Trust & verification

How we validate

Every number we show should be readable and checkable. Each free calculator below exposes its assumptions, carries a worked check in the test suite, and cites the government rule, formula, or primary research behind it. The planner's federal-tax computation is checked against an independent oracle too — see the full planning engine below. Here is exactly what each calculator is checked against.

By Steven, Founder · Updated September 3, 2026

The same plan gives the same answer

Every simulated market path is seeded from its own index rather than drawn from one shared stream, so the result never depends on how the work was divided up. Run the same plan twice — the seed is stored with the plan, so you do not have to do anything to keep it — and you get the same answer, to the last digit, however many workers your browser spread the simulation across.

That guarantee is scoped to one browser engine, and we would rather say so than imply more. The sampler uses logarithms and exponentials, which the JavaScript standard does not require to be rounded identically everywhere, so a single path sitting exactly on the line between lasting and running short can fall the other way in a different engine. We report the probability to the nearest 1%, which absorbs it: the difference can move the figure you see by at most one point, and only when the underlying number already sits on a boundary.

One projected year, worked all the way through

A single federal-tax figure is not checkable. Below is one year of one projection with every intermediate the engine computed on its way there — generated from the engine, not typed out — so you can follow it line by line against the published tables and see where each number came from.

What this does not cover

The 1,728 scenarios above check the engine's federal income-tax function — the one behind the free Roth-conversion calculator. They do not reach every line the planner computes: no scenario itemizes deductions, and none covers state tax, Social Security taxation, required minimum distributions, payroll tax, AMT, NIIT, or credits. Those are built on the published figures named in the cards above and tested against them directly, not against this oracle. The worked year is exactly what the planner's own tax path produces, and the oracle does not check that path end to end.

Models U.S. federal income tax plus one flat state rate at a time — including a move to another state at ages you choose, so each year is taxed on the state you live in that year — and prices your Medicare IRMAA surcharge from a two-year MAGI lookback. Before Medicare it models marketplace premium tax credits from household MAGI: gross premium comes from what you enter or, if blank, a stated national benchmark silver-plan premium; household size follows filing status (one or two); real premiums vary by age and county and are not modeled that finely; the year you turn 65 is treated as a full pre-Medicare year for this purpose. It does not model any state's actual rules: no per-state brackets, credits or exemptions, no preferential state rate on long-term gains — a state that taxes them below its ordinary rate is modelled at 0% — and no part-year or nonresident allocation, so the whole year you move is taxed on the state you move to. Excludes year-end ACA advance-credit reconciliation on your tax return, NIIT, AMT, and estate tax. Models two-earner wages: each spouse can have wages that stop at an age you choose. One Social Security benefit, and no survivor transition. Excludes a house, mortgage, or other debt. The 72(t)/SEPP penalty waiver is capped at $200,000/yr rather than verified against your actual SEPP schedule. HSA and 529 withdrawals are assumed to be for qualified expenses, so non-qualified spending from either isn't taxed or penalized here. Enabling the rule-of-55 exception waives the penalty on your entire traditional-account balance, since we don't track 401(k) dollars (eligible) separately from IRA dollars (never eligible) within it. Pre-tax contributions are deducted in full with no statutory contribution limit and no Traditional IRA deductibility phase-out for high earners with a workplace plan. Does not model the Social Security earnings test, which can reduce or withhold benefits if you claim before full retirement age while still earning above the annual threshold. Withdrawals default to cash, then taxable, then traditional, then Roth, then HSA, then 529 last. You can instead preserve Roth (Roth last) or tap traditional last. Calculator figures for IRMAA and ACA are current as of 2026 law; the ACA calculator assumes the enhanced premium tax credits expired at the end of 2025. By default the Monte Carlo model draws each year's return independently from a log-normal distribution, which does not capture autocorrelation, mean reversion or volatility clustering, and runs mildly optimistic against historical sequences. The planner also offers a historical block bootstrap, which replays runs of consecutive recorded years shifted onto your own expected return; it carries its own limits — a finite 1928–2024 record, sampled with a circular wrap at a fixed block length, and recorded returns paired with assumed rather than recorded inflation. Whichever one produced the verdict's probability is named beside it; the sustainable-spend, contingency and assumption-ranking figures are always run on independent draws.