Method
How to split expenses unequally
Splitting equally is a practical convention, not a universal rule. As soon as people consume different things, stay different lengths of time or have very different means, another key produces a result everyone recognises as fairer.
The four keys that actually get used
Percentages suit cases where the proportion is already settled: 70 / 30 between two people, 40 / 40 / 20 on a project. Their only constraint is adding to exactly 100%.
Weighted shares are easier to say out loud: two shares per adult and one per child, or three for the double room and two for the single. Nobody computes a percentage; they count shares.
Income ratios apply between people who live together on clearly different incomes. They equalise effort rather than amounts.
Fixed amounts take over when consumption is individual and known: each person pays for what they ordered, and only the shared extras get divided.
One expense, four keys, four answers
| Key | Alex | Bilal | Chloe |
|---|---|---|---|
| Equal shares | £100.00 | £100.00 | £100.00 |
| Percentages 50/30/20 | £150.00 | £90.00 | £60.00 |
| Shares 2 / 1 / 1 | £150.00 | £75.00 | £75.00 |
| Income 2,400 / 1,800 / 1,800 | £120.00 | £90.00 | £90.00 |
The income row uses a £6,000 total: Alex represents 2,400 / 6,000, so 40% of £300.00. Every row adds back to exactly £300.00.
None of these is objectively correct. They describe different situations, and it is the situation — not the arithmetic — that deserves the discussion.
The rounding problem
Unequal keys can land on fractions of a penny. Rounding each share on its own creates a gap with the total: whoever fronted the money is a penny up or down, and the error repeats on every expense.
The right method is to compute each share in whole pence, then distribute the leftover one penny at a time. On £100.01 between three people that gives £33.34 / £33.34 / £33.33 — never £33.33 for everyone.
Announce the key before paying
An unequal split decided after the fact lands badly, even when it is entirely fair. The same key announced before the expense makes the rule explicit and reduces the risk of disagreement: "the house splits by nights", said while booking.
That is the one social rule that really matters here. Everything else is arithmetic.
Frequently asked questions
Which key should I pick when in doubt?
Pick the one you can explain in a single sentence. A fair split nobody understands gets challenged; a slightly rough split that is obvious gets accepted. That is one reason to consider weighted shares.
Can one group mix several keys?
Yes, and that is the normal case. Groceries split equally, the house splits by nights, the boat trip splits between the people who went. What matters is that each expense has the right key, not that the whole group uses one.
How do I avoid rounding errors?
Work in whole pence and hand out the remainder one penny at a time. A 33 / 33 / 33% key applied by hand to £100 loses a penny; done properly it gives £33.34 / £33.33 / £33.33.
Editorial review — Written and reviewed by the Pivunio team. The methods described here are the ones the app itself uses, and they are covered by automated tests.