Splitting roommate expenses — fair, without anyone losing out

Why a flat-share kitty is simpler than a family one

A shared flat is simpler than a family holiday in one way: everyone's an adult, everyone eats roughly the same amount, nobody has to work out a child's share. The friction shows up somewhere else instead. It's rarely the individual purchase that annoys anyone — it's the feeling that builds up over weeks: "I feel like I buy the toilet paper more often than everyone else." Without a running list, that feeling can't be proven or disproven — and that's exactly what makes it drag on.

On top of that, shared flats change: someone moves out mid-month, a new room isn't rented again until the first of the next month, a sublet only stays for one semester. A kitty that handles these changes cleanly saves you the argument over who still owes money for the three days after moving out.

What belongs in the kitty — and what stays private

The line is usually clearer in a flat-share than on a group holiday, because everyday life draws it for you. Still, a short conversation when someone moves in is worth having before the first expense turns into an argument.

1. In the kitty: household supplies (cleaning products, toilet paper, bin bags), shared staples if you do that (oil, spices, coffee), replacements for shared spaces (a new pan, light bulbs, a shower curtain), shared parties and flat dinners
2. Stays private: your own weekly shop, individual streaming subscriptions, furniture for your own room
3. Worth clarifying up front: internet and utility bills usually run on their own standing order in most flat-shares, not through the expense kitty — the kitty is for the irregular, spontaneous purchases, not for fixed costs with their own split

Why splitting per person is almost always the fair rule here

On a family holiday, weighting by age matters because a toddler doesn't eat as much as an adult. In a shared flat that difference almost always disappears — everyone counts as an adult, so splitting evenly per person isn't just simple, it's fair. The exception is a one-off expense that doesn't concern everyone: if one person buys extra drinks for their own party, that doesn't belong in the shared kitty — it stays private, or is split only among the people at that party.

What matters more than the rule itself is agreeing on it once when you move in and sticking to it — a rule that gets renegotiated every time someone buys something isn't a rule, it's a recurring argument.

A note on the kitchen whiteboard, the group chat, or an app

The classic is a whiteboard or a piece of paper on the fridge: whoever buys something writes down the amount. It works as long as everyone actually writes things down and nobody wipes the board. In practice it usually fails because the ink fades, the paper ends up in the recycling, or an expense simply gets forgotten because nobody had a pen handy.

A group chat is the digital version of the same problem: amounts are scattered between a hundred other messages, and whoever wants to add it all up at the end of the month has to scroll through weeks of chat history. A shared kitty in the browser flips that around — every expense lives in one place, sorted, dated, and the tool does the arithmetic instead of whichever flatmate happens to be the most patient.

Moving in or out mid-month

If someone moves in or out partway through the month, the cleanest approach is: expenses from before the move stay split among the people who lived there at the time — retroactively adding a new flatmate who never used the shared pan would be unfair in the other direction. From the move-in date onward, the new person counts normally in every future expense.

In practice that means: a flatmate can only be removed from the member list as long as no expense has ever been logged for them — otherwise old balances would have gaps. If someone who's leaving has already paid for or been included in an expense, their row stays; you simply stop adding them to future expenses, so they no longer count there.

How the shared kitty works with La Caisse

La Caisse is a shared kitty that runs in the browser. For a flat-share, it's enough to enter each flatmate as their own row with one adult, no child factor. You then share a link in the flat's group chat — whoever has it can see the kitty and add expenses. No account, no sign-up, nothing to install.

Whoever buys something logs it right after: the amount and the type of expense — "groceries" or "other" covers most flat-share purchases. At the end of the month, the kitty shows each flatmate what they used versus what they actually paid, and suggests who owes whom. Money never passes through La Caisse itself — you settle up directly between yourselves; the kitty is just the shared ledger.

If someone moves out without ever having been part of an expense, you can simply remove their row. If they've already been included in one, their row stays — existing expenses and their split don't change; you just stop selecting them for new expenses. The kitty itself stays available for two years after the last visit, and that period renews every time it's opened, so it keeps working across an entire tenancy.