Living together
When your partner moves into an apartment you already share
Your bedroom is already paid for, so nothing about it changes. What changes is that the kitchen, the bathroom and the living room now hold three people instead of two, and that is the part the rent needs to follow. Here is the arithmetic, including the roommate’s rent cut nobody thinks to offer.
Updated September 20268 min read
The short version
Split the rent into two pools: the private rooms, priced by how much space each one is, and the shared space, priced per person. When a partner moves in, the room price does not move. The shared space is now divided by three instead of two.
Which means two things happen at once, and only one of them usually gets discussed. The couple start paying for a second share of the shared space. And the roommate who is not part of the couple pays less than they did before, because the space they are buying is now split more ways.
The number
Why the rent splits into two pools
An apartment is not one product. It is a private room per person plus a set of rooms everybody uses, and those two things are bought in completely different ways. You buy a bedroom by the square foot: a bigger one is worth more and that is the end of the argument. You buy a kitchen by the head, because two people in a kitchen get less of it than one person does.
Splitting the rent by area alone gets the bedrooms right and the rest wrong. Splitting it per person gets the shared space right and charges the same for a box room as for the double with the bay window. The two-pool split is what handles both, and it is the same method as splitting rent when the bedrooms aren’t equal: this post is what happens to it when the head count changes.
Nobody moving in asks to pay less. What they usually get charged is either nothing or half of their partner’s rent, and both of those are wrong in the same direction.
The worked example
Ada and Ben share a $2,400 two-bedroom apartment. Ada has the big bedroom at 180 square feet, Ben has the small one at 120. The kitchen, bathroom and living room come to 200. Ada’s partner Cass moves into Ada’s room.
- Private rooms are 300 of the 500 square feet, so they carry $1,440 of the rent. The big bedroom is $864.00 of that and the small one $576.00.
- Shared space carries the remaining $960, split per person: $480 each between two people, $320 each between three.
Ada
- Room share
- $432.00
- Shared space
- $320.00
- Pays now
- $752.00
- Paid before
- $1,344.00
Cass
- Room share
- $432.00
- Shared space
- $320.00
- Pays now
- $752.00
- Paid before
- Not there
Ben
- Room share
- $576.00
- Shared space
- $320.00
- Pays now
- $896.00
- Paid before
- $1,056.00
| Person | Room share | Shared space | Pays now | Paid before |
|---|---|---|---|---|
| Ada | $432.00 | $320.00 | $752.00 | $1,344.00 |
| Cass | $432.00 | $320.00 | $752.00 | Not there |
| Ben | $576.00 | $320.00 | $896.00 | $1,056.00 |
The couple pay $1,504 between them for a room worth $864 plus two shares of the shared space. Ada’s own share falls from $1,344 to $752, which is the actual reason people move in together, and Ben’s falls by $160, which is the part nobody raises.
Split the room however you like
The error this post exists to name
The intuitive move is to treat the couple as one unit, because they are one couple, one bedroom, one conversation. Do that and the shared space is divided by two: the couple pay one share and Ben pays one share, exactly as before.
So Ben pays $1,056, the number he was already paying, while a third person moved into the apartment and started using the kitchen he is buying half of. The error is worth $160 a month, which is $1,920 a year, and it never gets caught, because from Ben’s side nothing appears to have changed. That is the tell: something did change, so his rent should have moved.
A couple is two people for anything that scales with people. Shared space, hot water, the electricity bill, the internet if it is per-device rather than a flat fee. One person for anything that scales with rooms.
Work out your own apartment
Put in the rent, the shared floor area, and each room with how many people sleep in it now. The last column is the same apartment with one person per room, so you can see exactly what the change costs and who it moves.
The private rooms, and who sleeps in them now
The private rooms account for $1,440.00 of the rent and the shared space for $960.00, which is $480.00 per person now against $480.00 with one person per room.
| Room | In it | Room cost | Shared space | Room total now | Before |
|---|---|---|---|---|---|
| Big bedroom | 1 | $864.00 | $480.00 | $1,344.00 | $1,344.00 |
| Small bedroom | 1 | $576.00 | $480.00 | $1,056.00 | $1,056.00 |
Every column adds back to the rent exactly. “Before” is the same apartment with one person in each room, so the last column is what actually changes when somebody moves in. Nothing you type here leaves your browser.
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Split rent by room size
The standalone version for rooms of different sizes with one person in each. Free, no signup, inputs saved on this device.
The cases the arithmetic does not settle by itself
They are only there some of the time
Three or four nights a week is the common case and it deserves a fraction rather than a full share. Prorate the shared-space share by nights present: three nights of seven is roughly 3/7 of a resident, so in the example above that is about $137 rather than $320. Agree the fraction rather than deriving it to two decimal places, and revisit it when the pattern changes rather than counting nights every month.
The bills
Anything that scales with occupancy gets a third share for the same reason the shared space does. A flat fee like broadband does not care how many people are in the apartment, so it splits per resident, which now means three ways. Splitting utility bills when usage isn’t equal is the version of this for a household where somebody is home all day, which a new resident often is for the first few weeks.
The tenancy and the deposit
This is a paperwork question rather than an arithmetic one, and it is worth separating from the money. A new occupant usually needs the landlord’s agreement, and the deposit already lodged does not move: it belongs to whoever paid it. If the couple later takes over the tenancy, that is a new agreement, not a recalculation.
What if the roommate objects to a third person at all
Then the rent split is not the conversation you are having, and no calculator will help. Have the other one first. The numbers here are for households where everybody has agreed to the arrangement and wants it priced honestly, which is most of them.
If you are starting from scratch rather than adjusting, how to split shared costs fairly covers picking a method before any money moves.
Common questions
Should your partner pay rent when they move into your shared apartment?
Does the existing roommate’s rent go down when a partner moves in?
Does a couple count as one person or two?
How much should a partner pay if they only stay a few nights a week?
Should the couple split their own share evenly between them?
What about bills and the deposit?
Or keep the new split running
Halvy holds a household’s split as a rule rather than a monthly calculation, so adding a third person changes the shares once and every bill after that follows. Free on iOS and Android, no ads, and it never asks to connect to a bank.
Every figure here was produced by running Halvy’s own split engine on the worked example, which is why both the before and after columns reconcile to $2,400.00 exactly. Floor areas are a proxy for what a room is worth rather than a valuation: light, noise, storage and a private bathroom all move the number, and the calculator above is the starting point for that conversation rather than the end of it. Whether a new occupant is permitted at all is a matter for your tenancy agreement.