Fire Coast Calculator

Coast FIRE is the point where you could stop saving entirely and still reach your retirement goal, because investments already made have enough time to grow on their own. A Fire Coast Calculator works that idea backwards: give it your target retirement fund, the years until retirement, and an expected annual return, and it tells you the amount you would need invested today.

That amount is called the Coast FIRE number. The maths divides your target by a growth factor, which is (1 plus the return) raised to the power of the years remaining. Long horizons produce startling results: with decades of growth ahead, the amount needed today can be a small fraction of the final target.

Reaching the number does not mean retiring today. It means the heavy lifting of saving could, in theory, be done, and future earnings only need to cover current living costs. The calculator also shows the Coast number if you wait five more years, which rises steeply, and that contrast is the clearest advertisement for starting early that personal finance offers.

How the Coast FIRE Number Is Worked Out

The formula is target fund divided by (1 plus r) to the power of n, where r is the annual return as a decimal and n is the years of growth. For a $1,000,000 target, 20 years, and a 7 percent return, the growth factor is about 3.8697, so the Coast number is about $258,419. In other words, that sum invested today, untouched, would grow toward the target.

Every part of the formula is an assumption you control. A bigger target raises the number, a longer horizon lowers it dramatically, and a higher assumed return lowers it too, which is exactly why return assumptions deserve honesty rather than optimism. Small changes in the return compound into large changes over twenty or thirty years.

Coast FIRE Versus Full FIRE

Full FIRE means having the entire target invested now, enough to live on without further work income. Coast FIRE is an earlier milestone on the same road: the portfolio is not big enough to live on yet, but it is big enough, given time, to become the full amount without another contribution.

Many people use the Coast milestone to change how they work rather than whether they work: moving to a less demanding role, going part-time, or taking a lower-paid job they enjoy, while the portfolio quietly does the remaining growth in the background. It is a flexibility milestone, not a finish line.

How to Use the Fire Coast Calculator

Follow these steps in order, using your own figures. The whole process takes less than a minute, and you can repeat it as often as you like while you compare options.

1. Enter the retirement fund you ultimately want, such as 25 times your expected yearly spending.
2. Enter the years between now and the retirement age at which you want that fund complete.
3. Enter an expected annual return as a percentage, using a cautious long-term figure.
4. Click Calculate to see the Coast FIRE number for today, the growth factor, and the wait-five-years figure.
5. Compare the Coast number with your current invested savings to see how close the milestone is.

Worked Example 1: A Million Dollar Target Over Twenty Years

The target fund is $1,000,000, retirement is 20 years away, and the expected return is 7 percent a year.

Step 1: Enter the target retirement fund as 1000000.

Step 2: Enter the years until retirement as 20.

Step 3: Enter the expected annual return as 7.

Step 4: The growth factor is 1.07 to the power of 20, giving 3.8697.

Step 5: The Coast FIRE number is 1000000 divided by 3.8697, giving $258,419.08 needed today.

Step 6: If you wait 5 years, only 15 years of growth remain, and the Coast number rises to $362,446.02.

Waiting five years raises the required starting sum by more than $100,000 in this example, even though the target never changed. Time in the formula is powerful precisely because the growth factor shrinks as the years are used up.

Worked Example 2: A Smaller Target Over Twenty-Five Years

The target fund is $750,000, retirement is 25 years away, and the expected return is 6 percent a year.

Step 1: Enter the target retirement fund as 750000.

Step 2: Enter the years until retirement as 25.

Step 3: Enter the expected annual return as 6.

Step 4: The growth factor is 1.06 to the power of 25, giving 4.2919.

Step 5: The Coast FIRE number is 750000 divided by 4.2919, giving $174,739.78 needed today.

Step 6: If you wait 5 years, 20 years of growth remain, and the Coast number rises to $233,857.15.

A longer horizon and a smaller target pull the Coast number below $175,000 here, which shows why younger savers can reach this milestone surprisingly early, and why the goal deserves a place in a long-term plan.

The Return Assumption Decides Everything

No input deserves more care than the expected return. Historical long-run stock market returns are often quoted around 7 percent after inflation, but your actual figure depends on your investments, fees, taxes, and the sequence of good and bad years, none of which a simple formula can foresee.

Run the calculator at a cautious return, such as 5 percent, as well as at 7 percent, and treat the cautious answer as the planning figure. If your plan only works at an optimistic return, the honest reading is that the milestone is further away than the flattering number suggests.

What Coast FIRE Does Not Cover

The calculation assumes no further contributions and no withdrawals, a smooth return every year, and a target that itself stays fixed. Real life adds inflation in spending, career breaks, tax on growth in some accounts, and the possibility that your desired retirement spending changes as life changes.

Use the result as a compass, not a contract. Revisit the target and the horizon each year, keep costs and debts under control after reaching the milestone, and remember that covering current expenses still requires income until full retirement funding is in place.

Getting More From Your Result

A useful habit with any estimate is to run the calculation twice, once with the figures you expect and once with figures that are a little less favourable. If both versions point to the same decision, you can act with more confidence. If they point in opposite directions, the honest answer is that the decision is sensitive, and gathering one better input is worth more than any further arithmetic on the old ones.

Keep a short written record of the inputs you used, not just the answer you received. Circumstances change, and a result without its inputs cannot be checked, updated, or explained to anyone else later. A dated note with the three figures and the result takes seconds to write and turns a one-off calculation into something you can build on, compare against, and trust when the same question returns in a few months.

Keep a short written record of the inputs you used, not just the answer you received. Circumstances change, and a result without its inputs cannot be checked, updated, or explained to anyone else later. A dated note with the three figures and the result takes seconds to write and turns a one-off calculation into something you can build on, compare against, and trust when the same question returns in a few months.

Notice also how the result behaves at the extremes. Trying a very small input and a very large one shows you the shape of the relationship, whether the answer grows steadily or races away, and that shape is often the real insight. People who understand the shape of a calculation make fewer surprised decisions than people who only ever see a single point estimate from a single set of figures.

Finally, remember that a tidy number can create false confidence. The output of this page is only as grounded as the measurements, estimates, and assumptions fed into it, and the surrounding sections exist to keep that grounding visible. Read the result together with its assumptions, and you will get the full value of the tool instead of just its fastest answer.

A useful habit with any estimate is to run the calculation twice, once with the figures you expect and once with figures that are a little less favourable. If both versions point to the same decision, you can act with more confidence. If they point in opposite directions, the honest answer is that the decision is sensitive, and gathering one better input is worth more than any further arithmetic on the old ones.

Keep a short written record of the inputs you used, not just the answer you received. Circumstances change, and a result without its inputs cannot be checked, updated, or explained to anyone else later. A dated note with the three figures and the result takes seconds to write and turns a one-off calculation into something you can build on, compare against, and trust when the same question returns in a few months.

Tips for Getting the Most From the Fire Coast Calculator

1. Set the target from spending multiplying the yearly spending you expect in retirement by about 25 as a starting estimate.
2. Use a cautious return testing 5 percent as well as 7 percent so the plan survives a less generous future.
3. Count only invested savings toward the Coast number, not home equity or cash set aside for emergencies.
4. Start the clock early because each year of horizon lowers the amount needed today more than most raises can.
5. Recheck the wait-five-years figure whenever you feel tempted to delay, and let that rising number argue for starting.
6. Keep contributing after coasting if you can since extra deposits buy an earlier or larger finish than the bare milestone.
7. Review the plan yearly updating the target, horizon, and return as your life and markets change.
8. Protect the downside with an emergency fund and insurance, so a bad year never forces you to sell investments.

Frequently Asked Questions

1. What exactly is the Coast FIRE number?

It is the amount you would need invested today for growth alone, with no further contributions, to reach your target by retirement.

In this calculator it equals your target fund divided by the growth factor for your chosen return and years.

2. Why does waiting five years raise the number so much?

Waiting removes five years of compound growth, so a larger starting sum must do the same job in less time.

In the first worked example the figure rises from about $258,419 to about $362,446, purely from the shorter horizon.

3. What return should I enter in the Fire Coast Calculator?

Many planners test a range around 5 to 7 percent for a diversified portfolio measured after inflation.

Choose the cautious end for decisions you cannot easily reverse, and treat higher figures as best-case illustrations rather than plans.

4. Does reaching Coast FIRE mean I can retire now?

No. It means your current investments could grow into the target by retirement age if left untouched.

You still need income to cover living costs between now and then, which is why Coast FIRE is best seen as freedom to change work, not to stop it.

5. What if my years until retirement are five or less?

With five or fewer years of growth left, waiting five years leaves no growth period, so the calculator shows the full target as the wait figure.

That is the mathematics making a fair point: very close to retirement, there is little time left for growth to do the heavy lifting.

6. What does the Fire Coast Calculator do?

The Fire Coast Calculator takes your target retirement fund, the years until retirement, and your expected annual return as a percentage and works out your Coast FIRE number needed today, the growth factor over the period, and the Coast number if you wait five more years for you in one click.

It is designed for quick planning and checking, so you can test different figures in seconds instead of redoing the same arithmetic by hand each time.

7. How accurate are the results?

The arithmetic is exact for the figures you enter, using the standard formula described on this page.

How well the result matches real life depends on how realistic your inputs are, so the sections above explain every assumption the calculation makes.

8. What information do I need before I start?

You need your target retirement fund, the years until retirement, and your expected annual return as a percentage.

Gathering those figures first makes the result meaningful, because changing one guessed input can move the answer more than any rounding ever will.

9. Can I try different values to compare options?

Yes, and that is one of the best uses of the tool. Change one input at a time and watch how the result moves.

Comparing two or three sensible scenarios side by side usually teaches you more than hunting for a single perfect answer.

10. Why did the calculator ask me to correct an entry?

An alert appears when a required field is empty, not a number, or outside the sensible range for this calculation.

Correct the highlighted kind of value and click Calculate again. The check exists to prevent a quiet mistake producing a confident-looking wrong answer.

11. What does the Reset button do?

Reset refreshes the page, clearing your entries and hiding the result box so you can start again cleanly.

If you want to keep a result, note it down before resetting, because the page does not store your figures.

12. Can I use this calculator on my phone?

Yes, the layout is a single simple card with large fields and buttons that work on a small screen.

The calculation itself happens on the page, so it also works the same way on a tablet or a desktop computer.

13. Should I rely on this result for an important decision?

Use it as a well-explained estimate and a checking tool, especially while you are learning how the figures relate.

For decisions with legal, tax, medical, or safety consequences, confirm the result with a qualified professional who can see your full situation.

14. Why are the outputs shown to several decimal places in some cases?

Extra decimals keep rounding honest when you want to check the working by hand or feed the result into a further step.

For everyday reporting you can round the final figure to a sensible level, as the tips on this page suggest.

15. Where can I learn more about the ideas behind the Fire Coast Calculator?

The concept and deep-dive sections on this page explain the formula, its assumptions, and its limits in plain language.

Working through the two examples with your own figures alongside is the fastest way to make those ideas stick.

CONCLUSION

The Fire Coast Calculator brings your target retirement fund, the years until retirement, and your expected annual return as a percentage together and returns your Coast FIRE number needed today, the growth factor over the period, and the Coast number if you wait five more years in a form you can check, compare, and act on. Its real value is not speed alone, but the clarity it gives you about which input is driving the answer.

Use it to test options before you commit, to check figures someone else gives you, and to build an honest record of how a decision was reached. With sensible inputs and the limits on this page kept in mind, the result is a dependable guide for everyday planning.