FIRE Calculator

Work out your FIRE number — the portfolio size that could support your spending indefinitely — and estimate how many years of saving and investing it takes to get there.

How the estimate is built

Step 1 — FIRE number. Annual expenses divided by the withdrawal rate: FIRE number = annual expenses ÷ withdrawal rate. At a 4% withdrawal rate this is the same as expenses × 25 — the "25×" figure some people quote is just the 4% rule written the other way round.

Step 2 — annual surplus. Annual income minus annual expenses: the amount assumed to be invested at the end of every year, on top of whatever is already saved.

Step 3 — years to reach it. Starting from your current savings, the calculator adds one year's growth at the entered real return, then that year's surplus, and repeats until the running total reaches the FIRE number (interpolating within the final year for a fractional figure). Because the return you enter is real — after inflation — the FIRE number, income and expenses are all treated as staying level in today's money, so no separate inflation figure is needed.

FAQ

Where does the "4% rule" come from?

From two pieces of US retirement research, not from any regulator or law. Financial planner William Bengen's 1994 paper "Determining Withdrawal Rates Using Historical Data" tested US stock and bond returns since 1926 and found that starting at roughly a 4% withdrawal rate, then increasing the amount withdrawn with inflation every year after, never exhausted a portfolio within 30 years across any of the historical starting points he tested — a 50/50 stock/bond split worked best, though heavier stock allocations came close. The 1998 "Trinity Study" (Cooley, Hubbard & Walz, of Trinity University) ran a similar historical backtest across many stock/bond mixes and withdrawal rates from 3% to 12% and reached a similar conclusion for rates around 3–4%. Both are a rule of thumb built on US historical data, not a guarantee or personal advice, and researchers still debate the "right" number today — this calculator lets you enter any withdrawal rate you think is appropriate. Neither study accounts for your personal tax situation, investment fees, sequence-of-returns risk (poor returns striking early in retirement), or spending that changes as you age.

What counts as my "annual surplus", and does it need to grow with my income?

Annual income minus annual expenses, held constant in today's money for every year of the projection. In real life your income and spending will change — a raise, a child, a mortgage paid off — so treat the years figure as a snapshot based on today's numbers, and recalculate whenever something significant changes.

Why does the expected return make such a big difference to the result?

Because it compounds twice over — once on your existing savings and once on every year's surplus, for however many years that surplus has left to grow. Small changes to the return you enter (say 4% versus 6% real) can shift the years-to-FIRE figure by several years, which is exactly why the number is uncertain: nobody knows in advance what real return the next few decades of markets will actually deliver.