🧾 Flatmate bills — who pays what
Put in everyone sharing the place and the bills that arrive, then see what each person puts in — split evenly or by a percentage per person — read out per week, per fortnight, per month and per year.
Results
What each person pays, and how often
The stats and tables below are the same bills in other shapes: what leaves each pocket, each bill converted to the frequency on its own line. Rent and bills get talked about per week here, so the weekly figures are the ones to hold a listing up against — the month and the year are there because that is where the size of the thing shows.
Each person's share, week to year
One line per person, the same share read four ways. The share is what each of them pays of every bill on the list — a line reading 40% is paying two-fifths of the rent, the power, the water and the internet alike.
| Who | Share of the bills | Per week | Per fortnight | Per month | Per year |
|---|
Bill by bill
One line per bill, and after the year column, one column per person: what they put in for that bill each time it arrives, at the frequency on its own line. The rent line is what each person transfers each week; the water line is their share each quarter. The last line covers a whole year, where one figure per person says everything.
An estimate, not advice, and only as good as the numbers in the boxes. Each bill is turned into a year's worth — weekly × 52, fortnightly × 26, monthly × 12, every 2 months × 6, quarterly × 4 — and read back out at whatever frequency is being looked at, so a month is 52 weeks ÷ 12, a little more than four weeks. The shares are percentages of every bill rather than fixed amounts: when the boxes do not add to 100, each person pays their slice of the total in the boxes. Somebody on 0% pays none of it. Cents are rounded for display, so a column can sit a cent away from its total. One-off costs — a bond, a moving truck, the first shop for the house — do not fit a share of recurring bills and are left out. Neither does a flatmate who pays a fixed dollar amount rather than a share: work out what that comes to as a percentage first. And the split itself is an agreement between people, not a calculation — this page can say what each share comes to, never who should be paying it.
Why use this calculator?
Splitting bills goes wrong in two familiar ways: one person pays a fixed amount every month while the others pay a share, or the rent arrives weekly, the power monthly and the water quarterly, so nobody is comparing the same thing. This calculator takes everyone sharing the place and every bill that arrives, at its own frequency, and shows what each person puts in — a week, a fortnight, a month and a year at a time — so the split is a figure everybody can see rather than an argument that comes back every quarter. The page gets handed round a flat, so it names everybody rather than calling one person you.
Why splitting a bill is a maths problem
Splitting a bill fairly is a genuine problem, with a game-theory answer and a very Australian piece of context:
- The unscrupulous diner’s dilemma is the name for what happens when a group agrees to split the bill evenly. Ordering the expensive dish is the rational individual move, because the extra cost is shared — so everybody orders it, and everybody ends up worse off than if they had all ordered the cheap one. It is a prisoner’s dilemma with a menu.
- Fair division is a field of mathematics built for exactly this kind of thing: dividing costs or goods so that each person gets what they consider their due. The archetypal procedure is divide and choose — one person cuts the cake, the other picks a piece — which is fair by construction, because the person cutting will make the pieces as even as they can.
- In Australia the price on the bill is the whole price. The GST is 10%, it started on 1 July 2000, it applies to most goods and services (basic food, healthcare and housing are largely outside it), and it sits inside the amount shown rather than being added at the counter.
How does this calculator work?
Every bill is turned into a year's worth as it is read — weekly × 52, fortnightly × 26, monthly × 12, every 2 months × 6, quarterly × 4, every 6 months × 2, yearly × 1 — and that yearly figure is read back out at whatever frequency you are looking at:
a bill per year = the amount × times it comes round in a year
The shares are percentages of every bill rather than fixed dollar amounts, and the boxes are treated as a total rather than trusted to add up to 100:
a person's share = their % ÷ the total of all the % boxes
- Split evenly, every person counts as one unit, so the total is the number of people — the same arithmetic as 50%, 25% and 25%.
- If the boxes add to 105, each person pays their slice of that 105, so a half-typed split still divides every bill in full and nothing is quietly lost.
- Somebody on 0% pays none of the recurring bills, which is what the box is for.
- Each person's share is read out per week (the year ÷ 52), per fortnight (÷ 26), per month (the year ÷ 12) and per year, and the bill-by-bill table shows what each person transfers when that bill arrives, at the bill's own frequency.
Frequently asked questions
What if the percentages do not add up to 100?
Then each person pays their share of whatever the boxes do add up to. Type 55 and 50 and the second person pays 50 ÷ 105 of every bill, so the bills are still divided in full and nothing is quietly lost — the note under the headline says what the boxes came to. That also means a half-typed split still gives a sensible answer.
Can one person pay nothing?
Yes. A share of 0, or a box left empty, means that person pays none of the recurring bills — which is how a flatmate who is away for the season, or somebody whose rent is covered by a parent, is put into the list without them distorting the split for everybody else.
How do I set an uneven split?
Untick "split the bills evenly" and type each person's percentage. A bigger room, an ensuite, a couple sharing one room or a shorter lease are all reasons a flat might agree on something other than equal shares; the boxes take whatever you agreed, and the page does not care what the numbers are so long as everybody is happy with them.
Someone in the flat pays a fixed dollar amount, not a share. What then?
This page is built on percentages, so turn the fixed amount into its share first: their dollars ÷ the total of all the bills in the same period × 100. That percentage is what goes in the box, and the rest of the page follows. A fixed amount stays fixed while a percentage moves with the power bill, which is exactly why the two do not mix on one list.
Why does the monthly column show more than four weekly ones?
Because a month is 52 weeks ÷ 12, or about 4.33 weeks. Reading a weekly figure as four weeks a month understates the year by around 8%, which over a year is the difference between a bill being paid and a bill being missed. Every figure here comes from one yearly total, so the columns cannot drift apart.
What about one-off costs like a bond, a moving truck or the first food shop?
They are left out, because a share of a one-off cost is usually a different arrangement from a share of a recurring bill and the two do not belong on the same list. Work them out separately — the same percentages will usually do — and keep this page for what comes round every month.
Who is right when we disagree about the split?
That is not a calculation. The page can say what each share comes to once it is agreed — what a 40% share of the power bill is, in dollars, every quarter — but it cannot say what is fair. The split is an agreement between people, and the honest version of it is the one everybody says out loud before the bills arrive.
What does the share button actually send?
The sentence on the result card and a link to this page, and nothing else. The site sends nothing anywhere by itself: the button hands that sentence to your own device’s share sheet, or puts it on your clipboard when the device has no share sheet to offer. What you do with it after that is between you and whichever app you picked.