Methodology and Sources

Every calculator on MFJ Tools applies the same set of IRS-published tax tables and formulas to the inputs you provide. This page explains what each calculator does, what assumptions it makes, and where the underlying numbers come from. If you want to verify any single bracket or deduction yourself, the bibliography at the bottom links directly to the IRS Revenue Procedure that publishes it.

How federal tax is computed

All four calculators share the same core calculation, just applied to different inputs:

1. Adjusted Gross Income (AGI) = wages + other ordinary income + long-term capital gains + qualified dividends − pre-tax retirement and HSA contributions.
2. Taxable income = AGI − the larger of the standard deduction or itemized deductions.
3. Tax on ordinary income applies the bracket table to the ordinary portion of taxable income (everything except long-term capital gains and qualified dividends).
4. Tax on preferential-rate income stacks long-term capital gains and qualified dividends on top of ordinary taxable income, using the 0%/15%/20% LTCG brackets keyed off total taxable income.
5. Credits (currently just the Child Tax Credit) are subtracted from total tax owed, capped at the tax owed (we treat the CTC as nonrefundable for comparison purposes).

Brackets are applied marginally — each slice of taxable income is taxed at the rate that applies to that slice, not your highest rate.

Per-calculator assumptions

Two-Earner W-4 Withholding Calculator

Computes the gap between (a) your household's actual joint federal tax liability on combined wages and (b) what each employer would withhold by default if both spouses' W-4s say "Married filing jointly" with nothing else filled in. The shortfall divided by remaining pay periods is the per-paycheck amount you should add to Step 4(c) of the higher earner's W-4.

Includes: wages, MFJ standard deduction, MFJ tax brackets. Does not include: non-wage income, bonuses, itemized deductions, tax credits, pre-tax 401(k)/HSA contributions (enter your post-deduction wages if you contribute heavily), state withholding.

MFJ vs MFS Comparison

Computes total federal tax under Married Filing Jointly (one return) versus Married Filing Separately (two returns summed) and recommends the cheaper one. For each filing status, the calculator picks the deduction approach (standard or itemized) that produces the lower tax — with the MFS-specific constraint that if either spouse itemizes, the other cannot take the standard deduction. The 7.5%-of-AGI medical floor is applied against joint AGI on MFJ and against each spouse's individual AGI on MFS, which is the mechanism behind the classic medical-driven MFS scenarios.

The Child Tax Credit on the MFS side is assumed to be claimed by the higher-earning spouse, since the CTC is limited by tax owed and that spouse usually has more tax to offset.

Includes: wages, medical expenses, other itemized deductions, CTC, MFJ + MFS brackets and standard deductions. Does not include: EITC, education credits, Child & Dependent Care Credit, student loan interest deduction, IRA deduction phaseouts, Premium Tax Credit, AMT, NIIT, community property income allocation, state taxes.

Marriage Tax Penalty / Bonus

Computes federal tax under two hypotheticals and shows the delta. The "two singles" side runs each spouse through the Single brackets and standard deduction. The "filing jointly" side runs the same household through the MFJ brackets and standard deduction. A positive delta is a marriage penalty (MFJ costs more); a negative delta is a marriage bonus.

All qualifying children are assumed to be claimed by the higher-earning spouse on the singles side, since that maximizes CTC use under the per-spouse $200,000 single-filer phaseout.

Includes: wages, kids (CTC), Single + MFJ brackets and standard deductions. Does not include: Net Investment Income Tax, Additional Medicare Tax, SALT $10,000 cap, capital gains rate brackets, itemized deductions, EITC, retirement contribution phaseouts, state taxes. Several of these are real sources of marriage penalty for moderate-to-high earners that this calculator does not capture — see the calculator page for details.

Combined Income Tax Estimator

The most general-purpose of the four. Computes total federal tax for an MFJ household with optional inputs for non-wage ordinary income, long-term capital gains and qualified dividends, pre-tax retirement contributions, itemized deductions, and year-to-date federal withholding. Long-term gains and qualified dividends are stacked on top of ordinary taxable income for the 0%/15%/20% bracket math. If withholding is provided, the calculator estimates whether you'll owe a balance or get a refund.

Includes: wages, ordinary investment income, long-term capital gains, qualified dividends, pre-tax contributions, itemized deductions, CTC, MFJ ordinary brackets, MFJ LTCG brackets. Does not include: NIIT, Additional Medicare Tax, self-employment tax, QBI deduction, AMT, education credits, EITC, Premium Tax Credit, FICA payroll tax, state taxes.

What we deliberately leave out

Most omissions are by design. We chose to keep each calculator scoped to a specific question rather than build one mega-tool that captures every edge case. The IRS Tax Withholding Estimator and full-blown tax software (TurboTax, FreeTaxUSA, a CPA) handle the long tail. Our goal is to give a fast, transparent answer for the ~80% of married-couple situations where the math is straightforward, and to be explicit about when to seek a more thorough tool.

Sources

Every bracket, deduction, credit, and phaseout is sourced from official IRS publications. The most important is the annual Revenue Procedure that publishes inflation-adjusted figures (typically released in late October for the following tax year). When Congress passes mid-year tax legislation (like the One Big Beautiful Bill Act in 2025), we cite the IRS guidance that amends the Rev. Proc. figures.

Primary sources by tax year

• Tax year 2025:
  • IRS Rev. Proc. 2024-40 — Original 2025 inflation-adjusted brackets, LTCG thresholds, and CTC phaseouts.
  • IRS — One Big Beautiful Bill provisions (Individuals and workers) — OBBBA (2025) raised the 2025 standard deduction to $31,500 MFJ / $15,750 Single+MFS and the Child Tax Credit to $2,200 per child.
• Tax year 2026:
  • IRS Rev. Proc. 2025-32 — 2026 inflation-adjusted brackets, LTCG thresholds, standard deductions, and CTC phaseouts (incorporates OBBBA amendments).
  • IRS — Tax inflation adjustments for tax year 2026 — Plain-English summary of the 2026 figures and OBBBA changes.

Secondary references

Useful third-party sources that consolidate or explain the IRS data, particularly for the Married Filing Separately bracket table (which the IRS Rev. Proc. publishes in technical form):

• Tax Foundation — 2025 Tax Brackets and Federal Income Tax Rates
• Tax Foundation — 2026 Tax Brackets and Federal Income Tax Rates
• Fidelity — 2025 and 2026 tax brackets and federal income tax rates — includes the Married Filing Separately bracket table.
• IRS Tax Withholding Estimator — the official IRS tool for the more comprehensive case our W-4 calculator does not handle.

Updates

The supported years above (2025, 2026) are pinned at build time. When the IRS publishes the next year's Revenue Procedure (typically the October before the year begins), we add it as a new year while keeping the prior years available at their year-pinned URLs. If a tax-law change retroactively affects an earlier year — as the OBBBA did to 2025 — we update that year's data and note the change in the source list above.

Found an error?

If you spot a number that doesn't match the IRS source linked above, please email 44bsoftware@gmail.com with the URL and the discrepancy. Tax data corrections take priority.