An earlier hint and comments discussed using "units", a built-in UNIX tool for unit conversion. Here is a way to use the utility in a convenient way. Put this line in your aliases.mine file:
alias uni 'echo '\''scale=4; \!:1'\''`units \!:2 \!:3` |NOTE: Shown on two lines; put a space between the "|" and "awk" and remove the line break!
awk '\''{print $1, $2, $3}'\'' | bc -l'
% uni 200 nmile mileThis converted 200 nautical miles to statute miles. To find out which units are available, examine the file units.lib in /usr/share/misc/.
230.1558800
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020217092118165