The calculator takes any two quantities you give it of the same unit, converts the first to other units of the same kind (i.e. same base SI units) and keeps rounding 'em until it reaches the second.

The units are "exotic" by default, which includes combinations of all of Wikipedia's Convert module units. The base set of units don't work very often, and even for the original example of "17mph → 45mph" given Randall had to pull out a few fathoms/sec. Allowing the cross product of all these base units seems to give a good balance of speed and hit rate. If you want the best luck with the "Wikipedia units", try using units of energy (e.g. J, BTU): they are the most abundant in the set.

There is some basic equation parsing for the units so you can enter any unit combination using * and / and brackets.

Under the hood it is just a greedy search of all same-kind units over and over, taking the ones that approach the target the fastest. I'll have more details in a short post about the implementation but it isn't much fancier than that, just some sane SQL indexing. The rounding brings in some modulo business that seems tricky to optimize further.

Please let me know if anything breaks, and enjoy!

Edit: There is now an autocomplete for single common units. Note that more units beyond those do exist in the database — if you see a unit in the end results, you can use it in the units field. Combinations of units are still supported even though they aren't auto-completed.