If you and your co-parent earn meaningfully different amounts, a flat 50/50 split can start to feel lopsided fast — which is where proportional splitting comes in. The idea is simple: each parent covers the share of costs that matches their share of the combined income.
The calculation, in one line
Add the two incomes together. Divide each parent’s income by that total. Those percentages are what each of you covers on every shared cost. On $48,000 and $32,000, the combined figure is $80,000, so the split is 60/40 — and a $200 dentist bill is $120 and $80 rather than $100 each.
| Parent A income | Parent B income | Split | A pays | B pays |
|---|---|---|---|---|
| $40,000 | $40,000 | 50 / 50 | $365.00 | $365.00 |
| $48,000 | $32,000 | 60 / 40 | $438.00 | $292.00 |
| $60,000 | $20,000 | 75 / 25 | $547.50 | $182.50 |
| $70,000 | $10,000 | 87.5 / 12.5 | $638.75 | $91.25 |
The bottom row is worth sitting with, because it shows the limit of the method. As the gap widens, proportional splitting starts asking one parent to cover almost everything — which may be entirely reasonable, or may be the point at which you agree a floor so both parents retain some direct financial involvement.
What you actually need to write down before using this
- Which income figure counts. Gross or net, and from which sources. Gross annual is the most common choice because it is the easiest to evidence.
- What happens with bonuses, overtime and second jobs. Decide once whether variable pay is included or whether you use base only. Ambiguity here reliably resurfaces the first time someone has a good year.
- When you recalculate. A fixed month each year, plus a trigger — say, any change over 10% — so a new job does not wait eleven months to be reflected.
- What the percentages actually apply to. The method is worthless without an agreed list of shared categories. Proportional splitting of an undefined set of costs just moves the argument.
The split method is honestly only half of what makes this work. The other half is the record. Six months from now, the conversation will not be about the percentage — it will be about whether a particular cost was ever agreed, and that is settled by a dated entry with a receipt attached, not by either memory.
Bottom line: Income-proportional splits feel fairer when incomes are uneven, but the formula only works if you agree which income figure counts, when it gets recalculated, and which categories it applies to — then keep receipts against every entry.