Fair rent when two people share one room and others live alone
When a couple shares one bedroom and a single roommate has another, splitting the rent evenly per person overcharges the couple, while splitting it per room can overcharge the single. Fair Divider prices each room instead, and the couple counts as one party for their room — so the split stays envy-free no matter how many people sleep where.
Two common shortcuts both feel unfair here. Splitting the total by headcount makes the couple pay two-thirds of a three-person flat for a single room — even if that room is small. Splitting by room ignores that the couple's room may genuinely be the best one. The fair question isn't how many people are in a room; it's how much each party values the room it takes.
Set up the division with one party per bedroom — the couple together, and the single roommate — and one entry per room. Each party then picks the room it prefers at the prices shown, and the prices adjust until no party would rather swap. The couple pays their room's price as a unit; how they divide it between themselves is up to them.
Rent is €1,800 for two rooms: a large main bedroom and a smaller single. Split three ways by person, the couple pays €1,200 for the room they share and the single pays €600 — regardless of which room is nicer.
With an envy-free split, the couple's room might be priced €1,050 and the single's €750, because the couple values the extra space and the single is happy with less for less. The two prices still total €1,800, and neither party would trade.