How Income-Based Bill Splits Are Calculated, Step by Step

An income-based bill split works in three steps. First, work out each partner's share of the household's combined income. Second, apply that share to every shared bill to get what each person is expected to pay. Third, compare the expected amounts with what each person actually paid. The difference is the settle-up: one transfer that brings both of you back to your income shares. That is the whole idea, and it is what EvenUS does by default.

Below is the calculation with real numbers, the adjustments that matter in practice, and what the app shows you once the sums are done.

The worked example: Priya and Tom

Priya takes home £3,000 a month. Tom takes home £2,000. Over one month their shared spending looks like this:

Priya paid £1,400 of it. Tom paid £600. Nobody has done anything wrong here. Rent happens to come out of Priya's account, food out of Tom's, and that is how most households run.

Step 1: How do you work out each partner's income share?

Divide each person's income by the total. Combined take-home is £5,000.

• Priya: £3,000 ÷ £5,000 = 0.60, so 60%
• Tom: £2,000 ÷ £5,000 = 0.40, so 40%

Which income figure goes in?

The app works from what it calls contributable income: take-home pay, plus any support you receive, minus prior obligations you pay out, such as child support from a previous relationship, spousal maintenance, an older debt or a student loan. So if Tom paid £200 a month in child support, his figure would be £1,800, and the shares would become £3,000 ÷ £4,800 and £1,800 ÷ £4,800, about 62.5% and 37.5%.

This matters more than it sounds. A partner meeting a court-ordered payment is not contributing less to this household out of choice, and scoring them against gross pay would read their prior commitment as a shortfall. If you enter income weekly, fortnightly or yearly, it is converted to a monthly figure first.

Step 2: What is each person expected to pay?

Multiply every shared bill by each person's income share, then add the results up.

The app does this per bill rather than on the monthly total. With a pure income split the answer is the same either way, but doing it per bill is what lets individual bills carry their own rules, which comes up below.

Step 3: How do you compare expected with actual?

Subtract the expected share from what each person actually paid.

• Priya: paid £1,400, expected £1,200. Difference: +£200.
• Tom: paid £600, expected £800. Difference: −£200.

The two differences always cancel out, because every pound spent was paid by someone. So the settle-up is a single transfer: Tom sends Priya £200, and both are back at their income shares. In the app, that transfer is recorded as a settlement. It moves balance between you and is never counted as spending, so it does not inflate the month's total.

What each of them has left

After the split, Priya keeps £3,000 − £1,200 = £1,800 and Tom keeps £2,000 − £800 = £1,200. The ratio of what is left (60:40) matches the ratio of what came in. That is the property people usually mean when they call an income split fair. The app shows this "left with" line next to the split, because the money left over is the figure people feel, and the one most couples have never looked at.

How is the money balance figure worked out?

The app compares the share of spending each person actually covered with the share they were expected to cover, and turns the gap into a number from 0 to 100. The arithmetic for Priya and Tom:

1. Actual shares: Priya £1,400 ÷ £2,000 = 0.70. Tom £600 ÷ £2,000 = 0.30.
2. Expected shares: 0.60 and 0.40.
3. Add up the gaps: 0.10 + 0.10 = 0.20.
4. Divide by the largest gap possible. That is 2 × (1 − the smaller expected share), so 2 × 0.60 = 1.20. Here 0.20 ÷ 1.20 ≈ 0.167.
5. Apply a gentle curve (raise to the power 0.7): 0.167 to the power 0.7 ≈ 0.285.
6. Score: 100 × (1 − 0.285) ≈ 71.

Once Tom's £200 settlement is logged, both actual shares match the expected ones and the money figure goes to 100. Two things to notice. The maximum gap depends on your incomes rather than being a fixed number, which keeps the scale honest when incomes are very unequal. And the figure describes the month's arrangement. It never appears as a verdict on either person, and money is kept on its own axis: it is never converted into hours or blended with housework. How the EvenUS fairness score works covers the time side.

Which bills don't follow the income split?

Several, and handling them is most of what separates a usable split from a spreadsheet that drifts.

• Even or one-person bills. Any bill can be marked as split evenly, or as belonging to whoever paid it. A shared gift for a friend might be 50/50 even when everything else is by income.
• Custom amounts. If you agree "I'll cover £70 of this, you cover £30", a custom split overrides everything else for that bill, as long as the amounts add up to the total.
• Personal items in a shared shop. If £25 of a £180 supermarket trip was Tom's running shoes, that £25 goes wholly to Tom and only £155 is split.
• Reimbursements. A work expense your employer pays back, or a return, is subtracted first. Only what the household actually spent is shared.
• Children's costs. By default, a slice of each child-related cost (30%, adjustable in household settings) is split evenly, on the reasoning that both parents chose to have the child, and the rest follows the bill's normal split. In blended families you can set who is responsible for each child's costs, and that takes over. Fair division in blended families goes into that case.

What if you'd rather not split by income?

Income is the default, not a requirement. There are four methods, and the app shows what each one leaves you with before you choose. Using Priya and Tom's £3,000, £2,000 and £2,000 of shared costs:

"End up with the same" sets contributions so both are left with an equal amount: (£5,000 − £2,000) ÷ 2 = £1,500 each. "Split what's spare" lets each person keep a floor first, then shares costs in proportion to what is above it: £1,800 and £800 above the floor, so roughly 69% and 31%, or £1,385 and £615. The floor is a household setting, because one figure means very different things in different cities and currencies.

If you choose an even split with incomes this far apart, the app adds one plain line saying an even split takes a noticeably bigger bite out of one side, and that plenty of couples choose it on purpose. It names nobody and does not call the choice wrong. Splitting bills when you earn different amounts walks through choosing between the methods, and why 50/50 is often not neutral covers the even split in more depth.

Try it on your own numbers

You do not need the app to run the core sum. Add up your shared bills for a month, multiply by each person's income share, and compare with who paid what. It takes ten minutes with last month's statements. The free household fairness calculator does it in a browser with no account, and stores nothing.

Where the app helps is keeping it running: bills logged as they happen, the exceptions above applied automatically, and the settle-up worked out without anyone having to be the household bookkeeper.

Common questions

Should you use gross or take-home pay for an income split?

Take-home is usually fairer, because tax and pension deductions vary and take-home is the money actually available. Whichever you pick, use the same basis for both of you.

What happens if one partner earns nothing?

A pure income split puts all shared costs on the earner and leaves the other partner with no personal money at all. Many single-income couples set up a regular transfer so both have something of their own, and the "end up with the same" method shows what that transfer would be.

How often should you recalculate income shares?

Whenever either income changes meaningfully: a new job, a pay rise, parental leave, or reduced hours. In the app the shares update as soon as the income figure is edited.

Is an income-based split the same as a proportional split?

Yes. "Proportional", "income-based" and "split by income" all describe the same thing: each person contributes the same percentage of their income. Income proportional bill splitting covers the general method.