Every number we show you comes from a documented assumption, a published formula, or a named data source. This page explains all three — so you can check our work instead of trusting a black box.
All of our calculators share a small set of defaults. Every single one is adjustable — the defaults are a starting point for your own scenario, not a recommendation.
FIRE numbers are calculated from a 4% safe withdrawal rate by default — the classic baseline popularized by the Trinity study, which means your FIRE number is 25 times your expected annual spending. The withdrawal rate is a slider in every FIRE calculator: cautious planners often model 3.0–3.5%, optimists 4.5–5%, and your FIRE number updates instantly as you move it.
Projections compound at the real return — your expected nominal return minus expected inflation — so every result is expressed in today's money. With example inputs of a 7% nominal return and 2.5% inflation, projections grow at roughly 4.5% per year in real terms. Both the return and the inflation rate are inputs you control.
The portfolio analyzer doesn't draw a single line into the future. It runs 10,000 randomized market scenarios built from your portfolio's historical mean return and volatility, then reports the full spread of outcomes — including the probability of reaching your FIRE target, not just the average path.
Live quotes and historical price series for stocks and ETFs come from Yahoo Finance market data.
Fund composition used for look-through and overlap analysis comes from justETF and fund-provider disclosures.
US percentile brackets come from the Federal Reserve's Survey of Consumer Finances (SCF). UK, Canadian, Czech, and German brackets come from the respective national statistics offices and central-bank wealth surveys.
Assumption defaults and data tables are reviewed when their sources publish new releases — the SCF every three years, market data continuously — and whenever a calculation changes, the affected page's 'last updated' date changes with it. If we change a formula or a default, the change ships to both the English and Czech versions of the site at the same time.
These calculators are educational tools, not financial advice. They model simplified scenarios with assumptions you control; they don't know your taxes, your job security, or your risk tolerance. Past returns don't guarantee future results. Before making decisions that affect your financial future, consider consulting a qualified financial advisor.