Splitting bills, and the three answers people land on

There are really only three ways to split a shared cost, and almost every argument about money between two people who live together is a disagreement about which of them is fair.

Even means the same amount leaves both accounts. Proportional means the same share of each income does. By room, or by use, means the split follows something physical instead — square footage, who works from home, who actually watches the streaming service. None of these is correct in the abstract; they are three different definitions of fairness, and the useful thing is to notice which one each of you has quietly been assuming.

The other half of the problem is not the rule but the applying. Almost nobody gets the rent wrong: it is one number, it is the same every month, and both people know it. What drifts is everything else — the utility bill that moved, the four subscriptions on one person's card, the groceries that were meant to alternate. The posts below cover both halves.