Quick answer
You’re treated as married for federal tax purposes if you are legally married on December 31 of the tax year. That means the first time you file after marriage you may choose either Married Filing Jointly (MFJ) or Married Filing Separately (MFS), subject to IRS rules. If you pick MFS and later decide MFJ is better, you can generally amend your return to switch — but act quickly and get professional help. (See IRS guidance on filing status and amended returns: https://www.irs.gov and https://www.irs.gov/filing/how-to-file-an-amended-return-form-1040-x.)
Why timing matters
- Marital status on December 31 controls which statuses are available for that tax year. If you marry on December 31, you are considered married for the whole year for federal tax purposes (IRS Publication 501). This simple rule makes the timing of a year-end wedding especially important for tax planning.
- Filing status affects the standard deduction you claim, the tax brackets that apply, and whether you can take certain tax credits (for example, the Earned Income Tax Credit is generally not available if you file MFS). See IRS Pub 501 for details: https://www.irs.gov/pub/irs-pdf/p501.pdf.
- State tax rules don’t always mirror federal rules. Some states follow federal filing status, others treat married couples differently; check state guidance or our glossary entry on understanding state filing statuses.
Immediate actions after marriage (a short checklist)
- Update withholding: Submit a new Form W-4 to your employer(s) so withholdings reflect your combined situation and anticipated filing status. This prevents under- or over-withholding during the year. (IRS W-4 guidance: https://www.irs.gov/forms-pubs/about-form-w-4)
- Run both scenarios: Use tax software or a pro to estimate taxes as MFJ and MFS for the year you married. Include credits and deductions you expect to claim.
- Consider student loans and other liabilities: Income-driven repayment (IDR) plans and some federal/state benefits use your tax filing status to determine countable income; filing separately can sometimes reduce monthly payments but may cost you tax credits.
- Review state tax rules and file accordingly if your state differs from federal status.
Which factors should you weigh when comparing MFJ vs MFS?
- Credits and deductions: MFJ often unlocks credits disallowed for MFS (for example, the Earned Income Tax Credit and many education credits are limited or unavailable if you file separately). See IRS pages on credits and Pub 501.
- Standard deduction and tax brackets: Filing jointly usually gives a higher standard deduction and more favorable brackets. However, very unusual income shapes can sometimes make MFS advantageous.
- Liability and privacy: Filing separately can separate tax liability for each spouse’s items on separate returns, which may be desirable when one spouse has undisclosed tax issues or to limit joint liability. Note that filing separately does not automatically shield you from certain joint liabilities assessed by the IRS if a joint return had already been filed.
- Student loans and federal repayment plans: For many IDR plans, your spouse’s income is counted when you file jointly. Filing separately can reduce countable income for your loan repayment but may increase taxes or lose credits; run the numbers. See Federal Student Aid guidance: https://studentaid.gov.
- Medical expenses and casualty losses: Because these deductions are based on a percentage of AGI, filing separately can sometimes make it easier for one spouse to exceed the threshold if that spouse has large medical bills and a low AGI.
Real-world scenarios (illustrative)
- Married couple with similar incomes: In most cases, MFJ is beneficial because the higher standard deduction and broader access to credits outweigh disadvantages.
- One spouse on IDR with high student loan payments: Filing separately might reduce the payment amount because only the borrower’s income is used in some IDR calculations, but this must be balanced against lost credits and higher tax rates.
- One spouse with significant deductible medical expenses: If those expenses would exceed the AGI-based threshold on a separate return (but not on a combined return), MFS can enable larger deductible amounts.
How to switch after you file
- Changing from MFS to MFJ: If spouses initially file MFS and later agree to file a joint return, most couples can change to MFJ by filing an amended joint return (Form 1040-X) within the IRS time limits. Refer to IRS instructions for amended returns: https://www.irs.gov/filing/how-to-file-an-amended-return-form-1040-x.
- Changing from MFJ to MFS: This is harder. Once you file jointly, the IRS treats opening a joint return as the final filing for that year unless a special circumstance applies. If you need to split a joint return after filing, consult a tax professional immediately.
Practical step-by-step method to decide
- Gather last year’s W-2s, 1099s, interest and dividend statements, student loan statements, and records of major expenses (medical, childcare, education).
- Run two on-paper or software-based calculations: MFJ and MFS. Don’t forget to toggle credits that disallow MFS (EITC, some education credits, and certain child tax credit rules).
- Include state tax implications—some states treat married couples differently for income tax. Use our internal guide Understanding State Tax Filing Statuses to check how your state treats married returns.
- Factor in non-tax issues: student loan repayment plans, exposure to spouse’s tax problems, eligibility for health insurance subsidies, or legal protections.
- If the numbers are close or the situation is complex (e.g., business ownership, large itemized deductions, bankruptcy, or significant student loan debt), consult a CPA or tax advisor.
Common mistakes to avoid
- Assuming MFJ is always best: MFJ is usually advantageous, but not automatically so. Run the calculations rather than follow a rule of thumb.
- Forgetting the December 31 rule: Some couples assume the marriage date within the year doesn’t matter; it does for filing status.
- Ignoring state tax differences: Thinking federal outcome equals state outcome can create surprises at filing time.
- Not updating withholding: Not changing your W-4 after marriage can create an unexpected tax bill or an over-withheld situation.
When to consult a professional
Seek professional help if any of these apply:
- Combined incomes push you into a higher tax bracket and you have complex deductions.
- One spouse has complex business income, rental property, or significant investment income.
- One spouse has an open IRS issue or potential liability.
- Student loans are large and you’re enrolled in an income-driven repayment plan.
- You live in a state with different filing rules than the federal government.
Resources and authoritative references
- IRS Publication 501, Filing Status (for rules on marital status and filing): https://www.irs.gov/pub/irs-pdf/p501.pdf.
- IRS guidance on amending returns (Form 1040-X): https://www.irs.gov/filing/how-to-file-an-amended-return-form-1040-x.
- Federal Student Aid, Repayment Plans and IDR rules (how household income is counted): https://studentaid.gov.
- For a primer on filing jointly and when it makes sense, see our internal overview: Married Filing Jointly and the article When Married Filing Separately Makes Sense.
Bottom line
The rule that matters most: your marital status on December 31 determines what filing statuses are available for that tax year. After that, choosing MFJ or MFS should be an informed decision, backed by a side-by-side calculation that includes federal and state impacts, credit eligibility, student-loan consequences, and liability concerns. If in doubt, update your withholding, run both scenarios, and consult a tax professional before you file.
Disclaimer: This article is educational and does not constitute tax advice. Rules and deduction amounts change over time. For advice tailored to your situation, consult a CPA, enrolled agent, or tax attorney.
