How Taxtirement calculates

Every number in the app comes from a published rule, and you can see it. This page lists the 2026 values, where each one comes from, and the tests that check the calculations.

Principles

• Rules are data, with sources. Each tax year's values live in a file that names the IRS, SSA, CMS or state document behind every number.
• Every result explains itself. Tap "Why?" on any number to see each step, its formula, and the form line or code section it follows.
• Tests come from the sources, not the app. Expected values are worked out by hand from the published documents, never copied from the app's own output.
• Later years follow the law as written. Values that are indexed for inflation are projected with their own indexing method; fixed amounts (like the Social Security taxation thresholds) stay fixed; scheduled changes are applied in their year.
• Estimates, not advice. Results are scenarios to compare. Taxtirement doesn't recommend investments or allocations.

Federal income tax, 2026

Medicare premiums and IRMAA, 2026

Part B standard premium $202.90 a month. Premiums use income from 2 years earlier.

CMS 2026 Medicare Parts A & B premiums and deductibles fact sheet, https://www.cms.gov/newsroom/fact-sheets/2026-medicare-parts-b-premiums-deductibles (sources/cms-partb-irmaa-2026.txt; Part B, Part D and married-separate tables); checked 2026-10-02

Retirement accounts, Social Security and ACA, 2026

States and localities

All 50 states and DC, plus New York City and the Maryland counties. Values marked as awaiting the final form use the statute or the latest published worksheet until the state releases its 2026 form.

Every state with an income tax is checked against a second, independent calculation written from the state's forms and statutes without looking at the app, on 1,000 random households per state, to within $1. Government pensions are taxed by who pays them: federal civilian (FERS or CSRS), military retired pay, the state's own plans, or another state's, including rules tied to years of service (Kentucky before 1998, Oregon before October 1991, North Carolina's Bailey settlement and 20-year military rule). Where a state lets married couples file separately on one return (Arkansas, Delaware, DC, Kentucky), the lower tax is used.

After-tax comparisons

"What you'd have after tax" compares a scenario with no conversions at ages 70, 75, 80, 85 and 90 and at the end of the plan, in today's dollars. Savings count Roth, brokerage and cash in full, and pre-tax accounts after the tax rate set in Settings (24% by default). Both scenarios pay for the same spending, so the difference is money left over. Historical market sequences start in 1928, the first year with a Treasury bill series; earlier years aren't estimated.

The test suite (617 tests)

Every change to a calculation or a rule value needs a test that fails without it. Property tests also check rules that must always hold: more income never lowers tax (except where a state's law says so, each listed in the test), balances never go negative, and spending is always met while money remains.

Changes to the method

October 2026

• 2027 now uses the best numbers available instead of 2026's carried forward at the plan's inflation:
• Tax brackets, deductions and the Medicare surcharge thresholds are computed the way the IRS and CMS compute them, from the inflation data already published. The same method reproduces every 2026 figure.
• Amounts fixed in law are exact: the senior deduction, the state-and-local tax cap of $40,804, and the charity rules.
• Amounts already published are used as is: the 2027 ACA table and poverty guidelines, and the Part D base premium.
• The rest come from the Trustees Reports: the Part B premium, the Social Security wage base, and a COLA estimated from the inflation data to date.
• Each 2027 number is marked as law, published, or projected.
• Later years round each amount the way its law rounds it:
• joint brackets in $50 steps;
• Medicare joint thresholds at twice the single ones;
• the married-separate surcharge top tier tied to the single threshold;
• the alternative minimum tax to the nearest $100;
• the state-and-local tax cap growing exactly 1% a year;
• the Social Security wage base and earnings test following wages rather than prices.
• State law enacted for 2027 and later was checked against the session laws in every state. New this pass:
• Hawaii's capital-gain rate cap follows its new 2027 and 2029 brackets.
• Ohio resumes indexing in 2027.
• Idaho's retirement deduction cap is indexed.
• Maine's surcharge thresholds are indexed.
• 2026 corrections: the DC standard deduction ($15,000 / $30,000 / $22,500 under the 2026 Budget Support Act); Maine's itemized cap ($37,100) and its married-filing-separately pension phase-out.
• Every state with an income tax, plus DC and New York City, now agrees with an independent second implementation to within $1 on 1,000 random households each. Corrections from that comparison include:
• married couples filing separately on one return where allowed (Arkansas, Delaware, DC, Kentucky);
• low-income and family credits in Kentucky, Arkansas, North Dakota, West Virginia and New Mexico;
• Nebraska's tax limited to federal tax;
• Mississippi Roth conversions exempt at any age;
• 2026 amounts in Utah, Montana, Vermont, Maine and Rhode Island.
• Government pensions are taxed by who pays them: military retired pay (exempt or capped by state), federal civilian (FERS and CSRS), the state's own plans and other states' plans, and service-date rules in Kentucky, Oregon and North Carolina.
• Americans working abroad: the foreign earned income exclusion and its stacking rule (§ 911(f)), so a Roth conversion is tax-free only up to the deductions. The excluded income counts toward Social Security taxation, the senior deduction, the SALT phase-down, the investment income tax and IRMAA. The tax a country of residence charges on IRA income can be entered and is counted as a cost.
• New "What you'd have after tax" comparison at ages 70 to 90 and at the end of the plan.
• Historical market runs use 1928 onward only (no estimated cash returns before then).
• State taxes checked against a second, independent implementation for ten large retiree states (California, New York and New York City, Pennsylvania, New Jersey, Georgia, North Carolina, Virginia, Michigan, Arizona, Massachusetts); all now agree to within $1. Corrections:
• Arizona: 2026 standard deduction follows the federal amount, as Arizona law requires.
• New York and New York City: household credits added; New York City's school tax credits are refundable; the high-income worksheets use the amounts printed on the form.
• Virginia: spouse tax adjustment, filing threshold, the low-income credit, and the age deduction's "born on or before January 1, 1939" rule.
• Massachusetts: Limited Income Credit, the payroll-tax deduction, and the married-filing-separately rules.
• Michigan: the born-before-1946 tier and the age-67 standard deduction for those born 1953 or later.
• New Jersey: the unclaimed pension exclusion when only one spouse is 62 or older, and the filing threshold.
• Georgia: the low-income credit.
• Checked against an independent implementation: a second federal calculator, written separately from the IRS forms and statutes, now agrees with the app to within $1 on 20,000 random returns. The comparison found four corrections:
• The 2026 top-bracket limit on itemized deductions (§ 68) no longer reduces deductions for the alternative minimum tax (§ 56(b)(1)(E)).
• The AMT's capital-gain computation now follows Form 6251 Part III line by line.
• Married filing separately: the state and local tax cap is half of the joint phased-down amount (§ 164(b)(6)–(7)), and the AMT add-on above the exemption phase-out applies.
• The non-itemizer charitable deduction stays allowed under the AMT (it isn't the standard deduction).
• A Roth conversion is now limited to what the year can afford: it never leaves that year's spending unpaid. Before, a conversion rule could move the last of an IRA into a Roth in a year that then came up short.
• Property tests added: federal and state tax never fall as income rises, except where state law makes them (Alabama's deduction for federal income tax; Ohio's retirement income credit steps).

September 2026

• First version: 2026 federal, Medicare, ACA, Social Security and required-distribution rules; all 50 states, DC, New York City and the Maryland counties.