How MultiW2 calculates your taxes

Brackets are projected forward from 2025 published values (Rev. Proc. 2024-40) using the CPI-U inflation adjustment method prescribed by IRC §1(f). MultiW2 applies the standard deduction before calculating tax owed across brackets.

The calculator supports four filing statuses: Single, Married Filing Jointly, Married Filing Separately, and Head of Household. Rates are applied marginally — only income within each bracket range is taxed at that bracket’s rate.

We review and update brackets every January after the IRS publishes the Rev. Proc. for the coming tax year.

Social Security (OASDI): Employee rate is 6.2%, applied to wages up to the Social Security wage base. The 2026 projected wage base is $184,500, derived from the SSA COLA announcement (SSA.gov — October 2025 announcement). When the official 2026 figure is published, we update the wage base.

Medicare (HI): Employee rate is 1.45% on all wages, with no wage-base cap. This rate is statutory and not inflation-adjusted.

Additional Medicare Tax: An additional 0.9% applies to wages above $200,000 (Single / HoH) or $250,000 (MFJ). This threshold is also statutory under ACA §1411 and not adjusted for inflation. MultiW2 flags and quantifies this surtax because employers only withhold it after an individual employee crosses $200,000 — meaning multi-job earners frequently hit the threshold without any employer withholding.

Each employer withholds federal income tax as if their job is your only source of income. When you have two or more W2 jobs, each employer applies the full standard deduction to their own payroll — causing a withholding shortfall at filing.

MultiW2 aggregates income across all jobs, calculates the total federal tax owed on combined income, and subtracts what has already been withheld. The result is your projected balance due or refund. This is the core calculation that TurboTax and paycheck estimators cannot do mid-year.

To avoid the IRS underpayment penalty, you must have paid in at least the smaller of: (1) 100% of last year’s tax liability, or (2) 90% of this year’s liability. If your prior-year AGI exceeded $150,000 (MFJ: $150,000), the threshold rises to 110% of last year’s tax.

MultiW2 uses the $150,000 AGI threshold (IRC §6654(d)(1)(B)(ii)) and the 110% multiplier to flag whether you are inside safe harbor and by how much.

Projected 2026 standard deductions are CPI-adjusted from 2025 published values (Rev. Proc. 2024-40):

• Single / MFS: $15,700
• Married Filing Jointly: $31,400
• Head of Household: $23,500

MultiW2 does not model itemized deductions in the current version.

• State or local income tax
• Self-employment tax on 1099 income (the tool models 1099 income for bracket impact but does not calculate SE tax)
• Capital gains, dividends, or investment income
• Alternative Minimum Tax (AMT)
• Itemized deductions beyond the standard deduction
• Business income, pass-through entities, or Schedule C
• Tax credits beyond the Child Tax Credit and Other Dependents Credit used in the estimator

This tool is for planning, not filing. Use it to close withholding gaps during the year, then take your numbers to a tax professional or filing software.

We follow a January review cycle aligned with IRS publication schedules:

• Tax brackets + standard deduction: Updated when the IRS publishes the annual Rev. Proc. (typically October/November)
• Social Security wage base: Updated from the SSA COLA announcement (typically October)
• Medicare rate + Additional Medicare threshold: Statutory — updated only if Congress changes them
• Quarterly due dates: Updated each January for the full year ahead