you need NKRO on a laptop, use an external keyboard — and test it with the desktop layout. Switch the keyboard tester to its laptop layout before testing an internal keyboard.
Can software fix ghosting?
No. Ghosting is an electrical property of the keyboard circuit. No driver or setting can create a signal the hardware never sent.
Twenty seconds and eight fingers is all it takes. Open the keyboard ghosting test, hold a handful of keys, and read your real rollover limit off the peak counter.