The default assumption for most co-parents is 50/50. It feels fair — equal split, no favouritism. But 50/50 is often not fair at all. Fairness is about proportion, not equality.
If one parent earns significantly more than the other, a 50/50 split means the lower-income parent is contributing a much larger share of their disposable income to the same expense. That is not fair — it is just symmetrical.
Models for Splitting Expenses
The Equal Split (50/50)
Each parent pays half of every shared expense. Simple, clear, and easy to administer. Works well when both parents have similar incomes.
The Income-Proportional Split
Each parent pays a percentage that reflects their share of the combined household income. If Parent A earns $80,000 and Parent B earns $40,000, the combined income is $120,000. Parent A pays 67% and Parent B pays 33%.
This model is more equitable — but requires both parents to be transparent about their income, which can itself be a source of conflict.
The Categorical Split
Different categories have different splits. Medical might be 50/50, school fees might be 60/40, and one parent covers extracurriculars entirely. This allows for nuance — for example, if one parent signed the child up for expensive activities, it might be fair for them to bear a higher share of that cost.
The Per-Child Split
Different splits for different children. Perhaps one child has higher medical needs, or one parent has a stronger connection to a particular child's activities. Per-child splits allow for individual circumstances.
How to Choose the Right Model
The right split is the one both parents genuinely agree is fair — not the one imposed by default or by whoever argued longer.
Start with these questions:
- Is there a significant income disparity between parents?
- Are there categories where one parent makes most of the decisions (and should therefore bear more cost)?
- Are there children with significantly different needs or costs?
- What does each parent's custody arrangement look like? More custody time may mean more incidental costs.
Setting Rules That Apply Automatically
Once you have agreed on a split model, the next step is making it automatic. Manually negotiating the split for every expense is exhausting — and opens the door to disagreements.
Intelligent expense rules allow you to set your agreed percentages by category or by child, and have them applied automatically to every expense you log. When you log a dental expense, your pre-agreed 50/50 medical split is applied without any discussion. When you log a soccer registration, your 70/30 sports split is applied.
When to Revisit Your Split Agreement
Split agreements should not be set in stone forever. Review them annually, or when:
- Either parent's income changes significantly
- Custody arrangements change
- A child's needs change (new medical condition, change of school)
- Either parent remarries or has additional children
A split that was fair three years ago may not be fair today. Building in a regular review prevents resentment from accumulating around an outdated arrangement.
The Most Important Thing
Whatever split you choose, both parents need to genuinely agree it is fair — not just reluctantly accept it. An agreement reached under pressure is not an agreement; it is a grievance waiting to happen. Take the time to discuss it openly, consider multiple models, and document what you agree on.
Fairness is not just about the numbers. It is about both parents feeling respected and heard. Get that right, and the numbers become much easier to agree on.