Retirement Guides
A calculator answers one question with one number. Enter $10,000 today, $500 a month, and an assumed 7% annual return for 30 years into the Investment Calculator and it reports a balance of about $691,150 — $190,000 of it contributed, $501,150 of it growth. What the number cannot say is where it came from, how far it can be trusted, or what it would mean under different assumptions. The guides collected here fill that gap: each takes a rule or a piece of research the calculators lean on, explains the mechanism, marks where the model stops, and shows how to read the result.
The models share one set of conventions. Each projection assumes a single constant rate of return. The accumulation tools compound monthly at the annual rate divided by twelve; the Compound Interest and CD Calculators let the user choose the frequency, and the Inflation Calculator and the savings-runway mode step annually. Contributions arrive at the end of each month unless a begin-of-month option is offered and selected, and balances are reported in nominal, future dollars — entering a real (after-inflation) return is the way to see today's purchasing power. None of the models include taxes, Social Security, pensions, investment fees, contribution or income limits, or the sequence-of-returns risk that comes from returns arriving in an unpredictable order. The formulas themselves are documented on the methodology page; the guides sit one layer above, covering the tax rules, statutory ages, and published research that give the inputs and outputs their meaning.
401(k), Roth IRA, and Taxable Accounts: How They Fit Together
The growth arithmetic in a traditional 401(k), a Roth IRA, and a taxable brokerage account is identical; what differs is when the tax lands — at withdrawal for a traditional account, before the contribution for a Roth, or along the way for a brokerage account. This guide lays the three timings side by side, explains what changes at withdrawal time — the age-59½ line, ordinary-income versus tax-free treatment, and required minimum distributions from age 73 — and then walks through projecting all three accounts with a shared return and horizon so the pieces add into one pre-tax planning figure.
The Retirement Planner puts this guide into practice: a balance and a monthly contribution for each of the three buckets, all run at the same assumed return to the same retirement age, with each ending balance, the combined pre-tax total, and the 4% rule's first-year figure reported together. Its assumed-tax-rate field is an illustration control that discounts the traditional balance by whatever rate is entered — not an estimate of any future tax bill.
Retirement Age Milestones: 59½ to 73
Balances change every month; the rules change at fixed ages. Six statutory ages change what retirement accounts and benefit programs will do, each a hard line with one set of rules on either side: 59½, when the 10% additional federal tax on early distributions from most tax-advantaged accounts generally stops applying; 62, the earliest Social Security claim, at a permanently reduced amount; 65, when Medicare eligibility begins for most people; 67, full retirement age for anyone born in 1960 or later; 70, when delayed-retirement credits stop accruing; and 73, when required minimum distributions currently begin, scheduled to rise to 75 in 2033. The guide states what changes at each age, names the official source for every rule, and makes one central point: the milestones are boundaries, not recommendations.
The 4% Rule: Where It Comes From and What It Assumes
The rule is a compact summary of two historical back-tests: William Bengen's 1994 study, in which an initial withdrawal of about 4% of a stock-and-bond portfolio, adjusted for inflation annually, survived every rolling 30-year period in the U.S. market data he examined, and the 1998 Trinity study, which broadly reinforced that finding. The guide works through the exact two-step mechanic — the balance is consulted exactly once, in year one — then lists what the studies held fixed (a specific allocation, a 30-year horizon, no fees, no taxes) and treats the standard criticisms, sequence-of-returns risk and longer retirements, as the edges of what the research measured. The Retirement Withdrawal Calculator computes the first-year figure for any initial rate from 2% to 6%.
RMD Rules, Explained: Age 73, the Formula, and the Penalty
A traditional 401(k) or IRA defers tax rather than forgiving it, and the required minimum distribution is the mechanism that eventually collects it. The rules reduce to three pieces of arithmetic, and the guide walks through each: a start age, currently 73 under the SECURE 2.0 Act and scheduled to rise to 75 in 2033; a one-line formula, the prior December 31 balance divided by the IRS life-expectancy factor for the age reached this year (for most owners, the Uniform Lifetime Table divisor); and a penalty, an excise tax of 25% on any shortfall, reduced to 10% if corrected in a timely manner. A worked table shows the divisor shrinking and the required percentage rising with age, and the guide states where the RMD Calculator stops: inherited accounts, much-younger spousal beneficiaries, and multiple-account aggregation are outside it.
How to read a result from any of these tools
Three habits carry across every calculator on the site. Every rate in a projection is an assumption the user chose, not a forecast the tool made — a 7% return in a worked example means the example assumed 7%, and changing the assumption changes the balance by the same arithmetic. A projected balance is a nominal figure; the Inflation Calculator translates it into today's purchasing power. And each model has a stated boundary, and the common misreadings come from crossing it: reading the constant return as a promise about the order in which real returns will arrive, the 4% first-year figure as a permanent income rather than the opening step of a convention, a combined multi-account balance as spendable money before tax, or an RMD as a spending plan when it is a floor on what must come out.
Every tool the guides refer to is listed in the calculator directory, grouped as Retirement, Investing & Savings, and Planning & Protection. The methodology page holds the formula reference behind all of them and the reference figures the engine is tested against.
Not financial advice: an educational index of four explainers covering account mechanics, statutory ages, and published research. The worked examples assume a constant return and exclude contribution limits, taxes, and market volatility; they are computed at build time by the same tested engine as the calculators, so this page cannot drift from the tools. Statutory ages and rules are current as of September 2026 — confirm current figures at irs.gov and ssa.gov, and consult a licensed professional for your situation. See the methodology page.