How to test a keyboard: stuck keys, ghosting and key rollover

How do I test my keyboard? Open an online keyboard tester, click into the test area and press every key. Each key that works lights up. To check for ghosting, hold several keys at once and see whether they all stay lit. A key that never lights up may be broken, or the system may be catching it first. The keyboard tester shows each key's code, flags repeats that point to a stuck key, and lists the keys you have not pressed yet.

A keyboard fails in a few predictable ways. A key stops responding. A key types twice. A key seems to stay pressed. Or some key combinations stop working when you press many keys at once, which gamers notice first. This guide explains each problem, how to test for it, and what to try next.

What a keyboard tester can and cannot tell you

A keyboard tester is a web page that listens for key events from your browser. When you press a key, the browser reports it, and the page marks it. That tells you whether a key signal reaches the browser. It does not look inside the keyboard.

The toolsdocks tester shows these details for each press:

• code, the physical key, such as ShiftLeft.
• key, the character or name it produces with your layout, such as Shift.
• location, which separates left, right and numpad versions of a key.

For example, pressing the left Shift key reports code ShiftLeft, key Shift and location 1. No keystrokes are stored by the tool.

There is one important limit. Some keys never reach a web page. The Fn key, many media keys and operating system shortcuts are handled by the keyboard or the system first. If one of those does not light up, it does not prove the key is broken. Try it in another way, such as using the key to change the volume, before you decide.

Step by step: test every key

1. Open the keyboard tester.
2. Click into the test area so the page receives your keystrokes.
3. Press each key once. Tested keys turn green on the layout.
4. Look at the list of keys not yet pressed. Work through it.
5. For each key that does not light up, press it firmly a few more times, then note it down.
6. Check the event log to confirm that each press shows the right code.

It works for full-size keyboards and for compact layouts without a numpad, since the list only shows keys you have not pressed yet.

Find a stuck or repeating key

A stuck key behaves in one of two ways: it keeps typing the same letter, or it sends extra characters when you press it once.

To test it:

1. Press the suspect key once and release it.
2. Watch the event log. A healthy key reports one press and one release.
3. If the log shows many repeats, the key is held down electrically or is getting stuck.
4. Compare with another key. Operating systems repeat a key when you hold it, so repeat events during a deliberate hold are normal. Repeats after a quick tap are not.

On a mouse, a similar problem is called chatter, where one click is registered twice. The mouse tester flags two clicks less than 80 milliseconds apart. A failing switch, for example, might show two click events 12 ms apart after a single press. It also tests the back and forward buttons, scroll wheel direction and steps, and double-click speed.

What ghosting is

Most keyboards are built as a grid of rows and columns. Each key sits where a row and a column cross. The keyboard controller scans the grid to see which crossings are connected. When three keys in a rectangle of the grid are pressed at the same time, the controller can sometimes see a fourth, false connection. That false key is a ghost. Or the controller may ignore the real third key, which is called blocking.

Both problems are common on simpler keyboards that save money on wiring. Keyboards that prevent them add a small diode under each key, or use a different switch design. A keyboard sold as "anti-ghosting" may protect only some keys, such as the WASD cluster, so the term alone does not tell you how well the whole board behaves.

For ordinary typing, ghosting almost never matters. You press one or two keys at once. It matters for gaming, music software and shortcuts that need many keys together.

Key rollover, explained

Rollover is how many keys the keyboard can report at the same time. You will see terms like 2KRO, 6KRO and NKRO.

• 2KRO means two keys at once. Some basic designs stop there.
• 6KRO means six regular keys at once, plus modifier keys such as Shift, Ctrl, Alt and the Windows or Command key. This limit comes from the standard USB keyboard report format that computers use during startup and often afterward.
• NKRO, or n-key rollover, means every key you can press is reported at the same time.

Some listings count differently, such as 6 keys plus modifiers, and some keyboards offer NKRO only in a special mode, so read the spec. When a spec says "anti-ghosting," it may not tell you the number.

A worked example: a ghosting and rollover test

You want to know whether your keyboard handles the keys a game uses.

1. Open the keyboard tester.
2. Hold down W, A, S, D together. Check that all four stay lit.
3. Add Space and Shift. Shift is a modifier, so it does not count against the six regular keys. You should now see five regular keys and one modifier.
4. Add E. That is six regular keys. A keyboard with 6KRO still handles this.
5. Add Q. That is a seventh regular key. If it does not light up, you have likely reached a 6KRO limit, and that is normal for many keyboards.
6. Try other groups you actually use, such as the arrow keys with Shift, or the keys in a shortcut. If a key fails to light up in a small group, the keyboard may be blocking keys in its matrix.

Interpret the results carefully:

• All keys light up: your keyboard handles at least that many at once.
• The seventh key fails but others work: likely a 6KRO limit, not a defect.
• A key fails even in small groups: it may be a bad key or blocked by the matrix.
• A combination fails only with certain keys: it may be a system shortcut grabbing the keys before the browser sees them.

The tool's own guidance is the same: a key that does not light up while others are held may be ghosting, but some key combinations are taken by the operating system and never reach the page.

Laptop keyboards vs mechanical keyboards

	Laptop	Mechanical
Key travel	Short	Longer
Replaceable keys	Rarely	Often
Common failure	Dirt, crumbs, worn contacts	Worn switch, chatter
Rollover	Varies by model, check the specs	Varies by model, check the specs
Repair	Often needs a shop	Often possible at home

A laptop keyboard is built into the case, so one bad key can mean a repair or a replacement keyboard. A mechanical desktop keyboard is easier to test by plugging in a different one. Many mechanical keyboards let you swap one switch or keycap. Always check your warranty before opening anything.

What to do when a key fails

Work from easiest to hardest.

1. Test it in another place. Try the key in a text editor and in the tester. If it works elsewhere, the problem is the app.
2. Restart the computer. A stuck modifier or a driver glitch will clear.
3. Check accessibility features. On Windows, Filter Keys and Sticky Keys can change how keys behave. Holding the right Shift key for about eight seconds is a known way to turn on Filter Keys by accident. Look in the Accessibility settings and turn off features you did not choose.
4. Clean it. For a MacBook, Apple's guidance is to use compressed air with the straw that comes with the can. Hold the laptop at about a 75 degree angle. Spray the keyboard, or the affected keys, from left to right, then turn the laptop onto its right side and spray again, then its left side. Keep the end of the straw about 1.5 cm from the keys and do not turn the can upside down. For other laptops, check the maker's care guide.
5. Check the keycap. A badly seated keycap can feel stuck or fail to press the switch.
6. Try an external keyboard. If the external one works, the laptop keyboard hardware is the issue. If it does not, look at the software.
7. Update or reinstall drivers or firmware. This applies to gaming keyboards with their own software.
8. Replace or repair. If the key still fails, use the warranty or get a repair quote.

Do not pour liquids on keyboards to clean them, and do not use solvents that can strip printing from the keys.

Common mistakes

• Blaming the keyboard for a missing Fn key. Fn and media keys are often handled below the browser.
• Testing without clicking into the page. The tester only sees keys while the page has focus.
• Using one test to condemn the board. Test twice, and in another app, before you replace it.
• Pressing hard on a stuck key. You can damage the switch or keycap.
• Expecting a gaming feature on any board. Check the spec for NKRO and the connection mode it needs.
• Forgetting the mouse. A mouse that double-clicks can look like a keyboard problem in games.
• Ignoring spills. Disconnect power, tip out liquid and follow the maker's advice. Do not test a wet keyboard.

FAQ

Why is one key not working on my laptop? Common causes are dirt, a loose keycap, a worn contact or a software setting such as Filter Keys. Test the key in the tester and in another app, clean it, then try an external keyboard to see if the hardware is at fault.

How can I tell ghosting from a bad key? A bad key fails even when pressed alone. A ghosting or blocking problem shows up only when other keys are held. The tester helps you tell them apart.

What is key rollover and does it matter? It is how many keys the keyboard can report at once. For typing it hardly matters. For gaming and music, a higher number helps.

Can the tester see the Fn key? Often not. Fn and many media keys are handled by the keyboard or system before the browser sees them.

Is my typing recorded? No. The tester shows key events on the page. It does not store keystrokes.

Does an online test work for a Bluetooth keyboard? It works for any keyboard that your computer treats as a normal keyboard. Wireless lag can make the log look slower, though.

If a mouse is acting up too, check it with the mouse tester the same way.