Love what spliddit.org is doing! Great auction-based tools for splitting, not just for rent - they divide possessions too. See also the NYTimes rent calculator which takes a bit longer and is based on a series of discrete choices (Francis Su): nytimes.com/interactive/2014/science/rent-division-calculator.htm
Spliddit and the NYT approach are perfect if your roommates are willing to do an auction and people value things differently, like who likes to be upstairs vs. downstairs, etc. They are best if you haven't picked rooms yet because they assign people to rooms based on their reserve prices.
I find that using a fairness-based approach like Splitwise (splitwise.com/calculators/rent) works well when people already know which rooms they want, and are trying to agree on a fair differential from a neutral authority. (I'm the CEO of Splitwise and created our tool). A lot of non-nerds don't like auctions, so you may want to try Splitwise instead if your roommates wouldn't want to do an auction, or already know which rooms they want.
I have definitely moved into apartments with unevenly sized rooms / bathrooms. The split feels arbitrary and even if you are discussing weight functions for different amenities, people are basically just arguing about the amount of rent they should pay. This makes the process feel random and a little contentious. Next time I move in somewhere, it will be nice to start the rent split conversation with a third party tool.
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Great idea, how about Spliddit 2.0....an app that fairly splits dish duty and cleaning among roomates.
Splitwise
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