Splitting expenses by income: the calculation, step by step
Someone earning €1,500 and someone earning €2,500 don't split the rent down the middle: they split the effort evenly. The calculation behind it fits in three lines, and the arguments almost always start over the first one.
6 min read
Splitting by salary means everyone pays the same percentage of their income, not the same amount. With two incomes of €1,500 and €2,500, the shares aren't 50 and 50 but 37.5% and 62.5%, and the person who earns less no longer finishes the month with a third of what they have.
The principle fits in one line. What people argue about is always everything else: which income counts, what to do when salaries change every month, and whether pure proportional splitting is really the best choice or one of its two variants is. This piece is about the calculation. If you're looking for the bigger picture of methods for couples, we've covered it here: how to split expenses as a couple.
There are no references here to laws, marital property regimes or tax: they vary from one country to another, and the arithmetic doesn't.
How to split expenses by income: the calculation in three steps
- Add up the incomes. €1,500 + €2,500 = €4,000.
- Work out each person's share. 1,500 / 4,000 = 37.5%. 2,500 / 4,000 = 62.5%. The two shares add up to 100%: that's the only check you need.
- Apply the shares to the total of your shared expenses, rather than to each expense one by one, unless you'd prefer to.
Which income goes into the calculation
This is where the calculation gets stuck, not in the division. Agree on it once, and write it down:
- net, not gross. You divide what actually lands in the account, not what the contract says;
- regular income that isn't salary counts: rent received, benefits received, a side business;
- a thirteenth-month payment and bonuses are either divided by twelve and added in, or left out and dealt with separately when they arrive. Both choices are defensible; the one that isn't is leaving it vague;
- debts taken on before the relationship are not subtracted from income. They're personal expenses, not lower income: subtracting them would make the others pay off a loan they never signed.
The calculation is redone when something changes, not every month. A promotion, a period of leave, a lost job: you sit down again for ten minutes and update the percentage. A ratio worked out on salaries from three years ago is no longer a ratio.
Three variants, and what each one changes
Pure proportional
The one in the example: each person pays their percentage of income. Everyone keeps the same percentage.
It's the simplest to calculate and the easiest to defend. Its limit shows when incomes are very low: 65% of €1,500 isn't 65% of €2,500, and below a certain threshold the percentage you keep matters less than the money you keep.
Proportional after a guaranteed amount
You set aside the same minimum amount for each person, and split only what's left over in proportion.
With €800 guaranteed each, that leaves €700 and €1,700 to compare, that is 29.17% and 70.83%. On the same shared expenses, that comes to €408.33 and €991.67: the total is still €1,400, to the cent.
The person who earns less pays about €117 less than with pure proportional. It's the variant you need when one income is low in absolute terms, not just low compared with the other.
The same remainder for everyone
You start from the end: once everything is paid, each person should be left with the same amount. €4,000 minus €1,400 is €2,600, that is €1,300 each. So the first person pays €200, the second €1,200.
It's the fairest in the strict sense, and also the hardest to accept, because it puts the difference in income on display every month. It works for couples who genuinely pool everything; in a flatshare between friends, nobody suggests it.
When salaries change every month
If you're self-employed, paid on commission or working variable shifts, you don't have a stable percentage, and recalculating it every month turns the method into a job in itself.
Two approaches that hold up over time:
- the average of the last twelve months, reviewed once a year. It's the simplest, and it smooths out the seasons;
- quarterly recalculation, if income fluctuates a lot. More precise, and it means four conversations a year instead of one.
In both cases, the agreed percentage applies until the next review, even in a month when someone earns very little. Recalculate month by month and you end up arguing about money twelve times a year, which is exactly the opposite of what you wanted.
How it works in Kotisso
Kotisso keeps the tally; it doesn't touch your bank accounts and doesn't move any money. An income-based split is set on the expense, in two ways, and both are included in the free version:
- Percentages: enter 37.5 and 62.5. The percentages must add up to exactly 100%, and until they do, the expense won't be saved;
- By shares: 1,500 and 2,500 simplify to 3 shares and 5 shares. Same result, no decimals to copy out, and it's the handiest option when the split is a simple ratio.
On a €1,400 expense, both routes give the same figures, €525 and €875. Rounding is handled by the tally, not by you: the amounts are rounded so that they always add up to the total of the expense, to the cent. Not a single cent appears or disappears.
Rent and bills are recorded as recurring expenses: they come back on their own every month with the same split, and nobody has to think about them. At the end of the month, the balance shows who owes how much, and the breakdown can be exported to a spreadsheet and to PDF, free of charge.
One thing Kotisso doesn't do: apply a split to all of a group's future expenses with a single switch. The split belongs to each individual expense, and it's the recurrence that repeats it. In practice, you set it once on the rent and the bills, and those are the lines that carry the weight.
If you're still choosing a tool, our comparison also covers which ones are free and which aren't: Is Tricount free or paid?
The questions that always come up
How do you calculate splitting expenses by salary? Add up the net incomes, divide each person's income by that total to get their percentage, and apply that percentage to the shared expenses. With €1,500 and €2,500, the shares are 37.5% and 62.5%: on €1,400 of expenses, that's €525 and €875, and each person keeps 65% of what they earn.
Is splitting by income fairer than splitting down the middle? It depends on what you call fair. Split down the middle, everyone pays the same amount; split proportionally, everyone makes the same effort. A 50/50 split is perfect when incomes are close and becomes a burden as they drift apart: on €1,500 and €2,500, splitting down the middle leaves one person with 53% and the other with 72%.
And if one of you has no income? Pure proportional splitting assigns them 0%, which is mathematically correct and should be said out loud rather than applied straight away. Couples in this situation usually choose the same remainder for everyone, or a guaranteed amount for the person who isn't earning, so that they have money of their own they don't have to account for.
How often should the calculation be redone? Whenever incomes change, and at least once a year. With variable income, a twelve-month average reviewed annually saves you from reopening the discussion every month.
Does it also apply to a house shared between friends? Yes, with one difference: between flatmates, costs are usually split equally, because nobody wants to say how much they earn. A proportional split tends to come in when the rooms aren't the same or when someone stays fewer days. The method is the same; only the criterion changes: splitting expenses between flatmates.