What is n-key rollover?

N-key rollover is commonly cited as an advantage of choosing a mechanical keyboard over a rubber dome alternative, but the concept often isn’t well explained. In this article, we’ll explain what we mean by rollover, how keyboard rollover is classified and show you how to test rollover for yourself. We’ll also give some hints on what you should look for when you’re buying a new keyboard.

What does rollover mean?

Rollover is simply how well your keyboard can correctly register multiple keys being pressed simultaneously. When typing slowly, you’re probably typing one key after another, but if you’re writing more quickly or playing a game, it’s very likely that you’ll be holding down, pressing and releasing multiple keys simultaneously. Musical instrument emulation and Plover (a stenotype software) also require many keys to be pressed simultaneously, and therefore require sufficient rollover.

How are keyboards classified in terms of rollover?

Not all keyboards are created equal when it comes to rollover. Where one keyboard might be able to handle half a dozen or more inputs, others might struggle to correctly recognise three keys pressed simultaneously.

Rollover is quantified as n-key rollover, where n is the minimum number of keys that can be correctly registered when pressed simultaneously. This is often written in its short form KRO. For example, 2–key rollover becomes 2KRO.

Many inexpensive keyboards have only 2KRO, while higher-end keyboards (and many mechanical keyboards) have 6KRO or higher. If a keyboard can register all of its keys being pressed simultaneously, then this is called NKRO.

Note well that this limit doesn’t include modifier keys like Ctrl, Alt, Win or Shift.

Note also that not all 2KRO keyboards are equal, as their rollover may be quite different (or optimised towards different expected usages). For example, a 2KRO keyboard marketed towards gamers might take extra care to ensure that the commonly used WASD movement keys don’t cause rollover issues, allowing it to perform better when playing games that use these keys than keyboards built for general purpose use.

What happens if you exceed a keyboard’s rollover limit?

Two things can potentially go wrong if you press more keys simultaneously than a keyboard can handle.

The first is called ghosting, where a unpressed key is erroneously registered as having been pressed. For example, on a 2KRO keyboard, pressing down three keys simultaneously might cause four key presses to be registered.

Modern keyboards include ‘anti-ghosting’, a feature which essentially blocks additional keys from being registered once the rollover limit has been reached. Here, when three keys are pressed on our 2KRO keyboard, only two are registered, and the third one is blocked. This is why anti-ghosting is also called blocking or jamming.

How do I test rollover?

Rollover can be tested using apps that you download or run online. Run the program, and press and hold an increasing number of keys. Ensure that each key is correctly shown on-screen when pressed and released.

It’s important to test as many key combinations as you can. Keys next to each other are often prone to not being recognised, so these make good targets. It also makes sense to test for the combinations that you’re likely to use. An FPS gamer, for example, might hold W, D, Q, Shift and Space.

It’s important to remember that rollover is a minimum, not a maximum. If your keyboard can correctly register some six key combinations, but it can’t correctly register one three key combination, then it is a 2KRO keyboard, not a 6KRO keyboard.

We like Aqua Key Test, a free program for Windows, and Microsoft’s online app. You can find more programs here, courtesy of the mechanical keyboards subreddit. You can also find a list of keyboards and their KRO values here, or ask for help in the comments below.

How do I get NKRO on my keyboard?

For a long time, keyboards could only achieve NKRO over the older PS/2 connector. This is why some mechanical keyboards come with a USB to PS/2 adapter in the box, even as PS/2 ports started to disappear from computers. If your keyboard comes with a PS/2 adapter and is marketed as having NKRO, you’ll almost certainly need to use that adapter to achieve full NKRO on your machine.

Recently, more mechanical keyboards are able to offer NKRO over USB, often using the full-speed USB standard. These keyboards often come with a DIP switch or key that switches between 6KRO and NKRO, as the NKRO mode sometimes doesn’t work in non-Windows operating systems or in a computer’s BIOS.

If your keyboard comes with NKRO over USB, ensure that the NKRO mode is turned on and test it using the programs linked above to ensure it’s working.

What should I look for in a new keyboard?

6KRO is enough for rapid typists, competitive gamers and computer users of all types. There are some rare exceptions though — like two people using one keyboard, extremely rapid music games, music emulation or stenography. For these users, NKRO keyboards, whether over PS/2 or USB, may be worth looking for.

Most mechanical keyboards come with 6KRO or higher, while most rubber dome keyboards have less, but there are rare exceptions in both directions. If the rollover isn’t stated on the product page, try doing a search for NKRO or 6KRO plus the name of your keyboard, e.g. “Filco Majestouch-2 NKRO”. You can also ask us in the comments below, although we can’t promise to know this stat for every keyboard!

Here are some rollover figures for keyboards we sell:

Wrapping up

Thanks for taking the time to read through our article. We hope it answered your questions, and we welcome your comments or further questions below.