For this example, let's assume a 1000 square-foot apartment with two bedrooms and one bathroom is shared between two roommates. The rent is $800 per month and there are no private bathrooms. One bedroom measures 14 feet by 15 feet, for a total of 210 square feet. The other bedroom is 11 feet by 12 feet, for a total of 132 feet.
First, we calculate the common and private space. The two bedrooms, which account for all the private space, make up 342 square feet, or 34.2% of the total space in the unit. Accordingly, the remaining common space is 658 square feet, or 65.8% of the unit.
Since each roommate is assumed to use the common space equally, they split the portion of the rent which applies to the common space equally. In this case, the portion of the rent that covers common space is 65.8% of $800, or $526.40. So, each roommate's share of the common space rent is half of that, or $263.20.
Since the common areas account for $526.40 of the $800 rent, the private areas must account for the remainder, or $273.60.
Since the rooms are not equal in size, we need to calculate each bedroom's share of the private space. The larger bedroom is 210 square feet, which is 61.4% of the total private space. The other bedroom accounts for 38.6% of the total private space. So, the roommate in the larger bedroom would pay $167.99 (61.4% of $273.60) toward his bedroom's share of the private space. The roommate in the smaller room pays $105.60 (38.6% of $273.60) toward his bedroom's share of the private space.
Adding it all up, finally, we find that the roommate in the larger bedroom owes $431.19 ($263.20 + $167.99), and the roommate in the smaller bedroom owes $368.80 ($263.20 + $105.60). When we add those two rents together, we get $799.99, off by a penny due to rounding. In this case, the extra penny could simply be added to the rent on the larger room, to bring it to $431.20.
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