Every couple sharing a roof eventually has the conversation, and most have it badly — because it’s conducted in vibes. “We split everything half and half” sounds like fairness itself. Run it through a spreadsheet where one person earns £3,400 a month and the other £1,900, and a different picture appears: after equal shares of a £2,200 household, one partner has £2,300 left and the other £800. Same bills, wildly different lives.

There’s no universally right answer here — but there are three coherent systems, and a small model that shows both people what each one actually does. The spreadsheet’s job isn’t to pick; it’s to make the conversation honest.

The three systems, as formulas

Inputs: each person’s take-home (the payslip model feeds this), and the shared monthly cost — the household’s four-number budget plus the sinking funds.

Equal: =shared_cost / 2 each. Simple, and regressive — the lower earner pays a far larger share of their income.

Proportional: each pays in the ratio of their take-home:

=shared_cost * [@TakeHome] / SUM(TakeHome)

On the numbers above: £1,412 and £788. Equal sacrifice instead of equal pounds — both partners keep the same percentage of their own income for themselves.

Pooled: all income lands in one pot; the pot pays everything; personal spending comes as equal allowances. The fully-merged model — simplest to run, biggest ask in autonomy.

earns £3,400 earns £1,900 pays £1,100 pays £1,100 equal: keeps £2,300 equal: keeps £800 50 : 50 same pounds, unequal lives pays £1,412 £788 proportional: keeps £1,988 (58%) proportional: keeps £1,112 (58%) 64 : 36 same share of self, equal sacrifice
The same household, two definitions of "fair". The model doesn't pick one — it makes sure both people are looking at the same picture when they do.

What the model surfaces that vibes don’t

Build it with both systems side by side — a Data-Table-style comparison of leftover-per-person under each — and the discussable facts appear: the leftover ratio (equal splitting on unequal incomes routinely leaves one partner with 3–4× the personal money); the what-counts-as-shared boundary (the energy model, groceries, the car and its true cost — agreeing the list is half the argument, and a Table with a Shared? column settles it line by line); and the long-game asymmetries — if one partner’s pension stalls while covering household costs, “fair this month” can be quietly unfair over decades. Net worth is a household line too, and the proportional conversation extends to it.

Practicalities that make any system survivable: a joint bills account fed by standing orders (each person’s computed share, updated when salaries change — the model recalculates in one edit), personal accounts untouched beyond that, and a yearly review rather than a monthly renegotiation. The system you pick matters less than both people having seen the same numbers when picking it.

One sheet, three systems, both leftovers visible. The money conversation every couple dreads turns out to be about forty minutes long once it’s conducted in arithmetic — and the spreadsheet, as ever, is just the neutral table everyone can sit at.