Performing unit conversions on the CLI

I recently discovered¹ the units CLI tool which installed by default on macOS/Linux and comes with a ton of builtin conversions. For example you can do:

❯ units '12 tablespoons' cups
	* 0.75
	/ 1.3333333

This tells you that 12 tablespoons is 0.75 cups, or that a cup is approximately 1.3333333 times 12 tablespoons. I don’t really have any idea why it also shows the latter, but it’s easy enough to ignore.

It also has an interactive mode that reduces the need for quoting shell arguments, e.g.:

❯ units
753 units, 62 prefixes
You have: 190 degC
You want: degF
	374

It also handles compound units seamlessly:

❯ units '10 m/s' mph
	* 22.369363
	/ 0.044704

One gotcha you have to watch out for is that numbers need to be separated from units by spaces, otherwise you get a conformability error and a gibberish result:

❯ units 10mm cm
conformability error
	10
	0.01 m
❯ units '10 mm' cm
	* 1
	/ 1

One of the most amazing things is the file containing the builtin unit definitions. On macOS 26 it is located at /usr/share/misc/units.lib. It starts out simple defining primitive units and prefixes:

# primitive units

m			!a!
kg			!b!
sec			!c!
[...]

# prefixes

[...]
kilo-			1e3
hecto-			1e2
deka-			1e1
deca-			deka
deci-			1e-1
centi-			1e-2
milli-			1e-3
micro-			1e-6
[...]

But it also contains a lot of (at least to me) extremely obscure units. For example, did you know that it takes 16 US feet make 15 French feet?

frenchfoot		16|15 ft

You also can define your own custom conversions for your own unique use cases.

I frequently find it convenient to use bc as a calculator on the command line in my dropdown terminal instead of using a browser/search engine, so I imagine I’ll use units in the future when remembering or typing the conversion into bc is too inconvenient.