US tax methodology

Tax year 2025 is the default because final federal return values exist and those returns are filed in 2026. Tax year 2026 is labeled as a planning estimate everywhere it appears. The two datasets are stored separately so a threshold cannot silently drift across years.

1. Validate every annual dollar and dependent count as a nonnegative safe integer.
2. Add taxpayer wages, spouse wages, other ordinary income, qualified dividends, and long term gains.
3. Apply adjustments to income before adjusted gross income.
4. Select the standard deduction or supplied itemized amount, then allocate reductions against ordinary income before preferential income.
5. Apply progressive ordinary brackets and stack preferential income above ordinary taxable income.
6. Calculate Social Security for each spouse separately, then Medicare and Additional Medicare using filing thresholds.
7. Calculate NIIT and phase down basic dependent credits. Credits cannot reduce modeled federal income tax below zero.

The complete federal result includes ordinary income tax, qualified dividend and long term capital gain rates, employee and employer Social Security and Medicare context, Additional Medicare Tax, NIIT, and basic nonrefundable Child Tax Credit and Other Dependent Credit.

Dedicated focused calculators also model self-employment tax, bonus withholding reconciliation, potential excess Social Security withholding, refund or amount owed planning, retirement contribution tax savings, estimated tax safe harbor, and the supported Schedule 1-A deductions. Each focused page states its own narrower inputs and exclusions.

Afterax does not complete AMT, EITC, refundable ACTC, QBI, complex Schedule D ordering, foreign income and credits, complex residency, most other credits, or actual estimated tax penalties. These boundaries appear visibly beside affected results rather than as hidden assumptions.

All 50 states and DC have a 2026 information profile. Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, and Wyoming are the only profiles treated as zero broad state wage income tax. Washington also shows zero wage tax, with a separate capital gains warning.

Every wage-taxing state is marked profile only. The federal take home number never implies that state or local liability has been subtracted.

Public inputs use whole annual dollars. Intermediate tax calculations retain decimal precision, and public monetary outputs round to cents. Effective rates round to four decimal places in the engine and display with reader appropriate precision.

Tests cover every filing status dataset, both standard deduction years, final 2025 Tax Table and worksheet selection, top brackets, spouse and multiple employer Social Security caps, capital gain stacking, Additional Medicare, NIIT, self-employment tax, bonus and refund reconciliation, retirement and Schedule 1-A deductions, dependent credit phaseout rounding, nonrefundable limits, input safety, state counts, Washington treatment, and shared-engine parity with international salary comparisons.

Calculator arithmetic runs in the browser. Inputs can be stored locally and encoded in the page URL for a shareable scenario. Afterax does not require an account. Avoid sharing a URL if its amounts are sensitive.

• IRS Revenue Procedure 2024-40
• IRS Revenue Procedure 2025-32
• Social Security 2025 COLA facts
• Social Security 2026 COLA facts
• Tax Foundation 2026 state structures

State and federal values are reviewed on a dated schedule. If a source changes, the relevant tax year dataset and regression fixtures must change together.