How our calculators work
Every formula and assumption behind our Coast FIRE, Barista FIRE and Coast FIRE number calculators, so you can check our work.
Last updated October 6, 2026
Everything is in today's money
You enter spending in today's dollars, so we do the rest of the math in today's dollars too. Instead of growing your spending with inflation, we take inflation and fees out of the return. That gives the real return:
With our defaults (7% return, no fees, 2.5% inflation), the real return is 4.39% a year. Every result on the site, including the portfolio values on the chart, is in today's money.
The FIRE number
At a 4% withdrawal rate, $40,000 a year of spending needs $1,000,000 invested. The 4% figure comes from historical studies of 30-year retirements. For a retirement that could last 40 years or more, many planners use 3% to 3.5%, which raises the FIRE number. You can change the rate under Assumptions.
The coast number
This is how much you need invested today for growth alone to reach your FIRE number by your retirement age. A 32-year-old retiring at 60 with a $1,000,000 FIRE number and a 4.39% real return needs $300,277. The Coast FIRE number page shows it for every age.
The coast date
The coast number shrinks as you get closer to retirement, because there's less time left to grow. To find the month you can stop saving, the Coast FIRE calculator steps your portfolio forward one month at a time:
- The yearly real return is turned into a monthly one: (1 + real return)1/12 − 1.
- At the start of each month, we compare your portfolio with the coast number for the years left until retirement. The first month it's at or above that line is your coast date.
- Otherwise, the portfolio grows by one month of returns and your monthly contribution is added at the end of the month.
- If you set a contribution raise, your monthly contribution goes up by that percentage every 12 months.
With our default example ($185,000 invested, $1,500 a month), the coast date is 7 years 7 months away. If you never reach the line before your retirement age, we show how much more you'd need to save each month to reach your FIRE number instead.
Odds of success
The coast date assumes your investments earn exactly the expected return every year. Real markets don't, so we also test your plan against 1,000 simulated markets:
- Each year's return is drawn at random. Returns follow a log-normal distribution whose median is your expected real return, with a 15% yearly standard deviation, which is roughly the long-run swing of a stock-heavy portfolio.
- You keep contributing until the coast date, then stop.
- A market counts as a success if your portfolio reaches the FIRE number by your retirement age.
- The random numbers come from a fixed seed, so the same inputs always give the same odds.
Because the coast date assumes the middle outcome, stopping exactly on it works in about half of the simulated markets. With our defaults, the odds are 50% if you stop on the coast date, 57% if you save for 3 more years, and 78% if you never stop saving before 60. That's why the calculator shows both numbers, and why a buffer and a yearly check-in matter.
The simulation is a simplification: it moves in whole years, treats each year as independent of the last, and doesn't model the crashes that are more extreme than a bell curve predicts. It only tests whether you reach your FIRE number. How long that money lasts in retirement depends on the withdrawal rate you choose.
What-if scenarios
Each what-if row changes one input, re-runs the coast date and shows how many months sooner or later it lands:
| Scenario | What changes |
|---|---|
| Save more | +$250 a month |
| Retire later | Retirement age +2 years |
| Lower returns | Expected return −1 percentage point |
| Higher fees | Fees +0.5 percentage points |
Barista FIRE
The Barista FIRE calculator uses the same month-by-month projection, with you contributing every month, and reports the first month your portfolio reaches the Barista FIRE number. It also shows when you'd reach your full FIRE number and your coast date for the same inputs, so you can compare the three milestones.
Default assumptions
| Assumption | Default | Why |
|---|---|---|
| Expected return | 7% a year | A common planning figure for a mostly-stock portfolio, before inflation. |
| Inflation | 2.5% a year | A little above the 2% target most central banks aim for, to leave some margin. |
| Fees | 0% | Add your fund and advice fees. Even 1% can delay your coast date by years. |
| Withdrawal rate | 4% | The classic rule of thumb from 30-year retirement studies. |
| Contribution raises | 0% | Contributions stay level in today's money unless you add raises. |
| Market volatility | 15% a year | Used only for the odds of success. |
These are starting points, not predictions. Change any of them to match your own situation.
What we leave out
- Taxes. Enter retirement spending that includes the tax you expect to pay, and part-time income after tax.
- Pensions. Social Security, CPP, OAS, the UK State Pension and workplace pensions aren't added automatically. Subtract the yearly amount you expect, in today's money, from your spending.
- Account types. We treat all your investments as one portfolio, whether they're in a 401(k), IRA, RRSP, TFSA, ISA or a taxable account.
- Changing spending. We assume your spending stays the same, in today's money, for your whole retirement.
- Currency. Choosing a currency changes the symbol and formatting only. We don't convert amounts.
Changes
October 2026: first version of the methodology. If we change how a result is calculated, we'll describe the change here. Spotted a mistake? Email hello@coastfireplanner.com.