Data & Methodology

The historical datasets

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The backtesting tools (the SWR backtester and the Monte Carlo simulator) run on long-run monthly series covering 1871 through 2025, roughly 154 years of market history. Each series is a total-return index normalized to 100 at January 1871, so a value of 8,000 means that asset grew 80x over the period, dividends and reinvestment included.

The US equity, bond and inflation series derive from the long-run dataset compiled by Robert Shiller at Yale, the same 1871-to-present data used across the academic safe-withdrawal-rate literature (Bengen's original 4% work, the Trinity Study, and the deep retirement-research blogs). The ex-US equity, gold, commodity and cash series are proxy extensions assembled over the same window. Where a series is a reconstruction or proxy rather than a direct historical record, I have flagged it as such above.

How each calculator works

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SWR Backtester

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This is not a simulation. It runs your withdrawal plan across every historical rolling window in the 1871-2025 data and reports how the plan would actually have fared. Real returns, real inflation, real sequence-of-returns risk. If a 4% withdrawal rate failed in the 1906 or 1966 retiree's window, you will see it fail here.

Monte Carlo Simulator

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Where the backtester replays history, the Monte Carlo tool generates thousands of possible futures. It draws on the historical return, volatility and correlation of the asset classes above, then runs 10,000 simulated paths by default (selectable from 1,000 up to 100,000). Defaults: a 50-year horizon and a 3% withdrawal rate, both adjustable, with an optional advisor-fee drag. The output is a success probability: the share of simulated retirements that never ran out of money.

Savings Rate Calculator

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The "Years to FIRE" estimate assumes a 7% annual return and a 4% safe withdrawal rate (the 25x-expenses rule). These are deliberately simple round assumptions for a quick estimate. Your real timeline depends on actual market returns, taxes, currency drift and life changes, which is exactly why the backtester and Monte Carlo tools exist.

Compound Interest Calculator

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Standard compound-growth formula. You set the rate, the contribution amount and the compounding frequency; it projects the balance forward. No hidden assumptions beyond the inputs you provide.

FIRE Calculator

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Built on the 25x rule: your FI number is 25 times your annual expenses, the inverse of a 4% withdrawal rate.

Emergency Fund Calculator

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Months of essential expenses times your monthly burn. No market assumptions; it is a budgeting tool, not a projection.

Portfolio Rebalancer

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Compares your current allocation against your target, shows the drift, and lists the trades that bring you back in line. UI-only, nothing is stored.

Currency-Aware FIRE Calculator

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This one carries its own data, because it has to. Your FI number is 25 times your annual expenses (the inverse of a 4% withdrawal rate), but if you earn in one currency and plan to retire in another, that number lives in two currencies at once. The calculator prices your target in the currency you will spend, then converts it into the one you save in.

Two exchange-rate datasets sit underneath it, both refreshed when the site is built and then stored with the site, so the tool runs in your browser with no live calls, no tracking, and no account:

• Today's rates come from a free public exchange-rate feed (open.er-api.com), stored as the value of 1 AED in each supported currency.
• The historical reality check uses European Central Bank reference rates, published every business day since 1999, via the Frankfurter dataset. For each currency pair the tool reads year-end rates back to 2000 and fits a long-run trend through every annual point (a log-linear regression, so one unusual start or end year cannot skew the result). That trend is the real annualized drift it reports, which lets you check your own drift assumption against what the pair actually did over the last 25 years instead of guessing. The UAE dirham is not published by the ECB, so where it is involved the rate is derived from its fixed peg of 1 USD to 3.6725 AED.

The honest limit: real currencies move in jumps, not smooth lines. The historical drift is a long-run average, not a forecast, and the next 20 years will not replay the last 25. Treat it as a sanity check on your assumption, not a prediction.

UAE Gratuity Calculator

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This is a legal calculation, not a market projection, so it contains no return assumptions at all. It follows the UAE private-sector end-of-service rule under Federal Decree-Law No. 33 of 2021:

• Your daily wage is your monthly basic salary divided by 30. Housing, transport and other allowances are not counted.
• You earn 21 days of basic pay for each of your first five years of service, then 30 days for every year after that.
• You must complete one continuous year to qualify. Beyond that, part-years are paid pro-rata, and any unpaid leave is deducted from your service time.
• The total is capped at two years' basic wage, the legal maximum.
• Resigning and being let go pay the same under the current law. The old rule that docked a third or two-thirds of the benefit for resigning was scrapped in 2021.

The payout is a dirham figure by law, so the headline is always in AED. The optional conversion to another currency, and the inflation-adjusted "today's money" view of the invested figure, use exchange rates baked into the site at build time from a free public feed (open.er-api.com), stored as the value of 1 AED in each currency. As always this is the general rule, not tailored advice: specific contracts, free-zone employers and disputes can differ, so treat it as a close estimate.

UCITS Withholding-Tax Drag Calculator

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This is a structural tax comparison, not a full after-tax return model. It computes two costs that non-US-person investors pay for holding US-domiciled ETFs instead of Ireland-domiciled UCITS: the annual and 30-year-compounded dividend withholding drag, plus the US federal estate-tax exposure on US-situs assets.

Three sources drive the math, all baked into the site (no live tax-API calls):

• US treaty withholding rates on portfolio dividends come from the US bilateral tax treaties in force with the four v1 residencies. UAE has no US income-tax treaty so the statutory 30% under Section 1441 applies. Philippines caps at 25% under Article 11(2)(b) of the US-PH treaty. Switzerland caps at 15% under Article 10 of the US-CH treaty (portfolio dividends). US persons pay US income tax on qualified dividends separately (0/15/20% plus 3.8% NIIT above Section 1411 thresholds), which the tool does not model.
• Ireland-US treaty rate at fund level = 15%. Standard retail Irish UCITS (PLC or ICAV structures like CSPX, VUAA, VWRA) qualify for the 15% rate under the US-Ireland treaty on US-source dividends received into the fund. Ireland levies 0% outbound withholding on distributions to non-Irish residents, so the fund-level 15% is the total US-side drag for a non-Irish holder.
• US estate tax uses the graduated schedule from IRC 2001(c), not a flat top rate. Rates run from 18% on the first $10,000 up to 40% only on amounts above $1,000,000. Non-US persons get a $13,000 unified credit under Section 2102(b)(1), which mechanically shields the first $60,000 of US-situs estate. A flat-40% shortcut commonly quoted online overstates the actual tax by 15-30% at the portfolio sizes this calculator targets ($100,000-$3M US-situs).

All treaty rates require a valid W-8BEN on file with the broker. Without one, the US withholds the statutory 30% regardless of the investor's home-country treaty. The compounding model assumes reinvestment (a first-order approximation appropriate for accumulating UCITS). Global funds like VWRA face a weighted blend of source-country withholding rates at fund level and land closer to 8-11% blended drag rather than the flat 15% shown for US-equity-only funds.

The honest limit: this is a structural comparison, not tax advice. Country-of-residence tax on distributions and capital gains (UK reporting-fund status, German InvStG, Australian attribution rules) can materially change the picture in specific jurisdictions and is deliberately out of scope. Consult a cross-border adviser for your own case.

Honest limitations

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A few things I want stated plainly rather than buried:

• Past performance is not a forecast. 154 years of data is a strong guide but doesn't predict the future. The next 30 years will not be a clean replay of the last 150.
• The history is mostly US. It is the deepest, cleanest long-run dataset available, but a globally mobile investor faces currency moves, foreign withholding tax, residency rules and estate-tax exposure that no return series captures. Treat US history as a baseline, then adjust for your own situation.
• These are planning tools, not advice. They help you reason about ranges and tradeoffs. They do not know your tax residency, your broker, or your risk tolerance. Nothing here is financial advice. See the full disclaimer.

If you spot something that looks wrong in the data or the math, tell me.

Last reviewed: July 2026.