TaxtirementHow we check our numbers
The retirement tax planner that shows its work
Every number in Taxtirement comes from the law and published tax rules (for years not yet published, from the law's own indexing method, marked as projected), and every calculation is checked against sources outside the app. Open "Why?" on any result to see each step and the form line or law it follows.
This page is the same text the app shows under Settings › What's calculated and how it's checked. Counts as of October 2026.
What's calculated
Each year of the plan gets a full federal and state return, plus the Medicare and health-insurance costs that depend on income.
Federal income tax
The Form 1040: taxable Social Security, the standard deduction or itemized deductions on Schedule A (property tax, state income tax, mortgage interest, charity, and medical costs including premiums), the senior deduction, capital-gain rates, the 3.8% investment income tax and the alternative minimum tax. The plan itemizes in the years Schedule A is larger.
Sales tax instead of income tax
Schedule A uses general sales tax from the IRS Optional State Sales Tax Tables when that's larger than state income tax, which is always the case in states without an income tax. Local sales tax is added from the rate you enter.
State income tax, every state
All 50 states and DC, including Washington's capital-gains tax, plus New York City and Maryland's counties and Baltimore City. Each state's own retirement-income exclusions, treatment of Social Security and government pensions, senior credits, and its own itemized deductions and property-tax credits where it has them.
Social Security
Benefits by claim month, with early reductions and delayed credits; spousal and survivor benefits, including the widow(er)'s limit; the earnings test; and yearly cost-of-living increases. An optional setting cuts benefits from a chosen year.
Required distributions and charitable distributions
Required minimum distributions (RMDs) start at the age set by birth year and use the IRS life tables, including for inherited IRAs. Qualified charitable distributions (QCDs) come only from IRAs and lower the taxable amount. After-tax money in an IRA comes out tax-free in proportion (Form 8606).
Pay before retirement
Social Security and Medicare tax come out of pay, with the 0.9% Additional Medicare Tax on high wages. Your pre-tax 401(k) contributions, up to the IRS limits for your age, lower taxable wages and go into your plan; catch-ups for people who earned over $150,000 the year before go to a Roth 401(k). Social Security benefits taken before full retirement age are reduced by the earnings test and raised at full retirement age.
Withdrawals before 59½
The 10% additional tax on IRA and 401(k) withdrawals before 59½ (Form 5329). It doesn't apply to Roth conversions, or to 401(k) withdrawals after leaving work at 55 or older. States that treat 401(k) money differently from IRA money follow their own rules.
Roth withdrawals
Withdrawals come out in the IRS order: your contributions, then conversions (oldest first), then earnings. Earnings are taxed until you're 59½ and the Roth is 5 years old; before 59½ they and any conversion less than 5 years old also owe the 10% additional tax.
Health savings accounts
HSA withdrawals are tax-free up to the year's medical costs: the expenses you enter, long-term-care premiums within the limit, and Medicare Part B and D premiums. The rest is income, plus a 20% additional tax before 65.
Medicare surcharges (IRMAA)
Part B and Part D surcharges for each person on Medicare, based on income from two years earlier. After a death, the plan can assume the survivor appeals with form SSA-44.
Marketplace premium credit (ACA)
Before 65, the credit from your benchmark premium and income, including the cliff at 400% of the poverty line. From 2026, any excess advance credit is repaid in full.
How it's checked
Expected answers come from the published sources, never from the app's own output. The published examples and the second implementation run again before every release.
The IRS Tax Table
Every cell of the 2025 table in the Form 1040 instructions (2,062 rows, four filing statuses) matches.
IRS and SSA worked examples
Examples printed in IRS publications (915, 590-B, 554, 17, 974 and others) and by SSA (its operations manual and the Chief Actuary's tables). Every example the app models passes.
State worked examples
294 examples, worksheets and tax-table rows from 41 states, DC and New York City. None fail. 3 are known errors in the source or households the app can't represent, and 65 cover things the app doesn't model.
A second, independent implementation
A separate program, written from the same primary sources without reading the app's code, agrees to within $1 on 20,000 random federal returns and on 1,000 random households in each income-tax state, DC and New York City. A separate year-by-year model is compared over 349 years of 14 multi-year scenarios.
PolicyEngine
An open-source tax model. Federal AGI, taxable Social Security and taxable income agree on every household compared; itemized federal returns agree on 324 of 326; all 5,358 cells of the 2025 sales tax table agree. Every state difference has been traced to its cause.
Open Social Security
An open-source claiming calculator. 14,398 cases of own, spousal and survivor benefits agree, apart from one bug in that calculator and restricted applications (open only to people born before 1954, all now past 70).
Real returns, including yours
When you import last year's Form 1040, the app recomputes it and shows whether taxable Social Security, AGI, taxable income and tax match lines 6b, 11, 15 and 16. A difference usually means a misread number. On a real published 2025 return, AGI and taxable income matched exactly.
Published and projected amounts
Each amount carries its source, marked as set by law, published, or projected.
2026
The amounts the IRS, SSA, CMS and the states published. Where a state indexes an amount but hasn't printed it yet, it's computed with the state's own method.
2027: projected until published
Tax brackets, deductions and the Medicare surcharge thresholds are computed the way the IRS and CMS compute them, from inflation data already out; the same method reproduces the published 2026 figures. Amounts set by law are exact. The ACA table, poverty guidelines and Part D base premium are already published. The cost-of-living increase, Part B premium and Social Security wage base are forecasts. Each is replaced when SSA (mid-October), the IRS (late October) and CMS (mid-November) publish. State amounts not yet published are carried forward with inflation, and state law already enacted for 2027 is applied.
2028 and later
Indexed amounts grow with your plan's inflation rate and are rounded the way their law rounds them. Amounts fixed in law, like the Social Security taxation thresholds and the investment income tax threshold, stay fixed. Scheduled changes apply in their year: the senior deduction ends after 2028, and the state-and-local tax cap returns to $10,000 in 2030.
Conventions
Choices every projection has to make, and the ones Taxtirement makes.
Withdrawals at the start of the year
Growth, dividends and interest are earned on what stays invested.
The Tax Table in the first year
So the first year can be compared with a filed return. Later years use the IRS rate schedule.
Withdrawals in proportion
Within one type of account, money comes out in proportion to each account's balance.
Filling a bracket
"Fill the 22% bracket" fills taxable income, capital gains included.
Charitable distributions
QCDs come from the first giver up to the yearly limit, then from the next.
State tax is deducted when it's paid
Estimated payments are the smaller of this year's state tax and last year's (the usual safe harbor), so after a year like a big Roth conversion the rest is paid, and deducted on the federal return, the next April. The plan's first year is paid as it goes.
The year you reach 59½
Withdrawals that year are assumed to come after the half-birthday, so the early-withdrawal tax doesn't apply.
Medicare
Starts in the month someone turns 65.
Gains and losses on sales
Selling from a taxable account realizes gain or loss in proportion to the account's average cost. Net losses offset gains, then up to $3,000 a year of other income; the rest carries forward. After a spouse's death, half of the losses still carried (the deceased's share) are dropped.
Gifts of shares
Shares given to charity come out of the taxable accounts at the start of the year, in proportion to balance. They're taken to be held over a year and given to public charities, so they're deducted at market value (by itemizers) and no gain is taxed.
Extra cash
Goes to the first taxable account, or else the first cash account. A new taxable account opens only when there's neither.
Not included
Most local income taxes
Only New York City, Maryland's counties and Baltimore City, and Indiana's counties are included. Yonkers, and city or county taxes in Pennsylvania, Ohio, Michigan and elsewhere, are not.
Income-tested state relief and renters' credits
Property-tax relief that depends on household-income tests the plan can't compute, and renters' credits.
A few state items
Vehicle property tax, part-year residents, state earned income credits, and 529 plan deductions.
Some self-employment deductions
Self-employed health insurance, SEP and solo 401(k) contributions, and business losses carried forward for the 20% business income deduction.
Restricted Social Security applications
Open only to people born before 1954, who are all now past 70.
Investment choices
The app never selects investments or suggests an allocation. Returns are the ones you enter, or historical and simulated paths.
The full rules reference and change log
Every 2026 value the app uses, federal and state, with the document it comes from, is listed in the rules reference. It's generated from the same rules files the app reads, so it can't drift from what the app calculates.
Every change to the method, including every correctness fix, is in the change log.
For a plain-language summary of each state, see how each state taxes retirement income.
Found a number that looks wrong?
Please tell us. In the app, open "Why?" on the number and choose Report a number. It drafts an email with the calculation, which you can read and edit before sending. Or write to support@taxtirement.com. More on the support page.
Results are estimates based on your inputs and current law. They're scenarios to compare, not investment, tax or legal advice.