Splitting bills when a roommate is away for a month

Rent does not change, because the room is not available to anybody else while they are gone — that is what they are paying for. What households do sometimes adjust is the usage half: power, water, gas, occasionally groceries. The line most people land on is that a bill for having something stays, and a bill for using something can move.

The distinction that settles it

Internet is for having. It costs the same whether three people or two are in the flat, and the absent person's room still has their things in it and their name still on the account. Electricity is for using, and one fewer person for five weeks is genuinely visible on the bill.

Sorting your shared costs into those two piles once takes about ten minutes and answers this question and several others permanently.

Whether it is worth doing at all

Often it is not, and saying so plainly is better than a grudging adjustment. On a two-month absence the difference on a shared power bill can be real money. On a fortnight it is frequently less than the effort of working it out, and households that insist on it for a fortnight tend to be having a different argument.

Decide it before they go

A month away that was never discussed produces an awkward message on the return, usually from the person who has been paying for an empty room and has been thinking about it for four weeks. Two sentences before the trip removes all of that.