FIRE & Retirement Glossary
Plain-English definitions of every term used in RetireLab — from FI number and the 4% rule to Monte Carlo simulation and sequence of returns risk. Each term links to related concepts.
FIRE Fundamentals
FIRE — Financial Independence, Retire Early
FIRE is a movement built around accumulating enough invested assets to cover living expenses indefinitely — making paid work optional. "Financial independence" is the milestone; "retire early" is the optional consequence. Many FIRE adherents don't stop working entirely; they shift to work they choose rather than work they need.
FI Number
The FI number is the savings target that makes financial independence achievable. It is calculated as annual spending multiplied by 25, derived from the 4% rule. A household spending $60,000 per year targets a portfolio of $1,500,000. Once that threshold is reached, a 4% annual withdrawal is theoretically sustainable in perpetuity based on historical market returns.
4% Rule / Safe Withdrawal Rate (SWR)
The 4% rule originates from William Bengen's 1994 research and the subsequent Trinity Study. It states that a retiree can withdraw 4% of their initial portfolio value in year one, then adjust for inflation each subsequent year, and have a high probability of not outliving their money over a 30-year horizon. RetireLab flags withdrawals above 4% as elevated risk and below 3.5% as safe. The rule assumes a diversified stock-and-bond portfolio — results vary with different allocations and retirement lengths.
Lean FIRE
Lean FIRE describes retiring on below-average annual spending — typically under $40,000 per year for a single person in the United States. The trade-off is a lower FI number and earlier retirement date, in exchange for a frugal lifestyle with minimal margin for unexpected expenses. RetireLab models Lean FIRE as 80% of the base spending scenario.
Fat FIRE
Fat FIRE describes retiring while maintaining or exceeding a pre-retirement lifestyle, typically $100,000 or more per year in spending. The FI number is proportionally larger, requiring more years of savings or higher income. RetireLab models Fat FIRE as 125% of the base spending scenario.
Coast FIRE
Coast FIRE is the point at which existing invested assets, left to compound without additional contributions, will grow to cover retirement by a target age. Once you hit your Coast FIRE number you can "coast" — stop investing and cover only current living expenses from income. RetireLab calculates the Coast FIRE number as the present value of your FI number discounted at your chosen growth rate from today to your target retirement age.
Simulation Concepts
Monte Carlo Simulation
A Monte Carlo simulation runs thousands of randomized market sequences to stress-test a retirement plan. Instead of assuming a single fixed return each year, RetireLab builds 1,000 distinct 30-to-50-year scenarios by resampling real historical return sequences — so each scenario carries the kind of booms, busts, and multi-year inflation stretches markets have actually produced. The engine uses real (inflation-adjusted) returns, so all dollar values in results are in today's dollars — what your money can actually buy, not inflated future numbers. The output is a success probability — the percentage of paths where the portfolio survived to the planning age. This is more informative than a single-line projection because it reveals how plans perform under bad sequences, not just average ones.
Success Probability
Success probability is the percentage of Monte Carlo simulation paths in which the portfolio survives to the planning age without reaching zero. A 90% success probability means the plan worked in 900 of 1,000 simulated lifetimes. RetireLab generally considers 85%+ comfortable, 70–84% cautious, and below 70% elevated risk — but the right threshold depends on the individual's flexibility, income sources, and risk tolerance.
Sequence of Returns Risk
Sequence of returns risk is the danger that a string of bad market years early in retirement permanently damages a portfolio — even if the long-run average return is acceptable. When withdrawals are taken from a declining portfolio, fewer shares remain to benefit from the subsequent recovery. A retiree who experiences a 2008-style crash in year one faces a far worse outcome than one who experiences the same crash in year 15. Monte Carlo simulation explicitly models this risk by running scenarios with varying year-by-year return sequences.
Historical Sequence Backtesting
Historical backtesting tests a retirement plan against every rolling window of actual past market returns, rather than randomized simulations. RetireLab uses S&P 500 and 10-Year Treasury real total returns from Robert Shiller's dataset (1871–2025, 156 years), blended by your chosen stock/bond allocation. For a 30-year plan, this means testing against every 30-year window: 1871–1901, 1872–1902, and so on (~125 sequences). Withdrawals are taken at the beginning of each year before returns are applied — a conservative assumption that may produce slightly lower survival rates (2–4 percentage points) than calculators that withdraw after returns. The result is a survival rate — how many of those real historical sequences the portfolio survived. This complements Monte Carlo simulation by answering: “Would this plan have survived the actual 1929 crash, 1970s stagflation, 2000 dot-com bust, and 2008 financial crisis?” Note: the historical backtest uses the full withdrawal amount and does not subtract Social Security or pension income — this makes it a conservative worst-case test. Past performance is not predictive of future results.
Lean / Base / Fat Spending Scenarios
RetireLab runs every simulation across three spending levels simultaneously. The Base scenario uses the annual withdrawal amount you enter. Lean reduces it to 80% (frugal retirement) and Fat increases it to 125% (comfortable lifestyle buffer). Showing all three at once lets you see how much flexibility matters — a 5 percentage-point difference in success probability between Lean and Base tells you how much cushion a modest spending reduction would provide.
p50 / Median Portfolio Value
The p50 (50th percentile) figure is the median portfolio value at the planning age across all 1,000 simulated paths, shown in today's dollars. Half of simulations end with more than this amount; half end with less. It is a useful middle-ground estimate — less optimistic than the average (which can be skewed by extreme upside paths) and more constructive than worst-case outcomes. Because RetireLab uses inflation-adjusted returns, these values will appear lower than nominal-dollar projections from other tools — but they represent the same purchasing power. A p50 well above zero suggests the plan has real survivability, not just survival by the skin of its teeth.
CAPE Ratio (Shiller P/E)
The Cyclically Adjusted Price-to-Earnings ratio, developed by economist Robert Shiller, measures stock market valuation by comparing current prices to average inflation-adjusted earnings over the prior ten years. A high CAPE suggests stocks are priced expensively relative to historical earnings; a low CAPE suggests they are cheap. Historically, elevated CAPE readings have tended to precede lower-than-average long-run real returns — but the relationship is loose and not reliable on shorter horizons. RetireLab incorporates the current CAPE as context in its AI chat to frame return expectations. Past CAPE levels are not a reliable predictor of near-term market direction, and CAPE alone is not a basis for investment decisions.
Expense Ratio
The expense ratio is the annual cost of owning a fund, expressed as a percentage of assets under management. A fund with a 0.10% expense ratio costs $10 per year on a $10,000 investment; that cost is deducted from fund returns automatically. Over a long retirement horizon, expense ratios compound against the portfolio: a 1% annual drag costs roughly 20% of final portfolio value compared with a 0.10% fund over 30 years, all else equal. The RetireLab SWR Comparison tool includes an expense ratio input that adjusts the effective return assumption to show how fund costs affect safe withdrawal rate estimates.
Blended Return
The blended return is the weighted-average real (inflation-adjusted) return of a portfolio holding a mix of asset classes. For an 80/20 stock-bond portfolio, if stocks return 7% real and bonds return 2% real, the blended return is (0.80 × 7%) + (0.20 × 2%) = 6.0%. RetireLab's Show Math panels display the blended return formula to show how the stock/bond allocation translates into the single return assumption used in simulation.
RetireLab-Specific Terms
Risk Tolerance (LOW / MEDIUM / HIGH)
RetireLab's risk tolerance setting controls the real return assumption used in the Monte Carlo engine. LOW uses a 5% real return (conservative bond-heavy allocation or pessimistic market outlook). MEDIUM uses 7% (balanced portfolio, consistent with long-run U.S. equity history). HIGH uses 9% (equity-heavy, aggressive growth assumptions). These are real returns — inflation is not added on top. Choosing a higher risk tolerance is not a risk preference; it is an assumption about portfolio growth. Consult a financial advisor before selecting assumptions for real planning decisions.
Planning Age
Planning age is the age your portfolio must survive to — the finish line for the simulation. It is distinct from retirement age (when withdrawals begin). A person retiring at 50 with a planning age of 95 needs the portfolio to last 45 years. A longer horizon dramatically increases sequence of returns risk and requires a larger portfolio or lower withdrawal rate. RetireLab defaults to 95 and allows values up to 130.
One More Year (OMY)
"One More Year" describes the pattern of repeatedly postponing retirement by working one additional year — often because the decision feels too significant to commit to, or because the portfolio just cleared a milestone. Each extra year simultaneously grows the portfolio through new contributions and reduces the remaining retirement horizon. The RetireLab OMY button runs a what-if simulation showing the precise change in success probability from delaying retirement by one year, so the trade-off can be evaluated with actual numbers rather than intuition.
SWR% (Safe Withdrawal Rate Display)
The SWR% shown below each withdrawal scenario is the annual withdrawal as a percentage of the portfolio at retirement. It is calculated as annual withdrawal ÷ portfolio value × 100. RetireLab highlights this because the relationship between the withdrawal rate and the portfolio — not the dollar amount in isolation — determines long-run viability. The 4% rule sets the benchmark: below 3.5% is considered safe, 3.5–4% is cautious, above 4% is elevated risk.
Withdrawal Rate Danger Zones
RetireLab uses three zones to contextualize the withdrawal rate: Safe (below 3.5%) — historical portfolios at this rate almost never failed even over 40–50 year horizons. Caution (3.5–4%) — the original 4% rule territory; survivable in most historical sequences but vulnerable to long retirements or poor early returns. Elevated risk (above 4%) — a meaningful fraction of historical sequences fail at this rate, especially over 30+ years. None of this is financial advice; actual outcomes depend on asset allocation, spending flexibility, and other income sources.
Spending Guardrails (Guyton-Klinger)
Guardrails are a flexible-spending rule that cuts withdrawals when a portfolio falls far enough below its starting value, rather than spending a fixed amount regardless of market conditions. The approach comes from the Guyton-Klinger decision rules, which trim spending in bad years to help a portfolio recover. RetireLab models two settings through its spending flexibility input: Moderate reduces the annual withdrawal by 10% in any year the portfolio drops below 80% of its value at retirement, and Aggressive reduces it by 20% when the portfolio drops below 75%. Reducing spending in downturns directly counters sequence of returns risk and materially improves success probability, which is why a flexible retiree can often sustain a higher starting withdrawal rate than a rigid one. Flexibility is a modeling assumption, not a promise — the trade-off is a lower income in poor markets.
Traditional Retirement
Social Security Break-Even Age
The break-even age is when the cumulative benefits from delaying Social Security claiming (larger monthly payments) overtake the cumulative benefits of claiming early (smaller payments for more years). Claiming at 62 gives roughly 70% of the Full Retirement Age (FRA) benefit; claiming at 70 gives 124%. The crossover typically falls between age 78 and 82 depending on the benefit amount. If you expect to live past the break-even age, delaying is mathematically advantageous — but health, cash flow needs, and other income sources all factor into the real decision.
Healthcare Bridge
A healthcare bridge is the cost of private health insurance coverage between early retirement and Medicare eligibility at age 65. For a 50-year-old who retires early, this gap spans 15 years. ACA marketplace premiums vary widely by state, age, and income — and are income-sensitive, meaning lower portfolio withdrawals can reduce premiums through subsidy eligibility. Healthcare bridge cost is one of the largest and most variable expenses in early retirement planning.
Net Withdrawal
Net withdrawal is the annual portfolio withdrawal after subtracting guaranteed income sources — primarily Social Security benefits and pensions. If a retiree needs $80,000 per year and receives $24,000 in Social Security, the net withdrawal from the portfolio is $56,000. RetireLab uses the net withdrawal figure in its simulations, which reduces the required FI number and improves success probability for traditional retirees with meaningful Social Security or pension income.
Social Security Trust Fund
The Social Security Old-Age and Survivors Insurance (OASI) Trust Fund is the reserve from which Social Security retirement benefits are paid when payroll tax revenue falls short of benefit obligations. The SSA Trustees Report projects the OASI fund may be depleted around 2033–2034. If Congress does not act before depletion, continuing payroll tax revenue would cover only about 77% of scheduled benefits — resulting in an automatic reduction of approximately 23%. This does not mean Social Security “goes away” — it means benefits could be reduced to match incoming revenue. RetireLab's SS benefit reduction option lets users model this scenario by reducing their Social Security benefit by a chosen percentage before running the simulation. This provides a more conservative estimate of retirement income for those claiming benefits after the projected depletion date.
Tax & Withdrawal Planning
Traditional vs. Roth Accounts
The core difference between traditional and Roth retirement accounts is when taxes are paid. Traditional accounts (traditional IRA, 401(k), 403(b)) accept pre-tax contributions that reduce taxable income today; withdrawals in retirement are taxed as ordinary income. Roth accounts (Roth IRA, Roth 401(k)) accept after-tax contributions with no upfront deduction; qualifying withdrawals in retirement are tax-free. The choice of which account to draw from first in retirement — withdrawal ordering — affects lifetime tax burden, Medicare premiums, Social Security taxation, and estate planning. The RetireLab Withdrawal Planner models sequential withdrawals across taxable, traditional, and Roth accounts to estimate a blended effective tax cost on each dollar of spending.
Withdrawal Order (Drawdown Sequencing)
Withdrawal order is the sequence in which retirement savings are spent down across account types — taxable, traditional (pre-tax), and Roth. Common approaches include taxable-first (spend brokerage accounts before tax-advantaged ones), traditional-first (draw pre-tax accounts early to reduce later required minimum distributions), Roth-first, and proportional (draw from each account in proportion to its balance). Because each account is taxed differently, the order changes lifetime taxes, how long a portfolio lasts, Medicare premiums (IRMAA), and how much of Social Security is taxed. Required minimum distributions are always taken from pre-tax accounts first regardless of the chosen order. RetireLab's projection can compare all four orders side by side over a full retirement horizon; it shows the trade-offs rather than naming a single order as correct, since the right choice depends on a household's bracket, RMDs, and estate goals.
RMD — Required Minimum Distribution
A required minimum distribution is the amount the IRS forces you to withdraw each year from pre-tax retirement accounts (traditional IRA, 401(k), 403(b)) once you reach a set age — currently 73, rising to 75 for those born in 1960 or later under the SECURE 2.0 Act. The annual amount is the account balance divided by an IRS life-expectancy factor, so it grows as a percentage of the balance with age. RMDs are taxed as ordinary income and cannot be avoided by leaving the money invested; missing one carries a penalty. Because a large pre-tax balance can force sizable taxable withdrawals late in retirement — pushing a household into a higher bracket, taxing more Social Security, and triggering IRMAA surcharges — RMDs are a central reason households consider Roth conversions in lower-income early-retirement years. Roth IRAs have no RMDs during the original owner's lifetime. In RetireLab's projection, RMDs are always drawn from pre-tax accounts first, regardless of the chosen withdrawal order.
Roth Conversion
A Roth conversion moves funds from a traditional (pre-tax) retirement account — such as a traditional IRA or 401(k) — into a Roth IRA, with ordinary income tax paid on the converted amount in the year of conversion. In exchange, qualifying withdrawals from the Roth account in retirement are tax-free. Conversions are often considered during years when taxable income is temporarily lower: in early retirement before Social Security begins, after a job change, or when lower income keeps the conversion within a low tax bracket. The RetireLab Roth Conversion tool calculates how much can be converted before crossing into the next bracket (the "bracket gap"), and estimates the total tax cost now versus the taxes avoided on future withdrawals.
Provisional Income
Provisional income is the IRS measure used to determine what portion of Social Security benefits are subject to federal income tax. It equals adjusted gross income (AGI) plus any tax-exempt interest plus 50% of total Social Security benefits received. If provisional income exceeds $25,000 for single filers (or $32,000 for married filing jointly), up to 50% of Social Security benefits become taxable. Above $34,000 ($44,000 MFJ), up to 85% become taxable. These thresholds have not been adjusted for inflation since 1984, so a growing share of Social Security recipients pay taxes on benefits over time. Provisional income is the key input to understanding the Social Security tax torpedo and structuring Roth conversions that minimize lifetime taxes.
Social Security Tax Torpedo
The Social Security tax torpedo is a spike in effective marginal tax rate that occurs in the income range where Social Security benefits become increasingly taxable. Below the first provisional income threshold, Social Security is untaxed. As income rises through the phaseout range, each additional $1 of ordinary income makes an extra $0.50–$0.85 of Social Security taxable — effectively adding 50–85 cents to the marginal tax rate on top of the statutory bracket rate. For a retiree in the 22% bracket, the real marginal rate through the torpedo zone can reach 33–40%. The torpedo zone ends once 85% of benefits are fully taxable. Awareness of this range matters for Roth conversion planning: filling low brackets before the torpedo triggers can reduce lifetime taxes. The RetireLab Withdrawal Planner models the torpedo zone explicitly when estimating blended tax costs.
LTCG — Long-Term Capital Gains
A capital gain is the profit from selling an asset for more than you paid for it. When the asset was held longer than one year, the gain is a long-term capital gain and is taxed at preferential federal rates — 0%, 15%, or 20% depending on taxable income — rather than at the higher ordinary-income rates that apply to traditional-account withdrawals, wages, and short-term gains. This gap is why the order in which accounts are drawn matters: spending from a taxable brokerage account often realizes long-term gains taxed at 0–15%, while the same dollar pulled from a pre-tax IRA is taxed as ordinary income. Long-term gains also stack on top of ordinary income when determining which LTCG bracket applies, and they count toward the modified adjusted gross income that drives NIIT and IRMAA. The RetireLab Withdrawal Planner estimates long-term capital gains tax on taxable-account withdrawals separately from ordinary-income tax when computing a blended effective rate.
NIIT — Net Investment Income Tax
NIIT is a 3.8% federal surtax on net investment income — interest, dividends, capital gains, rental income — for taxpayers above a modified adjusted gross income (MAGI) threshold of $200,000 (single) or $250,000 (married filing jointly). Unlike most tax brackets, these thresholds are fixed by statute (IRC §1411) and have not been adjusted for inflation since the tax took effect in 2013, so it captures a growing share of taxpayers over time. The surtax applies to the lesser of net investment income or the amount MAGI exceeds the threshold. A Roth conversion itself is ordinary income, not investment income, but it raises MAGI and can pull existing investment income into NIIT exposure or push more of it over the threshold. The RetireLab Withdrawal Planner estimates NIIT using taxable-account withdrawals as a proxy for net investment income and folds it into the total blended tax cost; the Roth Conversion tool flags when a conversion's MAGI impact crosses the threshold.
IRMAA — Income-Related Monthly Adjustment Amount
IRMAA is a Medicare surcharge applied to Part B and Part D premiums for beneficiaries whose income exceeds certain thresholds. Medicare uses modified adjusted gross income (MAGI) from two years prior — so 2025 income determines 2027 IRMAA surcharges. For 2025, IRMAA begins above $106,000 for single filers and $212,000 for married filing jointly, with surcharges rising across five tiers. At the highest tier, the combined Part B and Part D surcharge can exceed $500 per month per person. A large Roth conversion or portfolio withdrawal in a single year can trigger or escalate IRMAA two years later, effectively adding thousands of dollars to the net cost of the conversion. The RetireLab Withdrawal Planner estimates IRMAA exposure based on projected income and flags when a withdrawal or conversion would cross a tier boundary.
RetireLab is an educational tool. Nothing on this page constitutes personalized financial, tax, or investment advice. Consult a qualified financial advisor before making retirement decisions.