Methodology
Exactly how the calculator gets its numbers, which assumptions it uses by default, and what it doesn't try to model.
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The formulas
The FIRE number is the portfolio that can support your yearly spending at your chosen withdrawal rate. With $60,000 of spending and a 4% withdrawal rate, that’s $1,500,000.
The real return converts your expected return into growth in today’s money using the exact relationship (1 + return) ÷ (1 + inflation) − 1, rather than simply subtracting inflation. With 7% returns and 3% inflation, the real return is 3.88%.
The Coast FIRE number discounts the FIRE number back to today at the real return, over the years remaining until your retirement age. For a 30-year-old retiring at 65, that’s $395,332.
Because everything is in real terms, all dollar amounts on the site are in today’s buying power. Your nominal account balance in the future will be higher.
When you reach Coast FIRE
The coast age comes from a month-by-month simulation rather than a single formula:
- Start with your current invested balance.
- Compare the balance to the Coast FIRE number for your age at that moment. The first month the balance is at or above it is your coast age. If you’re already over the line today, your coast age is your current age.
- Grow the balance by the monthly equivalent of the real return, (1 + real return)1/12.
- Add your monthly contribution at the end of the month, but only until you reach Coast FIRE.
- Repeat steps 2 to 4 every month until your retirement age.
If that never happens before your retirement age, the calculator says so. Contributions are treated as being in today’s dollars too, which is the same as assuming they rise with inflation.
The chart plots the same simulation once a year, alongside the Coast FIRE line and the FIRE target.
Default assumptions
| Input | Default | Why |
|---|---|---|
| Expected return | 7% | A middle-of-the-road long-run figure for a stock-heavy portfolio. US stocks have averaged roughly 10% a year before inflation over the past century; a mix with bonds has returned less. |
| Inflation | 3% | Close to the long-run US average. |
| Withdrawal rate | 4% | From William Bengen’s 1994 study and the Trinity Study (Cooley, Hubbard and Walz, 1998), which found that withdrawals starting at about 4% and raised each year for inflation historically lasted through almost every 30-year retirement for portfolios with at least half in stocks. |
| Retirement age | 65 | The traditional US retirement age, and the age Medicare usually starts. |
| Spending, balance, contribution | $60,000, $80,000, $2,000/mo | Example values so the first result isn’t blank. Replace them with your own. |
You can change every one of these in the calculator’s Assumptions panel. The return is the input that moves results the most, so it’s worth trying a lower number.
What the model leaves out
The calculator is deliberately simple, so it’s easy to check. That means it ignores some things that matter in real life:
- Sequence of returns. Real markets don’t deliver a steady average. A bad run early on can delay your date even if the long-run average holds. The Monte Carlo calculator tests this separately. It runs 2,000 simulated markets in which each year’s real return is drawn at random from a lognormal distribution, independent of other years, set so the median path grows at your real return, with 15% volatility by default. Contributions go in once a year until the age you stop, and the result is the share of paths that reach your FIRE number by retirement.
- Taxes. Withdrawals from pre-tax accounts are taxed. Build expected taxes into your spending figure.
- Investment fees. Subtract them from your expected return.
- Social Security and pensions. The main calculator doesn’t model these. If you expect them, lower your spending figure by the amount they’ll cover, keeping in mind when they start. This post walks through the adjustment, and the calculator with Social Security does it for you: it subtracts the income from your spending, and if the income starts after you retire, it sets aside a full year of it for each year you’re waiting, assuming that money earns nothing after inflation.
- Changes in spending over time, like healthcare costs rising later in life.
The other calculators
The 4% rule calculator looks at the years after you retire. It uses the same kind of simulated markets as the Monte Carlo calculator: 2,000 paths, each year’s real return drawn from a lognormal distribution whose median is your real return, 15% volatility by default. Your spending stays fixed in today’s dollars, it comes out at the start of each year, and the result is the share of paths that never run out. The “9 in 10” rate is the highest starting withdrawal rate that lasts in at least 90% of those same paths.
The compound interest calculator works month by month. Each month the balance grows by (1 + rate ÷ n)n/12, where n is how many times a year interest compounds, and then that month’s contribution is added. With monthly compounding this matches the standard future-value formula exactly.
Changelog
- September 24, 2026: Added the 4% rule and compound interest calculators.
- September 13, 2026: First version of the calculator and this page.