Every share house hits this moment eventually. A new housemate gets the keys on the 21st, the rent is due on the 1st, and the group chat goes quiet while everyone silently does — and re-does — the maths. Does the new person pay a full month? A half? "Whatever feels fair"? Someone proposes a number, someone else counters, and a person who's been in the house three days is suddenly negotiating money with people they barely know.
The move-out version is worse. The leaver has boxes in the car and zero interest in a spreadsheet debate, so they either overpay to escape the conversation or underpay and leave everyone quietly resentful. Neither is the housewarming — or the goodbye — anyone wanted.
Why the usual fixes fall short
An equal split is beautifully simple and plainly wrong here: it charges ten nights of living like thirty. A spreadsheet can get the proration right, but now one housemate owns a formula everyone else has to trust blindly — and it breaks again next time anyone's dates change. And most splitting apps only offer equal, percentage, or exact-amount splits, which quietly hands the mid-month problem right back to you: you still have to work out what the "exact amounts" should be.
Split by nights, not by negotiation
FairMate's house shares have a split most apps don't: By days. When you add the rent (or any bill), you choose it just like Equal or Percentage — and FairMate divides the amount by the nights each person was actually a member of the house during that billing month. Join dates and leave dates are already on each housemate's profile, so there's nothing to calculate and nothing to argue: the answer falls out of the calendar.
Add the expense → Split → By days — "splits by nights each person was a member in this period — fair when someone joins or leaves mid-month." Everyone sees their own share and exactly how it was worked out.
A worked example
| September rent | A$2,400.00 |
| Asha & Ben — all 30 nights each | 30 + 30 nights |
| Priya — moved in on the 21st | 10 nights |
| Asha pays (30 of 70 nights) | A$1,028.57 |
| Ben pays (30 of 70 nights) | A$1,028.57 |
| Priya pays (10 of 70 nights) | A$342.86 |
Total member-nights: 70. Priya pays for her ten nights — A$342.86 — instead of the A$800 an equal three-way split would have charged her. Nobody had to propose a number; the calendar did.
The same rule handles a mid-month leaver: their nights stop on their leave date, and the remaining housemates absorb the rest — proportionally, automatically. If everyone was there the whole month, By days gives exactly the same result as an equal split, so there's no downside to using it on every recurring bill.
What about the rest of the month's bills?
Electricity, internet and the cleaner can all use By days too — or stay on Equal if the house prefers simple. FairMate then nets the whole month into the fewest possible payments, so a mid-month change doesn't turn into six little transfers. One screen shows who pays whom, and everyone taps to settle directly — FairMate never holds your money.
Want to sanity-check a number before you move house? Try the free bill split calculator, or see how FairMate's share-house tools compare to other split apps on the comparison page — most of them, including Splitwise, don't do day-weighted splits at all.
Setting it up, step by step
The whole thing hangs on one piece of admin most houses already do badly: recording when people actually joined and left. In FairMate that's part of adding a housemate — their join date goes on their profile, and a leaver gets a leave date when they give notice. Get those two dates right and every By days split downstream is automatic.
- Add your housemates with real move-in dates — not "whenever", the actual key-handover day. New joiner mid-month? Set the date they moved in, even if you're adding them a week later.
- Add the rent as a monthly bill and pick By days as its split. Do the same for utilities if the house wants them prorated too.
- Check the preview — before saving, FairMate shows each person's share and the nights behind it, so the new housemate can see the logic before the number ever lands on them.
Two edge cases worth knowing. Someone who joins and leaves inside the same month simply gets their nights counted between the two dates — no special handling. And a bill that spans the change (a quarterly power bill, say) uses its own coverage dates, so each person is only weighted across the portion of the bill they were actually there for. The calendar keeps being the referee, which is exactly what you want: referees that nobody in the house has to be.