units: how many furlongs per fortnight is that?

Three thousand units, one command

Converting miles to kilometres usually means opening a browser tab. Your machine has had a tool for it since long before browsers, and it knows about three and a half thousand units, including several that exist only as jokes.

$ units -t "10 miles" "km"
16.09344

-t is terse mode: one line, just the number, ready to drop into a script. Without it you get an interactive prompt.

It understands compound units#

The interesting part is not converting one unit to another, it’s that units does dimensional algebra. You give it an expression, it works out whether the two sides are compatible:

$ units -t "1 acre" "m2"
4046.8564

$ units -t "2 GB / 3 minutes" "Mbit/s"
88.888889

That second one is a real question I have needed answered, and the arithmetic is the part I would have got wrong.

And since the post needs to earn its title:

$ units -t "furlongs/fortnight" "m/s"
0.00016630952

A furlong per fortnight is about a sixth of a millimetre per second. The unit database is 8275 lines defining 3480 units, and it contains things like smoot, defined as 5 ft + 7 in with the comment “Created as part of an MIT fraternity prank”:

$ units -t "1 smoot" "m"
1.7018

The trap that will get you#

Write the most natural thing in the world and it fails:

$ units -t "60 mph" "km/h"
conformability error
        26.8224 m / s

mph is fine. The problem is h, and the reason is beautiful:

$ grep -E '^h[[:space:]]' /usr/share/units/definitions.units
h         6.62607015e-34 J s # Planck constant (exact)

h is the Planck constant. You asked for kilometres per Planck constant, and units correctly told you that is not a speed. Spell it out and it works:

$ units -t "60 mph" "km/hour"
96.56064

This is what a tool looks like when physics matters more to it than your habits.

Temperature is a function, not a factor#

Converting temperatures is different from converting lengths, because the scales have different zeros. Multiplying by a factor gives the wrong answer, so units exposes them as functions:

$ units -t "tempF(72)" "tempC"
22.222222

Note the parentheses. degF also exists and means something else: the size of a degree, for temperature differences instead of temperatures.

What you get on each system#

Here the two implementations part ways. Linux has GNU Units:

sudo apt install units

macOS ships its own, identifying itself as Darwin units, and the temperature functions are missing:

$ units -t "tempF(72)" "tempC"
unknown unit 'tempF(72)'

Everything else in this post gave identical results on both, including the Planck trap and the smoot. So conversions are portable, temperatures are not. On a Mac, brew install gnu-units gets you the full version as gunits.

The honest caveat is that units answers the question you asked, and dimensional analysis is unforgiving about what that question was. A conformability error almost never means the tool is wrong.

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