How to work out rent when someone moves in or out mid-month

Somebody’s lease ends on the 11th and the replacement wants to move in on the 12th, and now four people are standing in a kitchen trying to work out one month’s rent. It is the most common household arithmetic there is, and it goes wrong in the same two places every time: the wrong divisor, and bills that get waved through as “roughly fair”.

Both have a plain fix. Rent is a daily rate multiplied by nights; bills are a bill divided by person-days. Everything else is the agreement you already have.

One thing first: the arithmetic below is how a household divides what it has decided to divide. Whatever your tenancy contract says about notice, liability and who is on the agreement comes first, and none of this replaces it.

Rent: the daily rate

The daily rate for a room is its monthly rent divided by the number of days in that particular month.

That divisor is where people improvise. Dividing by 30 every time feels tidy and is wrong every month except four: in a 31-day month it charges too much, in February it charges too little. The error is a few euros, which is small enough that nobody objects and exactly large enough that somebody notices, which is the worst possible size for a number in a shared house.

Then count nights, not days. A person is charged for the nights they have the room. The person leaving on the 12th slept there for the last time on the night of the 11th; the person arriving on the 12th sleeps there from that night. No overlap, no gap, and nobody has to argue about who owns the afternoon of the changeover.

A worked example

A three-bedroom flat, €1,500 a month, rooms treated as equal at €500 each. September has 30 days. Carla moves out on 12 September and Dan moves into her room the same day.

Carla has the room for the nights of 1–11: 11 nights. Dan has it for 12–30: 19 nights. The daily rate for that room in September is €500 ÷ 30 = €16.67.

Carla’s and Dan’s shares add to exactly €500, which is the check: the room is paid for once, by two people. If the rooms are not equal — and they usually are not — work out each room’s monthly rent first and prorate from there. Splitting rent when the rooms are not equal covers how to set those figures.

Bills: person-days

Utilities do not belong to a room, so they are shared by however many people were in the house, for however long. Person-days handles that in one step: multiply each person by the days they were there, add them up, and divide the bill by the total.

September’s electricity is €96. Ana and Ben were there all 30 days; Carla 11; Dan 19.

Person-days = 30 + 30 + 11 + 19 = 90. So each person-day costs €96 ÷ 90 = €1.0667.

The same method covers two people leaving in the same month, a guest who stayed a fortnight, or a housemate away for three weeks — as long as everyone agrees that being away does not stop the fridge running, which is a conversation to have once rather than every bill.

The things that are not prorated

The deposit. It is held, not spent. The outgoing housemate gets theirs back and the incoming one puts theirs in, at full value, between the people concerned. Folding it into the month’s rent arithmetic is how deposits get lost.

One-off joining costs. A new key, a name change on an account, the cleaning before the handover: these are shared by whoever agreed to share them, not divided by days.

Anything already bought. The bulk toilet paper, the shared spice rack. Prorating a cupboard is where a reasonable system becomes a joke.

The overlap, if there is one. If the incoming housemate needs three days of overlap to move their things, the room is genuinely paid for twice on those days. Somebody absorbs it — usually the person who wanted the overlap. Decide which before the van arrives.

Where Donget comes in

The month a housemate changes is the month a shared record earns its keep, because there are suddenly four different numbers instead of one.

Put the rent in as one expense under Add expense and split it by Amount, typing the four figures above — the app checks that they add up to the total, which catches the arithmetic slip before it becomes a disagreement. For bills, Multiplier is quicker than a calculator: give Ana and Ben 30, Carla 11 and Dan 19, and the person-day split is done. Scan receipt with AI handles the utility bill itself.

Everyone in the group sees the same Balances, so the person who did the maths is not also the person defending it. One Settle up at the end of the month turns the whole handover into a couple of transfers, and Carla’s departing balance is closed before she is gone rather than chased afterwards.

Frequently asked questions

How do you calculate prorated rent?

Divide the room’s monthly rent by the number of days in that specific month to get a daily rate, then multiply by the nights the person actually has the room. February and August give different daily rates for the same rent.

Should you divide rent by 30 days or by the days in the month?

By the days in the month. Dividing by 30 in a 31-day month quietly overcharges, and in February it undercharges — small on one month, and the kind of thing housemates notice.

Who pays for the changeover day?

Count nights, not days, and the question disappears. Whoever sleeps there that night pays for it, so the two stays never overlap and never leave a gap.

How do you split bills when someone was only there part of the month?

Use person-days. Add up how many days each person was in the house, divide the bill by the total, and multiply by each person’s days. It handles any number of partial stays at once.

Is the deposit prorated too?

No. A deposit is not rent — it is held and returned, so it is handled between the people it belongs to, at full value, separately from the month’s arithmetic.

What if the incoming and outgoing housemates overlap by a few days?

Charge both for the nights they each have, which means the room is paid for twice on the overlap. Decide before the move who absorbs that, rather than after.

The bottom line

Divide by the real number of days, count nights, and use person-days for anything that is not attached to a room. Do the sums before the boxes are packed and the handover is a transfer instead of a negotiation. The wider household system is worth setting up while everyone is paying attention anyway.

Download Donget free and let the changeover month settle itself.