Split expenses by income: formula, example and calculator

Quick answer

To split an expense in proportion to what each person earns, divide each salary by the sum of the salaries and multiply by the amount. If one person earns €1,700 and the other €1,225, a rent of €900 works out at €523.08 and €376.92: the higher earner pays 58.1% and the other 41.9%, and both are putting in exactly 30.77% of what they earn. You can run the numbers straight away in the expense splitting calculator, with no sign-up.

Why "down the middle" feels unfair when the salaries are not equal

Splitting in two is the easiest rule to apply and the hardest to keep up when there is a gap in income.

With a rent of €900 and salaries of €1,700 and €1,225, paying €450 each leaves one person with 73.5% of their salary and the other with 63.3%. On paper it is the same number; in practice one of them lives comfortably and the other is scraping through to the end of the month. And the conversation ends up sounding like an argument about money when it is really about effort.

A proportional split changes the question. Instead of "how much does each of us put in?", it asks "what share of their salary is each of us putting in?". With the same figures, both are putting in exactly 30.77% of what they earn. The amount each pays is different; the sacrifice is the same.

How it is worked out, with the numbers in front of you

The formula fits on one line:

what each person pays = (their salary ÷ sum of the salaries) × total amount

With the example above:

• Sum of the salaries: 1,700 + 1,225 = €2,925
• The one earning 1,700: 1,700 ÷ 2,925 = 0.5812 → 0.5812 × 900 = €523.08
• The one earning 1,225: 1,225 ÷ 2,925 = 0.4188 → 0.4188 × 900 = €376.92

Both are putting 30.77% of their salary towards the rent. Change the amount and the result changes, but the share each one gives up stays identical.

There is one detail almost nobody accounts for, and it is behind a lot of silly arguments: the cents. Splitting €900 between two percentages rarely lands on round figures, and if each person rounds on their own the total does not add up. A cent will be missing, or left over. It may sound ridiculous, but when the split happens every month and across several expenses, those cents pile up and somebody eventually notices.

The three splits people actually use

Equal shares. Everyone pays the same. It works well when the salaries are similar, and it is the one that needs the least explaining. It stops working as soon as the gap runs into a few hundred euros.

In proportion to salary. What we have just worked out. It is the fairest one in terms of effort and the easiest to defend out loud: nobody argues that someone earning twice as much can put in twice as much.

Proportional with a floor. A variant used when the gap is very wide. You agree that nobody goes below a certain amount, even if the proportional sum would have given them less. It avoids the feeling that one person "is not contributing", which is a real problem even when the numbers say otherwise.

There is no right answer for every couple or every flat share. What does help is to pick one and write it down, because the conflict almost never comes from the method: it comes from each person believing the agreement was something else.

What the calculator does

The splitting calculator does this sum without asking you for an account or an email address. You enter what each person earns and the expense to split, and it gives you back:

• What each person pays, with the cents already squared off.
• What percentage of their salary that comes to for each person, which is the number that really answers "is this fair?".
• The comparison with an equal split, so you can see the difference at a glance.

It takes money however you write it: you can type 1,225.50 with its thousands comma and its decimal point, and 1225.5 works just as well. What you type is shown back to you as the page has read it, so you can check which amount it understood before trusting the result.

And it stores nothing: the figures never reach a database and are not passed on to anyone. They travel inside the address of the page, so you can save it or share it if you want to keep a particular split — and deleting it is enough for no trace to be left.

When splitting stops being a sum and becomes a routine

A calculator settles today's expense. The problem shows up next month, when it all has to happen again with the rent, the electricity, the shopping, the insurance and the four things one of you paid for and has not mentioned yet.

What is needed at that point is not better arithmetic, it is not having to remember. That every expense gets logged with its split, that debts cancel each other out on their own, and that at any moment you can look up who owes what to whom without reconstructing the month from memory.

That is exactly what ControlarGastos does, and the calculator is the part you can try without creating an account.

Frequently asked questions

How do I work out the percentage each person pays based on their salary?

You divide each person's salary by the sum of all the salaries. With €1,700 and €1,225, the sum is €2,925: the first one contributes 58.1% and the second 41.9%. Those percentages then apply to any expense you want to split, with nothing to recalculate.

What if one of us has no income?

The proportional calculation would assign them zero, and that is rarely what anyone wants. In that situation most couples agree on a token minimum, or split only the variable expenses and leave the fixed ones to whoever has an income. It is a matter of agreement, not of formula.

Should bonuses be counted as part of the salary?

What matters is that both are counted the same way. If one of you uses a salary with the bonuses spread across the year and the other a monthly take-home figure without them, the split comes out skewed in favour of the second. The simplest approach is to use each person's average monthly take-home pay, bonuses included and divided by twelve.

Does a proportional split work for a flat share, not just for couples?

Yes, and the formula is the same with more people: each person divides their salary by the sum of all of them. In flat shares it is usually combined with another criterion for the rent — the size of each bedroom — leaving the proportional split for shared costs like electricity or internet.

Do we have to redo the calculation every month?

The percentages only change if the incomes change. As long as the salaries are the same, the split is the same and you just apply it to each new expense. What is worth doing is reviewing it when somebody changes job or gets a rise, because a split that no longer reflects reality is the fastest route to one person feeling hard done by.

Do you store what I type into the calculator?

It is not kept in any database and it is not passed on to anyone. What does happen is that the figures travel inside the address of the page: that is what lets you save or share a particular split, and simply not saving the link is enough for no trace to be left. No account and no email address needed.