Even splits are a decision, not a default
Dividing rent by the number of people is the fastest answer and often the right one. It stops being right the moment the apartment is not symmetrical: one bedroom has a window onto a wall, another has an ensuite, one person's room doubles as the office they work in five days a week.
The methods below are all defensible. The failure mode is not picking the wrong one. It is picking one without saying so, and then relitigating it every month.
By square footage
Measure each bedroom, add the numbers, and give each person their room's share of the total. Shared space is already priced in because everyone pays a slice of it through the same rent.
This is the easiest method to justify out loud, and the easiest to check. Its weakness is that it treats a large dark room as better than a small bright one.
Adjust it by adding weight rather than fudging the measurements. A private bathroom is commonly worth 10 to 15 percent on top of the room's raw area. Write the adjustment down next to the number so it survives being questioned in March.
By income
Each person pays the same percentage of what they earn. On a $3,000 rent where one person earns $90,000 and the other $60,000, that is $1,800 and $1,200 rather than $1,500 each.
This is common among couples and rare among roommates, for a reason worth naming: it requires disclosing income to someone you may only share a lease with. It also drifts, because raises are private and the split silently stops matching. Agree in advance how often you recheck it.
By auction
Everyone writes down what they would pay for each room, privately, with the constraint that their bids sum to the total rent. Rooms go to the highest bidder and prices settle where nobody would rather swap. This is the method behind the rent division tools that came out of fair-division research, including the New York Times calculator from 2014.
It produces the outcome people are least likely to resent, because each person chose their own price. It also needs everyone in a room together for twenty minutes, which is why most households do not use it.
Utilities are a separate question
Rent scales with the room. Electricity, internet, and water scale with the person, and mostly with how much they are home. Splitting utilities by the same ratio as rent means the person with the smaller room subsidizes the person who works from home.
Split rent by room and utilities evenly. Two rules, both easy to state, and the arithmetic still comes out to the total.
Whichever method you pick, keep the receipts
The argument that actually happens is not about the method. It is about whether March's payment cleared, and whether the internet bill that went up in June was ever resplit.
Footnote splits from the charge itself, so each month's rent line carries the bank row that created it and each person's share traces back to that row. The rent split calculator will run the numbers for square footage or income right now, without an account.