Formulas:

Molecular formulas use the exact number of atoms of each element present in the smallest unit of the substance.
http://www.ilpi.com/msds/ref/chemicalformula.html

Main Entry: 1for·mu·la
Pronunciation: 'for-my&-l&
Function: noun
Inflected Form(s): plural -las or for·mu·lae

http://www.m-w.com/cgi-bin/dictionary?book=Di...

Across the web "chemical formulas" is more popular than "chemical formulae" and "mathematical formulas" is more popular than "mathematical formulae".
But in British websites formulae is considerably more popular. So I would use formulae in BE and formulas in AE.

I understand that "formulae" is correct and "formulas" is vastly used - wouldn't even be the first time it happens.
Up to you the choice if bowing to a settled bad habit or playing the role of a cantankerous "laudator temporis acti". Take it easy, there's worse to the world :-)

"Formulas" is more common, of course, but I've been saying "formulae" ever since I started playing Diablo II.

My previous comment was only referring to mathematical and chemical formulae. In every other circumstance I can think of I would use formulas.
eg racing formulas (like Formula 1), formulas for success in management, infant formulas

Thanks everybody.

I think I will stick with formulae, as I'm trying to use BE consistently, and I'm indeed using the word in a mathematical context. If its less common, but more correct, that's fine with me :)