me and my roommate were thinking of diving the rent with the square feet of the area that we use ... but we are not sure how to do it ?

You can also draw lines through the house to make sure you both have an equal amount of footage

Naah, just kidding. 

Melody's formula will work. Be careful though that you calculate

[((common area/2)+your area)/ (total square footage)]  x  rent and not

[(common area/(2+your area))/ (total square footage)]  x  rent

It is a common mistake for people to make

Reinout 

Here's the way  to do it...

To figure your share, it's just...

[(square footage you use)/ (total square footage)]  x  rent

And your roommate's share is just

[(square footage he/she uses)/ (total square footage)]  x  rent

I think CPhill's idea is floored, he hasn't considered that some of the area is used by both of you.

I think it is because his his triplet siblings are perched on his head!  What a heavy weight to bare!  Poor CPhill. Maybe 6 eyes are better than two when you are seaching for a Roman 0.  

LOOK IN THE BOULDER SIR CPHILL AND SIB'S - 'TIS IN SISYPHUS'S BOULDER!!

(It is an in joke - don't worry - we are all crazy here!)

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 Let me see.

Here's the way  to do it...

Work out the

1) common area footage

2)your area footage

3)roommate's area footage

To figure your share, it's just...

[(common area/2+your area)/ (total square footage)]  x  rent

And your roommate's share is just

[(common area/2+roommate's area)/ (total square footage)]  x  rent

Do you know how to work out each area?