Splitting costs when one of you earns more.

Two people share a flat. One takes home 2,400 a month, the other 3,900. Rent, bills, food and the standing charges nobody enjoys come to 2,100. The currency does not matter and neither does the exact rent. What matters is that there are only three sensible ways to divide that 2,100, they produce very different lives, and almost nobody sits down and picks one.

The default, and what it costs

Straight halves. 1,050 each.

The person on 2,400 keeps 1,350. The person on 3,900 keeps 2,850. One of them has just over twice as much left as the other, from a gap in pay that was only about sixty percent.

That is the thing about splitting a fixed cost equally: it magnifies the difference in income rather than reflecting it. Shared costs are close to the same for both people, so they eat a much larger share of the smaller salary. The higher earner does not usually notice, because from where they sit the arrangement is obviously fair. Everyone paid the same.

Halves work when the incomes are close, or when both people would rather keep money entirely separate and are willing to pay for that with the arithmetic. They stop working the moment one person is saying no to things the other has stopped thinking about.

Proportional: everyone pays the same share of what they earn

Add the two incomes. 2,400 and 3,900 is 6,300. Work out each person's slice: 38 percent and 62 percent. Apply those to the shared 2,100.

• The person on 2,400 pays 800.
• The person on 3,900 pays 1,300.

Now the leftovers are 1,600 and 2,600, and the ratio between them is exactly the ratio between the salaries. Both people are spending the same third of their pay on the flat. Neither is subsidising the other in any sense that survives a conversation.

Two things to decide before this works in practice.

Which income figure. Use take home pay, after tax and after anything deducted at source. Gross pay flatters whoever has the bigger tax bill.

When it gets recalculated. Once a year, or whenever someone's pay changes by more than a little. Not every month, or the arithmetic becomes a monthly negotiation, which is the thing you were trying to avoid.

Equal leftovers: the version people arrive at when it is tight

Take the total income, 6,300. Take off the shared costs, 2,100. That leaves 4,200 of personal money, which is 2,100 each.

Work backwards. The person on 2,400 pays 300. The person on 3,900 pays 1,800.

This one is honest about what it is doing. It treats the household as one pot and gives both people the same spending money. It is common where one person earns much more, where one is studying or on parental leave, or where the gap is temporary and both of them know it.

It is also the arrangement most likely to be resented later, and usually not by the person you would guess. The higher earner is fine with it right up until something changes: a bonus that gets absorbed, a raise that produces no visible difference in their own life. If you pick this one, agree what happens to a raise before anyone gets one.

The three, side by side

Same flat, same bills, same month. Three arrangements, and the difference to the lower earner is 750 a month, which over a year is a holiday, or an emergency fund, or the deposit conversation happening a year earlier.

The parts that are not the split

Whichever you pick, four questions decide whether it survives contact with a real year.

What counts as shared. Rent, utilities, the internet, food eaten at home, cleaning, the boring insurance. Not clothes, not a phone contract someone chose, not a gym. Write the list down once. The arguments are almost never about the percentage. They are about whether a thing was on the list.

Savings. If you are saving for something joint, it goes in the shared column and gets split like everything else. If you are saving separately, it comes out of personal money and is nobody else's business. Mixing the two, where one person's saving is treated as a shared goal and the other's is treated as spending, is the fastest way to make one person feel supervised.

One offs. A washing machine, a flight for a funeral, a vet bill. Decide the threshold above which something is discussed rather than just paid for, and split anything above it the same way as everything else. A number that is unambiguous, like 200, beats a principle that is not.

Irregular income. If one of you freelances, the monthly percentage swings around and nobody wants to recalculate in February because January was quiet. Use a rolling average of the last three or six months, fixed for a quarter at a time. Being slightly wrong on purpose is better than being exactly right on a moving target.

Running it without a spreadsheet on Sunday

The arithmetic above takes two minutes. The work is everything after: someone pays the electricity, someone else buys the food, the numbers are on two different cards, and by the end of the month neither of you can reconstruct who owes what without going through statements.

That reconstruction job is the actual reason these arrangements fail. Not unfairness. Tedium.

What makes it stick is one place where shared spending lands as it happens, marked as shared, visible to both of you, with the rest of each person's money staying private. Then the monthly settle is a number you read rather than a number you build.

That is what shared money in Orla does. Accounts you choose to share, spending you mark as shared, a running balance of who is up and who is down, and one payment at the end of the month to flatten it. Everything you did not share stays yours, including the accounts it sits in. If it is you and one other person, the couples and family side is the shape of it.

Pick badly, but pick out loud

There is no split that is right in general. Halves are cleanest and hardest on the lower earner. Proportional is the one most people land on once they have done the arithmetic. Equal leftovers is the most generous and the most fragile.

The failure mode is not choosing the wrong one. It is never choosing, defaulting to halves because it needs no discussion, and finding out two years later that one of you has been quietly not going to things.