Agree what income means
Choose a consistent measure, often regular take-home income, and use the same period for both people. Decide how irregular bonuses, benefits or freelance receipts will be handled. Comparing one person's gross annual salary with another's monthly take-home pay creates a misleading ratio.
Then define the shared bills. Housing and household groceries may belong in the pool, while individual debts or personal hobbies may not. There is no universal household rule; the important part is making the agreement explicit.
Work out the ratio
If one person has $2,400 a month and the other has $1,600, combined income is $4,000. Their shares are 60% and 40%. For $1,500 of agreed shared costs, contributions would be $900 and $600 respectively.
Check the result against each person's other necessary commitments. A mathematically proportional split is not automatically affordable or fair in every situation. Caring responsibilities, irregular earnings and unequal access to savings may be reasons to choose a different agreement.
Decide when to recalculate
Review the ratio after a lasting income change rather than changing it over every small fluctuation. For highly variable earnings, a clearly defined average or a temporary contribution can reduce constant recalculation, provided both people understand its limits.
Keep a record of shared purchases made outside the usual account and settle them without counting the same expense twice. If someone pays a $120 shared bill directly, that payment may count toward their contribution rather than adding another obligation. Discuss disagreements before arrears build up. The calculation should support communication, not replace a conversation about what each person can realistically contribute.
