Every number MyFIRE shows you has a source. Every assumption has a reason. This page explains the math, the data, and the design decisions behind the model — so you can trust the output and understand its limits.
Most retirement calculators show you what you want to hear. MyFIRE is built to show you what you need to know — including the uncomfortable parts.
Three principles guide every design decision: completeness (model all three phases, not just the accumulation), honesty (show the success rate, not just the projected number), and transparency (explain every assumption so you can change what you disagree with).
The bridge fund gap — the years between early retirement and 59½ when your 401k unlocks — is real and most tools ignore it. Sequence of returns risk is real and a single projection line hides it. Healthcare before Medicare is real and most calculators pretend it doesn’t exist. MyFIRE models all of these.
MyFIRE is built around a principle most financial tools ignore: your financial data should be yours. Not ours. Not stored without your explicit choice.
In local mode, your complete financial plan is stored in a single JSON file. It contains everything MyFIRE knows about your plan — your inputs, saved scenarios, AI conversation context, and net worth history. The file is schema-versioned so future MyFIRE updates can load your data correctly even as the product evolves.
// myfire-2026-07-04.myfire { "schemaVersion": "V3.26.07.50", "exportedAt": "2026-07-04T00:00:00.000Z", "storageMode": "local", "plan": { // age, savings, contributions... }, "memory": { // AI conversation context }, "scenarios": [ // saved what-if scenarios ], "netWorth": { // check-in history } }
The schema version allows MyFIRE to detect when a file was created with an older version of the app. When breaking changes occur, the system maps old field names to new ones automatically — so your data survives product updates without manual re-entry.
Most retirement planners model one phase: the drawdown from a retirement account starting at 65. MyFIRE models all three phases simultaneously — because early retirement is fundamentally different.
From today until you stop working. MyFIRE calculates the monthly savings required across all account types simultaneously — 401k, Roth IRA, HSA, and taxable brokerage — to reach your FIRE number at your target age.
The accumulation model uses compound growth with your specified return rate (default 7% nominal). Inflation adjusts the required corpus over time.
The gap between stopping work and age 59½, when retirement accounts unlock without the 10% early withdrawal penalty. During this phase, you live on your taxable brokerage account (the bridge fund) while your 401k and IRA compound untouched.
MyFIRE calculates the required bridge fund based on years in the bridge phase, monthly spending, and the bridge return rate (default 5% — more conservative than accumulation, reflecting a more liquid, lower-risk allocation during drawdown).
From full retirement to age 100. MyFIRE draws down the portfolio using the 4% rule (or your specified safe withdrawal rate), adjusting for inflation annually. Social Security income reduces the required portfolio draw when it begins.
The retirement phase is stress-tested by 1,000 Monte Carlo simulations — not projected as a single line. The success rate shown is the percentage of historical scenarios where the portfolio survives to age 100.
A single projection line tells you the average outcome. Monte Carlo shows you the range — including the scenarios where things go wrong.
MyFIRE uses annual S&P 500 total return data from Aswath Damodaran / NYU Stern School of Business, covering 1928 through 2023. This dataset includes dividends reinvested and is one of the most widely cited sources in retirement-planning research. The data is updated annually and available publicly at pages.stern.nyu.edu/~adamodar/.
Most Monte Carlo simulators generate random returns drawn from a normal distribution centered on the historical average. MyFIRE uses historical sequences — actual rolling periods of real market history. This captures something random simulation misses: the clustering of bad years (1929–1932, 1966–1974, 2000–2002, 2008–2009) and the sequence of returns effect that makes the first five years of retirement the most critical to long-term survival.
A simulation ‘succeeds’ if the portfolio value is greater than zero at age 100. The success rate shown is the percentage of 1,000 simulations that succeed. A 75% success rate means 750 of 1,000 historical starting points resulted in a portfolio that lasted to age 100. It does not mean a 25% chance of dying broke — it means 25% of historical starting points would have depleted the portfolio before 100, assuming fixed inflation-adjusted spending throughout.
| Success rate | Verdict | What it means |
|---|---|---|
| 85%+ | Strong | Plan survives in 85+ of 100 historical scenarios. Strong buffer against bad sequences. |
| 70–84% | Good | Solid plan. Survives most historical scenarios with reasonable margin. |
| 50–69% | Needs attention | Meaningful failure risk. Consider retiring later, saving more, or spending less in retirement. |
| Below 50% | Fragile | More likely to fail than succeed historically. Significant adjustments recommended before retiring. |
Every return assumption in MyFIRE has a rationale. All defaults are adjustable in the Inputs tab under Return assumptions.
| Phase | Default | Rationale |
|---|---|---|
| Accumulation return | 7% | S&P 500 nominal average ~9.8% minus ~2.5% inflation minus ~0.3% fees. Conservative for a diversified index portfolio. |
| Bridge fund return | 5% | Lower than accumulation — bridge funds are held more conservatively (higher cash/bond allocation) to reduce sequence risk during the critical early drawdown years. |
| Retirement return | 6% | Between accumulation and bridge — reflects a moderately conservative allocation in retirement, consistent with the Trinity Study’s mixed portfolio assumption. |
| Inflation | 2.5% | Federal Reserve long-run target is 2%. We use 2.5% as a modest buffer. Healthcare inflation (typically 5–7%) is not separately modeled. |
| Safe withdrawal rate | 4% | The Trinity Study (Cooley, Hubbard & Walz, 1998). For retirements longer than 35 years, consider 3.5%. Fully adjustable in Inputs. |
Different accounts have different tax treatment, contribution limits, and withdrawal rules. MyFIRE tracks each type separately in the Inputs tab to produce an accurate withdrawal strategy.
In MyFIRE’s projection engine, Traditional 401k and Traditional IRA balances are combined into a single pre-tax bucket, and Roth 401k / Roth IRA are combined into a single Roth bucket. HSA and cash savings are currently combined with taxable brokerage into a single taxable bucket for simulation purposes, even though each is entered separately in the Inputs tab for your own record-keeping.
The Health Savings Account is uniquely valuable for early retirees — and consistently underused. Its tax treatment is fundamentally different from every other account type.
The optimal HSA approach for early retirees: contribute the maximum every year while working, invest it aggressively, and don’t spend it. Pay current medical costs out of pocket. Keep every medical receipt.
After retiring, reimburse yourself for any past medical expense at any time — there is no time limit on HSA reimbursement. A decade of receipts becomes a tax-free income source. This makes the HSA a stealth bridge fund specifically for healthcare costs — often the most expensive line item in early retirement.
After age 65, the HSA becomes a second Traditional IRA: withdrawable for any purpose, taxed as ordinary income, with no penalty. Combined with no RMD requirement, an HSA is strictly better than a Traditional IRA for anyone who can access one.
Individual: $4,400/year · Family: $8,750/year · Catch-up (age 55+): additional $1,000/year
To contribute, you must be enrolled in a High Deductible Health Plan (HDHP). 2026 minimums: $1,700 deductible (individual) / $3,400 (family). Maximum out-of-pocket: $8,500 (individual) / $17,000 (family).
HSA balance is entered separately in the Inputs tab under Account breakdown, alongside cash savings. In the current model, both are combined into the general taxable-account bucket for projection and withdrawal purposes — MyFIRE does not yet model HSA as a separately prioritized, medical-expense-only withdrawal source with its own sequencing rules. Treat the HSA strategy above as planning guidance to apply manually until that level of detail is added to the simulation engine.
MyFIRE incorporates 2026 federal tax brackets and a selectable state tax rate into the year-by-year withdrawal strategy.
Pre-tax withdrawals (401k/IRA) are taxed as ordinary income. The 12% bracket ceiling is the key optimization target for most FIRE retirees — the withdrawal engine tries to fill it before drawing from higher brackets.
| Rate | Single | Married filing jointly |
|---|---|---|
| 10% | $0 – $12,400 | $0 – $24,800 |
| 12% | $12,401 – $50,400 | $24,801 – $100,800 |
| 22% | $50,401 – $105,700 | $100,801 – $211,400 |
| 24% | $105,701 – $201,775 | $211,401 – $403,550 |
| 32% | $201,776 – $256,225 | $403,551 – $512,450 |
| 35% | $256,226 – $640,600 | $512,451 – $768,700 |
| 37% | $640,601+ | $768,701+ |
State tax rate is selected in the Inputs tab. MyFIRE includes a flat published income tax rate for all 50 states plus Washington D.C., applied to pre-tax withdrawals in the year-by-year withdrawal table. Several states have special exemptions or partial exemptions for retirement income specifically (Illinois is one well-known example) — MyFIRE’s current model does not yet special-case these state-specific retirement-income carve-outs, so the flat rate you select may overstate state tax owed in a state with such an exemption. Check your state’s specific treatment of retirement income before relying on this figure.
Withdrawals from taxable brokerage accounts are, in reality, only taxed on the appreciated portion of what’s sold, often at 0% for retirees with modest taxable income (up to $49,450 single / $98,900 MFJ in 2026). MyFIRE’s current withdrawal model does not calculate cost basis or capital gains explicitly — taxable-account withdrawals are treated as tax-free in the simulation, which approximates the favorable low-bracket case but will overstate your after-tax income if your realized gains would actually push you into a higher capital-gains bracket.
The order in which you withdraw from different accounts determines how much of your retirement spending goes to taxes. MyFIRE models a bracket-filling sequence.
The conventional wisdom is to spend taxable accounts first, then tax-deferred (401k/IRA), then Roth last. This preserves tax-free growth longest — but often results in a large 401k triggering RMDs at 73, pushing retirees into higher brackets when combined with Social Security.
MyFIRE’s withdrawal engine uses a bracket-filling approach: draw from pre-tax accounts (401k/IRA) up to the top of the 12% federal bracket each year, cover remaining spending from Roth IRA (tax-free), use taxable accounts for the rest, and after Social Security begins, shift more spending to Roth to avoid pushing into 22%+ brackets.
The years between early retirement and Social Security starting age (typically 67) are the lowest-income years of most FIRE retirees’ lives. This is the optimal window for Roth conversions: converting 401k to Roth at 12% now versus paying 22%+ later when SS income fills the lower brackets. MyFIRE includes a comprehensive guide to the Roth conversion ladder strategy — see our blog post for the complete playbook. Full pre-retirement conversion modeling (year-by-year optimal conversion amounts) is on the product roadmap.
Required Minimum Distributions begin at age 73 under current law (SECURE 2.0 Act). A large pre-tax balance at 73 can generate RMDs that push income into higher brackets even without additional spending. MyFIRE flags this risk in the withdrawal projection and treats early drawdown of pre-tax accounts as a mitigation strategy.
The FIRE number is a destination. The Progress tab shows how close you are — and updates your plan every time your net worth changes.
The Progress tab lets you record your net worth at any point in time across all account types: investments (401k, Roth IRA, taxable brokerage), cash savings, HSA, real estate equity, cryptocurrency, gold (with live price fetched automatically), and debt.
Each check-in is timestamped and stored in your account (cloud mode) or in your .myfire file (local mode). Over time, check-ins build a history of your financial progress toward your FIRE number.
Gold prices are fetched live from goldapi.io, and cryptocurrency prices (Bitcoin, Ethereum, Solana, XRP) are fetched live from the Coinbase public API, when you open the Progress tab. Enter your quantity (ounces of gold, units of crypto) and MyFIRE calculates the current USD value automatically.
Exchange rates for 7 major currencies (INR, GBP, EUR, CAD, AUD, SGD, JPY) are fetched live from open.er-api.com, converting foreign-currency entries to their USD equivalent automatically. MyFIRE supports labeling assets in 15 currencies in total — the remaining currencies are available for entry and display but do not yet have live conversion rates.
The Progress tab shows your current net worth as a percentage of your FIRE number — calculated from the same engine that powers the Plan tab. When you update your savings in the Inputs tab, your FIRE number and FI age recalculate automatically, and the Progress tab reflects the updated target.
For users tracking assets in Indian Rupees, MyFIRE formats values in the Indian numbering system: Lakhs (1 Lakh = ₹1,00,000) and Crores (1 Crore = ₹1,00,00,000). The currency toggle in the Progress tab shows only currencies currently in use across your entered assets.
In cloud mode, check-in history is stored in the net_worth_checkins table in Supabase, protected by row-level security — only your authenticated session can read your data. In local mode, check-in history is included in your downloaded .myfire file.
Social Security is not a small detail — it is a significant income stream that changes the math of retirement planning fundamentally.
Claiming at 62 results in roughly a 30% reduction versus your full retirement age (FRA) benefit. Claiming at FRA (67 for most people retiring today) is the baseline. Delaying to 70 increases the benefit by roughly 32% over FRA. The breakeven age for delaying from 67 to 70 is approximately age 82–83 — for someone retiring at 50 in good health, delaying to 70 is often the better bet.
SS monthly income is entered in the Inputs tab. MyFIRE adds it to the income stream starting at your specified claiming age, reducing the required portfolio withdrawal proportionally. For a retiree spending $6,000/month with $2,000/month in SS, only $4,000 needs to come from the portfolio — reducing the effective withdrawal rate and improving Monte Carlo success rates.
Your FIRE number is the portfolio size at which you can stop working and live indefinitely on investment returns.
The FIRE number equals 25 times your annual spending — derived directly from the 4% safe withdrawal rate. If you can withdraw 4% per year indefinitely, you need 25 times the annual amount.
At $60,000/year: $1,500,000. At $90,000/year: $2,250,000. At $120,000/year: $3,000,000.
For early retirements lasting 40–50 years, a 3.5% withdrawal rate (28.6x) is more conservative. For retirees with significant Social Security income, a higher rate may be appropriate. MyFIRE allows you to adjust the safe withdrawal rate in the Inputs tab, which changes the multiplier accordingly.
The FIRE number calculation in MyFIRE is based on your gross spending target, not after-tax spending. The tax burden is modeled separately in the withdrawal strategy engine. If you expect a low effective tax rate in retirement (common for FIRE retirees with controlled withdrawals), your actual portfolio requirement may be lower than the headline FIRE number suggests.
No model captures everything. These are the known limitations of MyFIRE’s current calculations — stated honestly rather than hidden.
Healthcare inflation. MyFIRE uses a single inflation rate for all spending. Healthcare costs in the US have historically inflated at 5–7% annually — significantly higher than the general 2.5% default. If healthcare is a large portion of your retirement budget, increase your monthly spend estimate to account for this, or add a separate healthcare line item to your spending.
Variable spending. MyFIRE models a fixed monthly spend adjusted for inflation. In practice, many retirees spend more in early retirement (travel, activity) and less later. A guardrails-style withdrawal strategy — spending less in down markets — is not yet modeled but is on the product roadmap.
International assets. Foreign investments, pensions, and accounts in other countries are not modeled in the tax calculations. Enter their USD equivalent in the relevant account fields for portfolio sizing purposes.
Real estate income. Rental income can be entered as an income stream but the tax treatment — depreciation, passive activity rules, cost segregation — is simplified. Consult a tax professional for rental property in your FIRE plan.
Behavioral risk. The biggest threat to most retirement plans is not market performance — it is behavior. Selling during downturns, lifestyle inflation, and panic at sequence-of-returns events are not captured in any model. The best plan is one you can stick to.
MyFIRE is provided for illustrative and educational purposes only. It is not financial advice, investment advice, tax advice, or legal advice.
MyFIRE is not a registered investment adviser under the Investment Advisers Act of 1940 or any state securities law. The outputs of MyFIRE are projections based on assumptions and historical data — they are not guarantees of future performance.
Past performance does not guarantee future results. Historical S&P 500 returns may not be representative of future market performance. All projections carry significant uncertainty. The range of outcomes shown in Monte Carlo simulations is based on historical data and does not represent the full range of possible future outcomes.
Before making any significant financial decisions — including decisions about when to retire, how to allocate investments, or how to structure withdrawals — consult a qualified, fee-only financial planner (CFP) who has a fiduciary duty to act in your interest. NAPFA.org maintains a directory of fee-only advisors.
This page and all MyFIRE content is governed by the laws of the State of Illinois, United States. Last updated: July 2026.