Coast FIRE calculator

How the number is worked out

Two steps. First, the portfolio you need on the day you retire, which is your annual spending divided by your withdrawal rate. At 4%, that is twenty-five times what you plan to spend in a year. Second, that figure discounted back to today over the years between now and then, at the return you expect to earn after inflation.

coast number = (annual spending / withdrawal rate) / (1 + real return) ^ years to retirement

The real return is the part most calculators get quietly wrong. Your spending figure is in today's money, so the growth rate has to be in today's money too. Here that is (1 + nominal) / (1 + inflation) - 1, which at 7% and 3% gives about 3.9%, not the 4% a simple subtraction suggests. Small difference on one year. Over thirty-five years it moves the answer by thousands.

Then the date. If you are not there yet, the calculator walks forward month by month, growing what you have at the real rate and adding your monthly saving. The target moves too, and it moves upward, because every month you wait is a month less of compounding to do the work. You reach Coast FIRE at the first month where your balance, left alone from that point, would still land on your retirement figure.

The assumption doing the heavy lifting is the return. Run it at 7% nominal and again at 5% and the gap between the two answers will be larger than most people expect. That is not a flaw in the arithmetic. It is what thirty-five years of compounding does to a one point difference, and it is the reason to treat any single number here as one scenario rather than a plan.

Questions people ask

Is Coast FIRE the same as retiring early?

No, and the difference is the whole point. Coast FIRE means you stop saving for retirement. Your job still has to cover your rent and your groceries until the retirement age you entered, because the portfolio is being left alone to grow rather than drawn down. Retiring now is a much larger number, and it is the one in the "portfolio needed" box above.

What counts as invested?

Money in the market that you will not touch before retirement. A 401(k), an IRA, a taxable brokerage account, a workplace pension pot you control. Not your emergency fund, not a house you live in, and not cash earmarked for something in the next few years. Including any of those makes the answer look better than it is.

Why is my number so much bigger than a friend's?

Usually years, not dollars. Someone ten years younger needs far less invested today for the same retirement, because their money has an extra decade to compound. Retirement age and current age move this figure harder than the spending number does. Change your retirement age by five years and watch what happens.

Should I really stop saving once I hit it?

That is a decision this arithmetic cannot make for you. Hitting the number means the retirement problem is solved under your assumptions, and if any of those assumptions are wrong the buffer is gone. Plenty of people who reach Coast FIRE keep saving at a lower rate, or use it to move to work they prefer rather than to spend more.

Do you store what I type?

No. This runs entirely in your browser. Nothing is sent anywhere, nothing is saved, and closing the tab loses it. That is also why the spreadsheet exists.

Educational tool, not financial advice. It assumes a constant real rate of return, contributions that never change, no taxes on growth or withdrawal, and no sequence-of-returns risk. Real markets deliver none of those things. The 4% default comes from Bengen's 1994 study of historical US data over thirty-year retirements, and it is a starting point rather than a guarantee. Confirm anything that matters with a qualified professional.

Related reading: what the Coast FIRE calculation actually assumes, the arithmetic behind the 4% rule, and the 2026 contribution limits.