How do I calculate whether I provide 50% of my domestic partner's support?

The test is whether you provide more than 50% of your partner's own expenses, not 50% of household expenses. For example, if you have $100,000 in combined household expenses split evenly ($50,000 each), you'd need to cover at least $25,001 of their $50,000 share to meet the requirement.

Support includes housing, food, utilities, medical care, clothing, transportation, and other living expenses. Keep records of your contributions in case you need to substantiate this for tax purposes.