Though I hardly use these type of tools, but, if you’re someone like a student or a teacher for instance, then having a software utility that lets you convert between units of measurement can be very useful. Most operating systems don’t ship those type of tools by default, but nice & free tools such as ‘qMetrics’ are always available, if you know where to look :).
It runs on both GNU/Linux and MS Windows platforms and lets you convert between a huge number of units of measurements with ease. Has pre-built packages for Ubuntu and Windows (installer) so the installation under those platforms is pretty easy too.
As you can see from the below screenshot, it doesn’t seem to ‘miss’ any units at all and has a pretty simple and easy to use user interface (with unit categories to the left). You can add/remove the units that you don’t need from its main window because having all the supported units displayed all the time can be a bit of a distraction as well.
If it’s a useful tool for you, then please get it from this ‘qMetrics’ home page (including links to Windows installer package and the source code).
If you use Ubuntu, then first download the ‘qmetrics …deb’ package, double click on it after the downloading completes and follow the on screen instruction with Ubuntu Software Center to install it. That’s it.