What to do when a roommate moves out mid-month

Rent, power and internet bills get charged for a fixed period (usually the whole month), but real life isn't that tidy — someone decides to move out on the 12th, and the month's bill only closes on the 30th. Splitting it equally among everyone, including someone who only lived there 12 days, isn't fair. Splitting it only among whoever stayed until the end isn't fair either — because the person who left still used the house for almost two weeks.

The right answer is a proportional split: everyone pays based on how many days they actually lived there during that period, not an equal fraction for everyone.

The principle is simple: take the total bill, divide it by the number of days in the period, and multiply by the days each person actually lived there. Whoever stayed the full month pays the full fraction; whoever left partway through only pays for the days they used.

In practice, this is easy to get wrong doing it by hand (especially with several bills and different move-out dates in the same month) — and it's exactly the kind of calculation that tends to cause friction, because nobody wants to look like they're overcharging (or undercharging) someone who's leaving.

Recurring bills, like rent, follow the proportional rule. A one-off expense — say, a grocery run on the 20th, after the person already said they were leaving — is usually left out, or negotiated separately, since it isn't an ongoing cost for that period.

The important part is agreeing on this as soon as someone gives notice, not leaving it to figure out only when the bill closes.

• Splitting equally among everyone, without factoring in how many days each person actually lived there that month.
• Leaving the money conversation for after the person's already gone — it gets harder to sort out from a distance.
• Not keeping a record of who lived in the house and when — without it, nobody remembers the exact move-out date a few weeks later.