Most co-parenting agreements include language about 'shared child expenses' without precisely defining what 'shared' means. Equal halves? Proportional to income? Proportional to custody time? When a dispute arises, the ambiguity of 'shared' is often the entire dispute.
The 50/50 model
Equal split — each parent pays half of every qualifying expense. This is the default assumption most co-parents start with and the easiest to calculate. It doesn't account for income differences, which is why courts in higher-conflict or higher-income-disparity cases often move away from it.
The proportional (60/40 or similar) model
Many agreements base the expense split on each parent's proportion of combined gross income. If Parent A earns $60,000 and Parent B earns $40,000, the split is 60% / 40%. The calculation changes annually as incomes change, which creates its own documentation requirements. Some agreements use custody time proportion instead of income proportion.
What your agreement probably says vs. what it means
- —'Shared equally' — 50/50 split on qualifying expenses. Simple but income-blind.
- —'In proportion to income' — requires annual disclosure of income documentation from both parents.
- —'As agreed' — no default; requires affirmative agreement before each major expense.
- —'Extraordinary expenses require prior written consent' — the most common source of disputes. What's extraordinary? What counts as consent?
The documentation problem with proportional splits
A 50/50 split is easy to document — every expense is just halved. Proportional splits are harder: you need a record of the agreed ratio, the calculation applied to each expense, and confirmation from both parents that the ratio is current. Without documentation of the agreed percentage, a co-parent can simply claim the ratio you used isn't the right one.
How to lock in the split before it becomes a dispute
Define the split percentage explicitly in FairSplit when you set up your co-parenting pair. Every expense request automatically shows the dollar amount each parent is responsible for based on that ratio. When your co-parent approves the request, they're approving both the expense and the split calculation. That approval is logged with a timestamp. There's no 'I didn't agree to 60/40' when their approval is on record.
Updating the ratio when income changes
If your agreement requires annual income-based adjustment, document the update. When the ratio changes, both parents should acknowledge the new split in writing — email, FairSplit message, or better, a legal amendment to the agreement. For the tax year transition, be clear which expenses fall under the old ratio and which fall under the new one. This is exactly the kind of ambiguity that generates expensive attorney time.
What happens when you disagree about which model applies
If you and your co-parent have a genuine disagreement about whether 50/50 or a proportional model applies — and your agreement is ambiguous — that's a matter for a family law attorney or mediator, not a software tool. What FairSplit can do is give you a complete record of what split was used for every expense going back to day one, which is often what a mediator needs to establish what the parties treated as the norm.