Fair when roommates earn — and can spend — very differently
When roommates earn very differently, someone usually suggests splitting rent by income. Fair Divider doesn't do that — it splits by how much each person values each room, not by their salary. In practice that often achieves what you actually want: the person on a tighter budget can take the cheaper room they prefer and pay less, without anyone means-testing anyone.
Charging rent as a percentage of income sounds fair, but it's awkward in practice: it means sharing salaries, it penalises the higher earner for no extra space, and it can breed resentment either way. Most roommate friction isn't really about income — it's about who gets the good room, and for how much.
Each roommate chooses the room they prefer at the prices shown, and the prices adjust until no one would swap. Someone watching their budget will naturally gravitate to a cheaper room and pay less for it — by choice, not by disclosure. The result is envy-free: whatever anyone earns, no one would rather have another person's room at that person's price.
Two roommates, rooms of different sizes, rent €1,200. The higher earner is happy to pay for the big room; the other is watching their budget and prefers to save.
An envy-free split might price the big room at €720 and the smaller at €480. Nobody compared payslips — but the person who wanted to spend less did, and both prefer their own room at its price. The two still add up to €1,200.