Coast FIRE Calculator: When Can You Stop Saving?

Coast FIRE is the point where the money you already have invested will grow into your full retirement target on its own, so you can stop saving for retirement and only earn enough to cover today's bills. Enter your age, the age you want to retire, what you expect to spend a year in today's dollars and what you have invested now. The calculator shows your Coast FIRE number today, how far you are from it, and the age you reach it if you keep contributing. It works in today's dollars throughout, so inflation is built in rather than ignored.

The Coast FIRE formula

First the full FIRE number: annual spending divided by the safe withdrawal rate, which at 4% is 25 times spending. Then discount it back to today: Coast FIRE number = FIRE number ÷ (1 + real return)^years to retirement. The real return is the return after inflation, (1 + expected return) ÷ (1 + inflation) − 1. At a 7% expected return and 3% inflation that is 3.88%, not 4%, and over thirty-odd years the difference adds up. Using the real return is what keeps the answer in today's dollars: a portfolio that grows at the real rate keeps pace with prices by definition.

A worked example

A 30-year-old who wants to retire at 65 and spend $40,000 a year in today's money has a FIRE number of $1,000,000. At 7% returns and 3% inflation the real return is 3.88%, so the Coast FIRE number today is $263,555. With $60,000 invested they are $203,555 short; left alone, $60,000 grows to about $227,657 in today's money by 65. Contributing $1,500 a month, they reach Coast FIRE at age 44.8, after 14 years 10 months. From then on the balance should reach the target with no new money, as long as returns and inflation behave as assumed. In the dollars of 35 years from now the same target is about $2,813,862, which is why the calculator never shows a bare nominal figure without the real one next to it.

Why the inflation input matters

Many Coast FIRE calculators discount the target with the full nominal return. With the same example that gives $93,663 instead of $263,555, which looks easier but would leave you short by the whole of 35 years of price rises. A few tenths of a percent in the return assumption move the answer by tens of thousands of dollars, as the table shows, so run a cautious case as well as your central one.

Coast FIRE for couples

For a couple, use your combined retirement spending and your combined invested balance, and the age at which you both plan to stop drawing a paycheck. If your ages are far apart, run it for the younger partner's retirement date as well: more years to grow means a smaller number today, but the older partner may want to stop sooner. Accounts in either name count, as long as they are earmarked for retirement.

What Coast FIRE does not include

Social Security, pensions and home equity are left out, which keeps the answer conservative; if you want to count a pension, reduce the annual spending figure by what it will pay. Taxes on withdrawals from pre-tax accounts are not modelled, so include an allowance for them in spending. Returns are assumed smooth: real markets are not, and a bad decade just before retirement matters more than one just after you start coasting. Revisit the number once a year, and treat reaching it as permission to save less, not as a guarantee.

• FIRE number calculator
• Barista FIRE calculator
• Compound interest calculator
• Retirement savings calculator