Equal splitting is simple. Everyone pays the same amount, no calculation needed. But equal isn't always fair, especially when the people sharing a bill earn very different amounts.
When one person earns €4,000 a month and another earns €1,500, a €700 rent contribution hits them completely differently. For the higher earner, it's 17.5% of income. For the lower earner, it's nearly half their monthly pay. The same number on paper, very different in practice.
Splitting bills proportionally based on income is the alternative. Each person pays a share that reflects what they earn, so the cost takes up roughly the same slice of everyone's income. Here's how to calculate it, when to use it, and how to have the conversation without it getting awkward.
The formula
Proportional splitting is straightforward once you have the numbers.
Your share = (Your income / Total combined income) x Total shared bill
Example: Two roommates, combined rent and bills of €1,800/month. One earns €3,000/month, the other earns €1,200/month.
Combined income: €4,200
Person A (€3,000): 3,000 / 4,200 x €1,800 = €1,286/month
Person B (€1,200): 1,200 / 4,200 x €1,800 = €514/month
As a proportion of income, both are paying roughly 43%. The amounts are different. The burden is the same.
For three people, the logic is identical: add all three incomes together, divide each person's income by the total, and multiply by the shared bill.
What counts as income?
Before you can run the formula, you need to agree on what income means. This sounds obvious but it's worth being explicit, because the answer affects the numbers significantly.
Gross income vs net income: Gross is your salary before tax. Net is what actually lands in your bank account. Net income is usually the more meaningful number for a proportional split, because it reflects what you actually have available to spend. If one person is in a higher tax bracket, using gross income slightly overstates their real financial position.
Regular income only, or everything: For most arrangements, stick to regular employment income. Occasional freelance work, a one-off bonus, or an inheritance are harder to account for and create unnecessary complexity. If someone has a significant and consistent secondary income, include it. If it's irregular, leave it out.
What about savings or investments: Generally, no. The point of a proportional split is to reflect take-home income, not net worth. Including investment income or savings complicates things without meaningfully improving fairness for most people.
A practical default: Use monthly net income (take-home pay after tax and pension contributions). It's the number everyone knows, it's easy to verify, and it reflects actual spending capacity.
When proportional splitting makes sense
Income-based splitting isn't right for every situation. It works best when:
There's a meaningful income gap. A 10 to 15% difference in income rarely justifies the complexity of a proportional system. When one person earns roughly twice what another earns, or more, the difference in bill burden becomes significant enough to matter.
The arrangement is ongoing. Proportional splitting is worth setting up for regular recurring costs: rent, utilities, shared groceries. For a single group dinner, equal split is almost always the right call.
Everyone is comfortable sharing their income. The system requires people to disclose what they earn, which not everyone is comfortable with. If anyone is reluctant, don't push it. A negotiated equal split that both people feel okay about is better than a proportional split that creates awkwardness.
The relationship supports it. Proportional splitting is most natural between close friends, long-term couples, or long-term housemates. It implies a level of trust and mutual goodwill that doesn't fit every arrangement.
Applying it to different scenarios
Couples: Income-based splitting is increasingly common between couples, particularly for shared rent and household bills. It tends to reduce the low-grade stress that comes when one partner is stretching to keep up with a 50/50 arrangement. For a fuller look at how to approach shared finances as a couple, see the guide to splitting bills fairly as a couple.
Roommates: Works well when there's a clear income gap and the group trusts each other enough to share salary information. One practical option for roommates who don't want to share exact figures: each person states a range (€1,000 to €1,500, €2,500 to €3,000) and you use the midpoint of each range for the calculation. Less precise, but it removes the discomfort of stating exact numbers. For more on splitting rent specifically, including room-size formulas, see how to split rent fairly with roommates.
Friend groups: Less common, but occasionally relevant for longer or more expensive trips where the cost is a real stretch for one or two people. On a ten-day holiday, the difference between equal and proportional can be significant. If the group is close and the income gap is real, it's worth raising. The guide on planning a group trip without the money drama covers how to have that conversation before the trip rather than after.
Edge cases worth thinking about
One person's income changes. Someone gets a pay rise, goes part-time, or takes parental leave. A proportional system needs to be revisited when this happens. Set a default review point: annually, or whenever anyone's income changes by more than 20%. Make it a habit rather than a negotiation each time.
One person is temporarily between jobs. This is where the system needs flexibility. A strict proportional calculation might suggest the unemployed person pays almost nothing, which can feel uncomfortable for everyone. A reasonable approach: the employed person covers costs temporarily, and the arrangement is revisited once the other person is working again. Treat it as a short-term adjustment rather than a permanent renegotiation.
Bonuses and windfalls. A December bonus, a tax refund, a freelance payment. Exclude these from the regular calculation. If someone wants to contribute extra in a good month, that's a choice, not a system requirement.
Significant wealth disparity without income disparity. Two people earning similar incomes but with very different levels of savings or family support. Proportional splitting by income doesn't capture this, and probably shouldn't try to. Income is the right proxy for ongoing financial capacity. Wealth is a different conversation.
How to have the conversation
Raising income-based splitting can feel like you're asking for a favour or making an assumption about someone's finances. It helps to frame it as a practical proposal rather than a request.
Something like: "I've been thinking about how we split things, and I wonder if proportional splitting might work better for us given our different salaries. I ran some numbers and I'm happy to share them if you want to see."
A few things that make the conversation easier:
Come with the calculation already done. Showing the numbers takes the abstraction out of it. People respond to concrete amounts much better than hypotheticals.
Make it mutual. The system only works if both (or all) people agree. If someone is uncomfortable with it, don't push. Look for a middle ground: a modified equal split where the higher earner covers a slightly larger one-off cost, or a split somewhere between 50/50 and fully proportional.
Agree on a review cadence. Knowing the arrangement isn't permanent makes it easier to agree to in the first place. Annual reviews, or a review whenever circumstances change, gives everyone an exit point if the system stops feeling fair.
Tracking a proportional split
Once you've agreed on the formula, the day-to-day tracking works the same as any other split. Log shared expenses as they happen, note each person's share, and settle regularly.
The main difference is that you're not splitting every expense 50/50. tricount lets you set custom split ratios per person, so you can log an expense and have it automatically calculate each person's proportional share rather than doing the maths manually each time.
Splitting bills based on income is the fairest approach when there's a meaningful gap in what people earn. The formula is simple: divide each person's income by the combined total, then multiply by the shared bill.
What makes it work is agreeing upfront on what counts as income, building in a review cadence so the arrangement stays current, and being willing to have the conversation directly rather than letting an unspoken imbalance build up over time.
Use tricount to track proportionally split expenses. Set custom splits per person, log costs as they happen, and settle without the mental arithmetic.
