Key takeaways
- A flat is two things: private bedrooms and shared space. They are fairest divided by different rules — bedrooms by floor area, shared space per head.
- Work out your shared fraction from two numbers you already have: the flat's total area, and the bedroom areas added up.
- Splitting by bedroom size alone charges people for the kitchen in proportion to their bedroom. In the example below that costs the smallest room about €75 a month.
- Adjustments for an ensuite or a windowless room work best as a fixed amount moved between people, so the total still equals the rent.
- Per-head shared space assumes everyone uses it about equally. When that is plainly untrue, say so before you sign, not in month four.
Three people, three visibly different bedrooms, one rent. Someone says "we should split it by room size", everyone nods, and then nobody is quite sure what that sentence means. It has at least two meanings, and they produce different numbers.
This is a worked method for getting to a figure everyone can check. It is not the only defensible answer — splitting equally or by income can both be right for a particular household — but it is the one that follows from what you are actually renting.
The two questions you are really answering
Your lease buys one thing: the whole flat. But the flat is used in two quite different ways, and that is the whole basis of the calculation.
- Private space — the bedrooms. One person has exclusive use. A bigger bedroom is straightforwardly more of the flat, so floor area is a reasonable measure of it.
- Shared space — kitchen, bathroom, hallway, living room. Everyone has the same claim on it. Nobody gets less kitchen for having a smaller bedroom.
So the two parts get divided by two different rules: private space in proportion to bedroom area, shared space per head. That distinction is what the rest of this guide is built on.
Step 1: work out how much of the flat is shared
You need two numbers, and you almost certainly already have both. The flat's total floor area is on the lease or the letting advert. The bedroom areas are on the floor plan, or are two minutes with a tape measure.
- Add up the bedroom areas. That is your private area.
- Subtract it from the flat's total area. What is left is the shared area.
- Shared area divided by total area is the fraction of your rent that belongs to space nobody owns privately.
Take a 90 m² flat at €1,800 a month, with bedrooms of 20, 18 and 12 m². The bedrooms come to 50 m², so 40 m² — a little under half the flat — is shared. That means €1,000 of the rent is buying private space and €800 is buying the kitchen, bathroom, hall and living room.
Measure the rooms, not the argument
Step 2: split the two pools
The private pool is divided in proportion to bedroom area. The shared pool is divided by the number of people. Then you add each person's two parts together.
Continuing the example — €1,000 private, €800 shared, three people, so €266.67 of shared space each:
- Ana, 20 m²: €400.00 of bedroom + €266.67 of shared space = €666.67
- Ben, 18 m²: €360.00 of bedroom + €266.67 of shared space = €626.67
- Cleo, 12 m²: €240.00 of bedroom + €266.67 of shared space = €506.66
Those add up to €1,800.00 exactly. That is worth checking every time you do this — a method that comes out a euro short is a method somebody will quietly stop trusting.
What this changes, compared with splitting by bedroom size alone
Plenty of rent guides describe the shorter version: add up the bedroom areas, work out each person's percentage of that, apply it to the whole rent. It is quicker, and for flats where the bedrooms are similar it lands in much the same place.
When the bedrooms are not similar, it does something you probably did not intend. Applying bedroom percentages to the whole rent charges each person for the kitchen and living room in proportion to their bedroom — so the person in the box room buys less kitchen than the person in the large room, despite both using it the same way.
- Ana: €720.00 by bedroom area alone, against €666.67 here — €53.33 more.
- Ben: €648.00 against €626.67 — €21.33 more.
- Cleo: €432.00 against €506.66 — €74.66 less.
Whether that difference is worth having a conversation about depends on your flat. The bigger your shared area, the more the two methods diverge; if your flat is nearly all bedrooms, they nearly agree. Both figures are in the calculator side by side, so you can see the gap for your own numbers rather than take this on trust.
Do it with your own numbers
Adjustments: the ensuite, the box room, the one with no window
Floor area does not capture everything. An ensuite bathroom, a balcony door, a room facing the bins, a bedroom you cannot fit a wardrobe into — these are real differences that square metres miss.
The reliable way to handle them is to agree a monthly amount rather than a vague sense of unfairness. Someone says the ensuite is worth €60 a month; if everyone agrees, that person pays €60 more. The important part is what happens to the other €60: it comes off everyone else's share, split between them. The rent has not changed, so the total must not change either.
In a three-person flat, a €60 ensuite adjustment means the person with it pays €40 more than their area-based share and the other two pay €20 less each. Agree the amount first and the arithmetic second — the number is a negotiation, not a calculation.
Keep adjustments small
When splitting shared space per head is the wrong call
Dividing the shared pool per head assumes everyone uses the shared rooms about equally. That assumption is usually close enough, and sometimes obviously false. It is worth saying out loud before you sign rather than discovering it in month four.
- Someone works from home. One person is in the living room forty hours a week while the others are out. Per head is still defensible, but it is a choice, not a fact.
- A couple share a bedroom. Two people in one room use the kitchen as two people but occupy one bedroom. The usual treatment is to count them as two heads for shared space and split their bedroom's cost between them — which is how Splitwise's own rent calculator handles it too.
- The shared space is not really shared. A dining room nobody uses, or a second bathroom only one bedroom can reach, is not shared space in any useful sense. Count it where it is actually used.
Bills are not rent
Rent buys space, so floor area is a sensible basis for dividing it. Electricity, water, internet and council tax buy consumption, and consumption has nothing to do with the size of your bedroom. Splitting them by room size is the one genuinely common mistake in this whole exercise.
Per head is the usual convention for bills and it is a reasonable default, but it is a convention rather than a result — worth revisiting if one person has electric heating in their room, or if someone is away half the year. Keep bills on their own line rather than folding them into the rent figure, so the two can be argued about separately.
Making the numbers add up
Percentages of a rent almost never divide into whole cents. €800 between three people is €266.666…, and if everyone rounds their own share you end up paying €1,799.99 or €1,800.01 to a landlord who wants neither.
The fix is to round the shares and then hand the leftover cents to whoever was rounded down hardest — the largest-remainder method. It is what the calculator does, which is why Cleo's share above is €506.66 rather than €506.67. One person is a cent out in their own favour, the group pays exactly the rent, and nobody has to think about it again.
Agreeing it, and what to do with the numbers
Write the result down somewhere all of you can see, along with the inputs it came from: the areas, the adjustments and who agreed to them. Almost every rent argument a year into a tenancy is really an argument about what was agreed, not about arithmetic.
If you track shared costs in an app, the shares go in as they are. Dolio takes fixed amounts per person — useful here, because you have already worked out the exact figures — or weighted shares if you would rather express the split as a ratio and let the amounts follow. Weights can be fractional, so an 18 m² room next to a 20 m² one is not a problem.
Whatever you use, revisit it when something changes: a rent increase, someone moving out, two people swapping rooms. The method above takes about five minutes the second time.
Common questions
Should a couple sharing one bedroom pay double?
Not double, but more than one share. The usual treatment is to count them as two people for shared space — they use the kitchen and bathroom as two — while splitting the cost of their single bedroom between them. Splitwise's rent calculator FAQ works the same case the same way, with the bedroom cost shared by the people sleeping in it.
Who pays for the living room?
Everyone, equally, under this method. The living room is shared space, so its share of the rent is divided per head rather than by bedroom size. That is the main thing that separates this from splitting by bedroom area alone.
How much is an ensuite worth?
There is no correct figure — it depends on how much the alternative bathroom is contested. Pick a monthly amount everyone accepts, then make sure it comes off the other shares rather than being added to the rent, so the total still matches what the landlord charges.
Do balconies, closets and storage count as bedroom space?
Count them as private if only one person can use them, and as shared if everyone can. A balcony reached through one bedroom is private; a balcony off the kitchen is shared. The rule is exclusivity of access, not what the floor plan calls the room.
What if the bedrooms add up to more than the flat's stated area?
Then one of the numbers is wrong. Usually the advertised total excludes hallways or is measured differently from the floor plan. Re-measure rather than proceeding — with no shared area left, the method collapses back into splitting by bedroom size alone.