📚 LPT: units is a unit conversion and calculation program for the command line. It even does currency conversions — an accompanying command, units_cur, updates exchange rates.

Both DuckDuckGo and Google have the same functionality, but units lets you do it without having to open the browser.

It does simple conversions:

You have: 30km You want: mile * 18.641136 / 0.0536448 

tells you definitions:

You have: watt You want: Definition: J/s = 1 kg m^2 / s^3 

and even does calculations: at 2.5 MiB/s, how much time would it take to download 70 GiB of data?

You have: 70 GiB / (2.5 MiB / second) You want: hours * 7.9644444 / 0.12555804 

The * lines are answers to what's asked for (30 km = 18.64 miles); the / lines are the inverse of that, useful when there is an integer relationship between the two units.