Small 40% keyboards with 45 keys or under have a lot of attention around the custom keyboard community. Noting low finger travel, it’s commonly suggested that 40% keyboards are ergonomically beneficial. This is questionable advice for people looking to improve their typing comfort. While smaller keyboards do reduce finger travel, they make other things worse. On the whole, 40% keyboards are not a clear win.

To be clear, I’m not saying 40% keyboards aren’t practical. People can and do use them practically. If you use a 40% or below and enjoy doing it, don’t let me stop you.

However, if your interest in custom keyboards is ergonomics, as it is for me, then 40% keyboards are a questionable direction.

The ergonomics argument for 40% keyboards is that “all keys are at most 1u away from home row position, so finger travel is lower than it would be on larger keyboards.” This is good. The compromise made though, with so few keys, is that extensive use of layers, combos, or other tricks is unavoidable to represent frequently used keys. That means a lot more key presses and stress on the hands (to change the layer, to make the combo, …) to accomplish the same typing.

More key presses = more work, counteracting the benefit of lower finger travel.

The base layer gets much better even with just a handful more keys. For instance 56 keys is enough to fit a traditional layout alphas area (30 keys), number row (10 keys), and outer columns (8 keys) on the base layer plus a few more keys (8 keys) on the thumbs or bottom row. This isn’t necessarily optimal, but likely closer to the sweet spot between low finger travel vs. avoiding layers and combos for frequent keys.