Equal and fair aren't always the same thing. One of the first money questions you'll work through as coparents is how to divide the costs your kids generate — and the honest answer to 50/50 vs. proportional for child expenses depends on your two incomes, the category of the expense, and what you can both apply consistently without re-deciding it every month. This guide walks through the three common approaches, two worked examples, and how to land on a split that actually holds.
It's tempting to default to 50/50 because it sounds fair and the math is easy. But a clean even split can quietly leave one household stretched thin while the other has room to spare — and kids feel the difference between two homes that both feel steady and two that don't. The goal here isn't a perfect calculation. It's a rule both parents can fund without resentment, write down once, and stop renegotiating every time a receipt shows up. Our broader guide to co-parenting shared expenses covers the full system — categories, tracking, settling up — and this post zooms in on the one decision that shapes every entry in it: the split itself.
What are the three common splits for child expenses?
Almost every coparenting arrangement lands on one of three approaches. None is automatically the right one — each fits a different income picture and a different tolerance for bookkeeping. It helps to know all three before you decide, because the best choice for your family is usually obvious once you can see them side by side.
The good news is this isn't a decision you have to make blind. The right split usually reveals itself once you put two things on the table — your incomes and the categories you've agreed to share — and look at them honestly. Here's how that conversation tends to go.
50/50 vs. proportional: how to tell which fits
Take it in two passes. First the income picture, which sets your default ratio. Then a quick category sanity-check, which tells you whether that one ratio is enough or whether a couple of categories want their own treatment — and if you haven't yet pinned down which categories are even shared, our guide to the expense categories most coparents forget to define is the place to start. Most families find the answer falls out of those two steps without much debate.
Start with the income picture
Before anyone argues ratios, put the two numbers next to each other. Add both parents' incomes, then work out each parent's share of the total. If you each earn within roughly ten percent of the other, 50/50 is the honest answer — proportional would just be 50/50 with extra steps. If one income is meaningfully larger, that gap is the case for proportional: the parent with more room contributes more, and neither household ends the month underwater. Use take-home pay if your incomes are steady, or a recent average if one of you works variable hours.
Then sanity-check by category
Once you have a default ratio, run it past the categories you've agreed to share. Some costs feel right to split proportionally no matter what — medical bills, tutoring, anything large and non-negotiable. Others feel natural at 50/50 because they're discretionary and roughly even over time, like entertainment or a friend's birthday gift. If your default ratio already feels fair across every category, keep it simple and use the one number. If only a few categories clearly want different treatment, that's your case for a category-based split.
Worked example: two similar incomes
Parent A takes home $4,000 a month; Parent B takes home $3,600. Combined, that's $7,600 — technically a 53/47 split. But that's close enough to even that tracking it as 53/47 buys a precision nobody will actually feel, and it adds a calculation to every single entry. This family is better off calling it 50/50 across the board: a $200 winter coat is $100 each, soccer registration is split down the middle, and the month-end settle-up is a thirty-second total. When incomes are close, the simplest rule is also the fairest one — and the one least likely to get quietly abandoned.
Worked example: a real income gap
Parent A takes home $6,000 a month; Parent B takes home $4,000. Combined, that's $10,000 — a 60/40 split. Here the gap is large enough that 50/50 would land very differently on each household: the same $300 expense is a rounding error for one parent and a real squeeze for the other. Proportional keeps both homes stable — on that $300, Parent A covers $180 and Parent B covers $120. Some families in this spot still keep small, everyday costs at 50/50 for simplicity and apply the proportional ratio only to the bigger categories like medical, childcare, and activities. Either way, the ratio gets written down once and applied the same way every time, so the gap never has to be re-litigated expense by expense.
Make the split stick
The whole value of a split ratio is that it's already settled by the time the dentist's bill arrives. If you find yourselves debating the ratio expense by expense, you don't have a system yet — you have a recurring negotiation wearing a system's clothes. Pick the rule in a calm moment, write it somewhere both parents can see it, and let it do the deciding from then on.
A 50/50 split applied to every expense, every month, will create less friction than a perfectly calculated proportional split that one parent quietly stops trusting. Pick the ratio you'll both actually honor without re-checking each other's math. The point of the number is to end the conversation, not to start a new one each month.
Incomes change — a raise, a layoff, a return to full-time work. That's a real reason to adjust the ratio, but it's a once-or-twice-a-year conversation, not a per-expense one. Put a check-in on the shared calendar, and until that date arrives, the agreed split stands even on the months it feels slightly off.
Whichever split you choose, the rule is the same: name it once, write it down, and apply it the same way to every shared expense until you both agree to change it. Equal or proportional matters far less than consistent. A split your kids never hear argued about — because it was decided long before the receipt showed up — is doing exactly what it's supposed to do: keeping the money quiet so the two of you can spend your attention on them instead.
coparent lets you set your split ratio once — 50/50, proportional, or your own number — and applies it automatically to every expense you log, so the running balance is always current and the month-end settle-up takes seconds, not an argument.
Try coparent free — set your split once and let it do the math