How to keep a record of shared spending you both trust

A shared record is trusted when both people can see it without asking, both can add to it, every entry says who paid and when, and neither person has to remember anything for it to stay accurate. A record that only one person can see is not a record — it is that person's word, and it puts them in the position of being believed rather than checked.

Both people can open it, any time

This is the one that matters most and the one a spreadsheet on somebody's laptop fails. Not because anybody is hiding anything, but because the other person's only route to the number is to ask, and asking twice in a month starts to feel like an accusation to both of them.

When the record is open, nobody has to ask. The question "where are we?" gets answered by looking, which costs nothing and implies nothing.

Both people can add to it

If one person is the only one who can enter things, they become the household's bookkeeper by default — and that job is invisible, unpaid and much bigger than it looks. It is also the reason most shared spreadsheets die: the person maintaining it gets tired, and there is nobody else with the habit.

Every entry says who paid

A list of amounts is a list of amounts. A list of amounts with a name and a date next to each one is something either person can check against their own bank app in a minute. That is the difference between a record you believe and a record you verify, and verification is what makes the trust cheap rather than effortful.

It does not depend on anybody's memory

Anything that needs somebody to remember at the end of the week will be incomplete, and an incomplete record is worse than none, because it looks authoritative while being wrong. Entries made at the moment the money moves, in ten seconds, are the only ones that reliably exist.

The test for any system: if both of you stopped trying for a fortnight, would the record still be right? If not, it is being held up by effort, and effort runs out.