Get out your calculator and find a measuring tape. Your first challenge is to put a price tag on each bedroom. Measure the length and width of each one, then multiply the numbers you come up with. This is the square footage of the room. One may be 200 square feet, another 100, and the third bedroom might be 125. If any of the rooms have a private bath, include that square footage in the room's total -- the roommate who gets these quarters should pay more for that perk. Likewise, if one room has a balcony, the extra square footage should be factored in.
Divide each room's square footage by the rental's overall square footage. If your entire apartment is 1,200 square feet, the 200 square foot bedroom takes up 16 percent of the overall living space. Calculate how much of the rent each room represents. If your rent is $1,750 a month, the largest bedroom would be worth $280, or 16 percent of $1,750.
Figure out who takes which bedroom. This can be a matter of flipping coins if there are just two roommates sharing a two-bedroom apartment. Otherwise, you can draw straws or negotiate. If one roommate earns less income than the others, it might make the most sense for him to take the smallest bedroom. If one works the grave shift, he might want a room that's in the quietest area of the dwelling so he can get some sleep at times when everyone else is up and moving about.
Calculate the rent for the common areas of the dwelling. If the bedrooms make up 425 square feet of the total 1,200 square feet, this represents 35 percent of the rent, or about $612. If there are three of you and three bedrooms, this would leave $1,138, and each of you would take responsibility for one-third of this. Add this number to the amount paid per bedroom. For example, if you take the largest bedroom, your share of the rent would come out to about $659 a month -- your bedroom's value of $280 plus $379. If three of you are squeezing into a two bedroom unit and two of you share a room, you can each take one half of that room's percentage of the rent total. You can split the cost of the common area in half if there are just two of you.