What it counts, and what it leaves out

It counts wages only, one tax year, for three filing statuses. The defaults are the guide's own example: $125,000 of wages, single, nothing saved or given; press 10% tithe to see the example it describes.

• Federal income tax, 2026. Brackets and standard deductions from Rev. Proc. 2025-32. The 2026 rules the One, Big, Beautiful Bill Act added for giving are modelled from the statute: non-itemizers deduct up to $1,000 ($2,000 joint) of cash gifts on top of the standard deduction; itemizers deduct gifts only above 0.5% of AGI; cash gifts count up to 60% of AGI; the state-and-local-tax cap is $40,400, shrinking by 30% of AGI over $505,000 to no less than $10,000; and itemized deductions are cut by 2/37 of the amount by which income reaches the 37% bracket. Tax is figured with the rate schedules, not the IRS tax table (which works in $50 steps below $100,000), so it can differ from a filed return by a few dollars.
• No credits. The child tax credit, education credits like the Lifetime Learning Credit, the earned income credit, and California's personal exemption credit all reduce tax dollar for dollar and are left out, so tax here is what you owe before credits.
• Itemized deductions are only giving and state income tax. Mortgage interest, property tax, medical costs and the other Schedule A lines are not asked for. California SDI is not counted toward state and local taxes.
• Giving is all cash to churches or other public charities, the kind both the $1,000/$2,000 deduction and the 60% limit cover.
• FICA. Social Security 6.2% up to the $184,500 wage base (per worker: with two equal earners each gets the full base), Medicare 1.45%, and the 0.9% Additional Medicare Tax on wages over $200,000 ($250,000 joint). 401(k) deferrals are still FICA wages; HSA contributions through a payroll cafeteria plan are not. The employer's matching half isn't shown.
• California uses the 2025 schedules, the latest the Franchise Tax Board has published (2026's arrive with next year's forms), with the 2025 standard deduction, the 2025 itemized-deduction limit for high incomes and the 1% Behavioral Health Services Tax on taxable income over $1,000,000. California does not recognise HSAs, so they are added back. SDI is the 2026 rate, 1.3% of all wages, applied here to gross wages.
• Not modelled: other states, investment income and capital gains, self-employment tax, the alternative minimum tax, the new 2026 deductions for tips, overtime, car-loan interest and seniors, catch-up contributions, married filing separately, and dependents.
• The staircase draws deductions and pre-tax savings as your first dollars. That is a drawing convention; the totals, the rates and every number in the tables come from the full calculation. Where a deduction shrinks as income grows (the 0.5% floor, the SALT phase-down, California's itemized limit), your true rate on the next dollar can sit a little above the bracket rate; the sidebar's marginal figure is measured on the next $100, so it includes that.

Sources

Every rate and dollar threshold is parsed from these documents by scripts/build_data.py, which writes data.js; nothing is typed in by hand. Fetched 8 October 2026.

• Internal Revenue Service, Revenue Procedure 2025-32 (tax year 2026 inflation adjustments, as amended by the One, Big, Beautiful Bill Act), §4.01 Tables 1–3 (rate schedules) and §4.14 (standard deduction). Cross-checked against the IRS news release IR-2025-103, 9 October 2025.
• Public Law 119-21 (One, Big, Beautiful Bill Act, 4 July 2025): §70111 (IRC §68, the 2/37 limit on itemized deductions), §70120 (IRC §164(b)(7), the SALT cap), §70424 (IRC §170(p), the deduction for non-itemizers), §70425 (IRC §170(b)(1)(I), the 0.5% floor, and §170(b)(1)(G), the 60% cash limit). All effective for tax years beginning after 31 December 2025, except the SALT cap (2025 on).
• Internal Revenue Service, Publication 15 (2026), (Circular E), Employer's Tax Guide, “What's New”: Social Security 6.2% to a $184,500 wage base, Medicare 1.45%. (The same wage base is in the Social Security Administration's 2026 COLA announcement; ssa.gov refused the script's requests, so the IRS guide is the cited copy.)
• Internal Revenue Service, Questions and answers for the Additional Medicare Tax: 0.9% above $200,000 single or head of household, $250,000 married filing jointly.
• Internal Revenue Service, IR-2025-111 (13 November 2025): 401(k) limit $24,500 for 2026; and Revenue Procedure 2025-19: HSA limits $4,400 self-only, $8,750 family. These cap the sliders.
• California Franchise Tax Board, 2025 California Tax Rate Schedules (Schedules X, Y, Z), and the 2025 Form 540 Personal Income Tax Booklet: standard deduction chart, Schedule CA (540) line 29 itemized-deduction thresholds and worksheet, line 62 Behavioral Health Services Tax, and the HSA add-back.
• California Employment Development Department, Contribution Rates, Withholding Schedules, and Meals and Lodging Values: SDI withholding rate 1.3% for 2026, no wage limit.

Method. The page follows the forms: wages less pre-tax savings give AGI; the larger of the standard deduction (plus the non-itemizer gift deduction) and itemized deductions gives taxable income; tax is the schedule's base amount plus the bracket rate on the excess. The script checks every printed base amount against the brackets beneath it and IRS's thresholds against its news release, and scripts/test.mjs checks the calculator against five hand-worked returns, among them the guide's $125,000-with-a-tithe example and the FTB's own worked example ($125,000 of California taxable income, married filing jointly, owes $4,768.10).

It illustrates the lesson Taxes: brackets, deductions, credits in the personal finance guide. It shows arithmetic on published rates, not tax or financial advice; a filed return depends on much more than it asks.