Marriage Tax Calculator

Combined Tax (Filing as Singles) is computeFederalTax(income1) plus computeFederalTax(income2, "single") -- each spouse's tax is computed independently on their own income (line 19-21). When both incomes sit in the same marginal bracket, as they do at the default $75,000 each, a given percentage change to either income moves the combined total by essentially the same amount, since neither spouse's income structurally outweighs the other's in this additive formula.

When one spouse earns little or nothing, filing jointly lets that spouse's unused lower brackets absorb part of the higher earner's income, since the MFJ bracket boundaries at the lower rates are exactly double the single boundaries (compare the 10% and 12% bands in lib/reference-data). That extra room pulls some income that would have been taxed at a higher single-filer rate down into a lower married rate, producing a bonus (a negative marriagePenaltyOrBonus).

Because the top MFJ bracket threshold isn't simply double the single threshold -- the 2026 37% bracket starts at $640,600 for single filers but only $768,700 for MFJ, well under double. Two similarly high, equal incomes that would each stay in a lower single bracket can combine into taxable income that crosses into the 37% MFJ bracket sooner than doubling the single threshold would suggest, producing a real penalty.

Combined Household Income is simply Spouse 1 Income plus Spouse 2 Income added together (line 24), the taxable income figure the married-filing-jointly bracket calculation is applied to. It moves up whenever either spouse's income rises, and it's used only as the input to the MFJ tax computation -- it doesn't reflect any deductions or adjustments either spouse might otherwise claim.