Calculate the baseline
Divide each monthly net income by the combined monthly net income. Apply those percentages to the shared amount. Use net income unless both partners agree that gross income better reflects their situation.
Look beyond salary
A formula cannot measure unpaid care, irregular income, debt obligations, family support, or ownership of a shared asset. Use the calculated split as a visible baseline, then name any agreed adjustment.
Plan for change
Income-based percentages should not become permanent by accident. Review after a job change, parental leave, relocation, major debt payoff, or at a regular interval such as every six months.
Keep exceptions exceptional
If every expense uses a different rule, the system creates more mental work than it removes. Keep one default and reserve overrides for purchases with a clear reason.