Three ways to split shared costs
An example: Partner A takes home $4,000 a month, Partner B $2,500, and together they spend $3,000 a month on rent, bills, groceries and the rest of the shared life.
| Model | A pays | B pays | A keeps | B keeps |
|---|---|---|---|---|
| 50/50 | $1,500 (38%) | $1,500 (60%) | $2,500 | $1,000 |
| Proportional to income | $1,846 (46%) | $1,154 (46%) | $2,154 | $1,346 |
| Equal leftover | $2,250 (56%) | $750 (30%) | $1,750 | $1,750 |
- 50/50. Each pays half. Simple, and fair when incomes are close. With a gap, the lower earner hands over a much bigger share of their pay.
- Proportional to income. Each pays the same percentage of take-home pay. The higher earner pays more in money, and both give up the same share.
- Equal leftover. The split is set so that both keep the same amount for themselves after the shared costs. It treats the household as one unit, which suits couples with a big income gap or where one partner does more unpaid care.
Why 50/50 often isn’t fair
In the example, 50/50 takes 38% of A’s pay and 60% of B’s. A ends the month with $2,500 for themselves, B with $1,000. Nobody chose that gap; it comes from paying the same amount out of different incomes. Proportional narrows it, and equal leftover closes it.
What counts as shared
Most couples share the home first: rent or mortgage, bills and groceries. After that it varies: eating out together, the car, children, holidays. Subscriptions and hobbies are often personal. The calculator starts from CapKin’s couple categories (Rent & bills, Groceries, Transport, Eating out, Fun, Health, Other) so you can enter each one, or just a single total.
If groceries are the number you are unsure of, the grocery budget calculator works it out from official data for your household.
How to talk about it
- Start with what you both want: fewer arguments, a fairer feel, or one shared budget. The right model follows from that.
- Look at what each of you keeps, not only what each of you pays. That number is what decides whether a split feels fair.
- Pick a date to look at it again. Incomes change, and a split agreed once tends to drift.
For running the shared money day to day, see how a budgeting app for couples keeps shared and personal spending in one place.
Frequently asked questions
- How do you split bills by income?
- Add up both take-home incomes and divide each by the total. That is each partner’s share. With $4,000 and $2,500, A’s share is 62%, so on $3,000 of shared costs A pays $1,846 and B $1,154.
- Is splitting expenses 50/50 fair?
- When your incomes are within about 15% of each other, usually yes. With a bigger gap, 50/50 takes a larger part of the lower earner’s pay and leaves them with much less for themselves, which is why many couples move to a proportional split.
- What is the equal leftover method?
- You set the contributions so that both of you have the same amount left for yourselves after the shared costs. Each partner pays their income minus half of what is left over together. It works only while both contributions stay above zero.
- Which split should we use?
- Our rule of thumb: incomes within 15% of each other, 50/50; 15–40% apart, proportional; more than 40% apart, or one of you doing unpaid care at home, equal leftover. It is a starting point for the conversation, not a rule.
- Should personal debts come off before splitting?
- Many couples take fixed personal costs, such as a student loan or child support from a previous relationship, off that partner’s income first. The calculator does this for the proportional and equal leftover models when you enter them.
- How does 50/30/20 work for couples?
- 50/30/20 puts half of take-home pay into needs, 30% into wants and 20% into savings. As a couple you can apply it to your combined income for the shared budget, then decide the split of the “needs” part with one of the three models above.
- Do you see our incomes?
- No. Everything is calculated in your browser. The share link carries only the country, the model and the split as a percentage; the email carries the split and, only if you tick the box, your shared costs. Incomes are never sent.
How we calculate this
All three models are plain arithmetic on what you enter. 50/50 halves the shared costs. Proportional splits them by each partner’s share of combined income. Equal leftover sets the contributions so that both keep the same amount. The suggested model is CapKin’s rule of thumb, based on how far apart your incomes are; it is not financial advice.
Full methodology and sources
- 50/50: each contribution = shared costs ÷ 2.
- Proportional: A’s contribution = shared costs × A’s income ÷ (A’s income + B’s income); B pays the rest.
- Equal leftover: each contribution = that partner’s income − (A’s income + B’s income − shared costs) ÷ 2. If either comes out below zero, the model is marked as not possible.
- What each keeps: income − personal fixed costs − contribution.
Incomes are take-home pay, after tax. Weekly or yearly figures are converted to monthly (× 52 ÷ 12, or ÷ 12). Personal fixed costs (a student loan, child support, debt repayments) come off that partner’s income before the proportional and equal leftover models are worked out; 50/50 ignores them in the split but they still reduce what that partner keeps.
Rule of thumb. We compare the incomes as (higher − lower) ÷ higher. Under 15%: 50/50. 15–40%: proportional. Over 40%, or when one partner works less to look after children or the home: equal leftover, if it is possible with your incomes. These thresholds are our judgement, not a published standard.
What this leaves out. Tax allowances and benefits that depend on the couple, savings and assets each partner brings, debts in joint names, and pension contributions. The split covers spending you agree is shared; it does not decide who owns what.