A nice, even split down the middle might seem like the easiest way to divide rent and utilities when you take on a roommate, but it isn't always fair. One of you might earn twice what the other does, or you might already have taken over the master bedroom and now you're looking for someone to use the smaller room down the hall. You can take a few approaches to make the situation more equitable.
Charge Based on Income
If you out-earn your roommate, consider including something other than money in the equation. If your rent is $1,500, you might consider charging her only $600 or $700 instead of a full half if she's willing to take over laundry, cleaning or cooking duties – chores you might hate. The more chores and responsibilities she takes on, the less of a percentage she should contribute. That way, there's something in it for both of you.
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Charge Based on Square Footage
Consider charging your roommate based on your dwelling's square footage. If your rental unit's total area is 1,200 square feet and your bedroom takes up 300 square feet of that while hers is 200, start by figuring out what each of those square feet costs each month. If you're paying $1,500 a month, it works out to $1.25 per square foot – $1,500 divided by 1,200.
Now, multiply each of your bedrooms' square footage by $1.25. This makes your room worth $375 and hers worth $250, for a total of $625 dedicated to the bedrooms. The balance of the rent – $875 – would cover the areas of the dwelling you both enjoy. Divide this in half, which comes out to $437.50 for each of you.
Then add the cost of your respective bedrooms. You would contribute $812.50 to the rent and she would pay $687.50.
Add Money for Amenities
You can add the square footage of amenities to the space you're paying for if only one of you gets to enjoy them, such as if your master bedroom also has a balcony or a private bath. You also can negotiate cooking or cleaning duties in exchange, as you might if one of you earned more.
Divide the Utilities
Utilities can be divvied up the same way as rent if they just cover basic needs like heat and electricity. Each of you can pay for the percentage of these bills that covers your living space, plus your share of common areas. However, this might not be fair with services like television and Internet. If you really need top-notch cable TV while your roommate hardly ever watches TV, or if she can't function without high-speed Internet but you rarely go online, consider putting each of these services in the name of the roommate who uses it most.
Considerations for Homeowners
It may not make sense to assign your roommate a percentage of the mortgage based on square footage if you own your own home. Your mortgage payment probably includes interest and possibly escrow for property taxes and insurance – which you benefit from as the homeowner but that a tenant wouldn't necessarily be responsible for if she rented a place of her own.
Depending on your city and what rentals go for there, charging a percentage of your mortgage payment might result in a ridiculously high or low rent amount. Find out what fair market rate is for rentals comparable to your home in your area, then base the percentages on that instead.
Get an Agreement in Writing
Whatever approach you take, confirm it in a signed agreement, particularly if chores are involved. Decide who's going to be handling the rent payments. Are you each going to give separate checks to the landlord or will you collect from your roommate and then make the payment yourself? Try to anticipate things that might go wrong, then plan for them in writing.