I've been prodded by limbic region to comment here, the module/programming language i've made called Math::Farnsworth can do this. It's on cpan, but because of a bad test in the dist it doesn't get indexed for installation, i'm currently working on it to get it ready for another release that isn't so prealpha quality. it actually ships with support for doing exactly that (and hundreds of other units).

http://search.cpan.org/~SIMCOP/Math-Farnsworth-0.5.1/lib/Math/Farnsworth.pm and http://search.cpan.org/~simcop/Math-Farnsworth-0.5.1/lib/Math/Farnsworth/Docs/Syntax.pm

if you need more help i'm best reached on either email (simcop2387 ta cpan.org) or on IRC (freenode, rizon, efnet, undernet, magnet), or on AIM (same name as here).