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Splitting in proportion to income
Fifty-fifty is simple, and sometimes unfair: the same amount does not weigh the same on two different budgets. Splitting in proportion to income fixes that. It comes down to one division — as long as you know which incomes to put in it, and when not to use it at all.
The short answer
Each share is that person’s income divided by the two incomes together. With £2,000 and £3,000, the total is £5,000: the shares are 40% and 60%. You then apply those percentages to every shared cost.
The ratio is worked out once, not for every expense. You set it, you write it down, and it holds until the incomes really move.
The calculation, on one example
Clara earns 2,000 a month, Thomas 3,000. Together, 5,000.
- Clara’s share: 2,000 ÷ 5,000 = 40%
- Thomas’s share: 3,000 ÷ 5,000 = 60%
A 300 sports club membership therefore splits into 120 and 180, instead of 150 each. Thomas pays 30 more than an even split — and that difference represents the same effort for both of them.
If Clara put the 300 in, Thomas owes her 180. The ratio changes how a cost is divided; it never changes the fact that whoever pays up front gets paid back.
Which income do you use?
This is where disagreements start, far more than over the division itself.
Income after tax. It is what actually lands in the account, so it is what can actually pay for school lunches. Pre-tax figures show gaps that no longer exist once tax is taken.
All income, not just salary. Pensions, rent received, benefits, self-employed earnings: if it arrives every month and it pays for living, it counts.
Irregular income is averaged over twelve months. Someone who invoices 6,000 in March and nothing in August does not have two ratios. They have one, worked out across the year.
The “money left to live on” variant
The plain ratio has a known limit: on a low income, 40% of a cost can still be unpayable, because rent and food already take most of the budget.
The correction is to subtract a living minimum from each income before working out the ratio. With a minimum of 1,000:
- Clara: 2,000 − 1,000 = 1,000
- Thomas: 3,000 − 1,000 = 2,000
- New shares: 33% and 67%
The gap widens, and that is the point. This variant needs you to agree on one more number — the living minimum — which makes it fairer but more arguable. Use it only when the income gap is wide.
When the ratio is the wrong tool
When the cost does not benefit both equally. A train ticket for one parent is not a shared cost, whatever the ratio says.
When child support is already being paid. It was usually set with incomes in mind. Applying a ratio to everyday costs on top of it counts the same gap twice. In that case an even split of the costs is generally right — the support has already done the rebalancing.
When the ratio has not been reviewed in two years. A ratio built on out-of-date incomes is less fair than an honest fifty-fifty.
Making it last
A splitting method is only worth as much as its chance of surviving a tired evening. Three things help.
Write the ratio down, with its date. “40/60 since January 2026” saves you from reconstructing it from memory six months later.
Review it on a fixed date, once a year, rather than in the middle of an argument. A real change — losing a job, a rise — justifies reviewing it sooner.
Apply the same ratio to everything, not case by case. Choosing the split one expense at a time is the surest way to reopen the discussion every time.
With WeSplit
The shares mode does exactly this calculation: you enter 40 and 60 once, and every expense then divides itself, down to the penny. The balance says who owes what, at any moment.
Nothing to recalculate by hand, and nothing to remember: the ratio lives in the group, where both parents can see it.