We're not entirely sure when the feature rolled out, but Android's Smart Text Selection has picked up the ability to do unit conversions. It's not as seamless as it could be, since results are displayed in the Google app rather than your current view, but it is faster than manual copying and pasting to do the conversion in another app yourself.
In case you aren't familiar, Android has a pretty snazzy tool called Smart Text Selection. First revealed back in 2017, it lets you pass content quickly between apps rather than having to manually copy and paste. Tap and hold part of a phone number, for example, and Android will not just intelligently highlight the full number, it will offer to pass it to the Phone app to make a call. The same goes for addresses and many other types of content. It's incredibly convenient.
We don't know precisely when it happened, but Smart Text Selection has picked up a new trick: unit conversions. That means if you select part of a measurement, it will automatically select the full measurement (both number and unit) and offer a button to convert it, handing the selected content to the Google app as part of a search, prefaced by "convert." This works for most units Google's conversion can handle, including length, volume, and even currency in our testing.
You still have to select the units you want it converted into, but that's just a couple of taps from Search's drop-down list. Though it would be a little more convenient to see unit conversions in some kind of overlay, this is a big time-saver over copying text, opening the Google app or a browser, pasting it, and further modifying the query to ask for a unit conversion.
We don't know exactly when the feature landed, and it may have been in testing for a while. There's at least one indication (Thanks: Mishaal Rahman) that it's new-ish, as Google only recently confirmed its existence on the issue tracker two weeks ago, however one of our readers (Thanks: Ibrahim) has proof it popped up for some folks as early as July.