Cost Fire Calculator

The Cost Fire Calculator answers one of the most freeing questions in personal finance: if you stopped adding money today, what would your current savings grow into by retirement? Enter what you have saved, the annual return you expect, and the years left to grow, and the tool projects the result.

This idea sits at the heart of Coast FIRE. Once your investments are large enough that compound growth alone will carry them to your retirement target, you have reached your coast point. From there, extra saving becomes optional: you only need to earn enough to cover today's spending.

Reaching that point changes how work feels. People use the freedom to move to part-time hours, choose lower-paid but more enjoyable work, take career breaks, or simply stop worrying about aggressive saving targets. The calculator shows how far along that road your current pot already is.

This page explains the compound growth maths in plain language, works through two full examples, and covers the assumptions that matter, especially the return rate and inflation. The figures are projections, not guarantees, and real markets never grow in a straight line.

How Compound Growth Does the Heavy Lifting

Compound growth means your returns themselves start earning returns. In the early years growth looks painfully slow, because the pot is small. Later, the same percentage growth on a much larger pot produces far bigger yearly gains, which is why time matters more than almost anything else.

The maths behind the calculator is the standard future value formula: your current amount multiplied by one plus the return rate, raised to the power of the number of years. That single calculation is why starting ten years earlier can be worth more than doubling your monthly saving later.

The second result, total growth, is simply the projected value minus what you started with. For long timeframes it often surprises people: over 25 years at a healthy return, the growth earned can be several times larger than the original pot itself.

What the Coast Point Really Means

Your coast point is the pot size at which growth alone, with no further contributions, reaches your retirement target by your retirement age. This calculator shows what any given pot becomes, so you can compare the projected value with the retirement number you have in mind.

If the projected value is above your target, you have already passed your coast point for that target and timeframe. If it falls short, the gap tells you roughly how much more needs to be added, or how much longer the money needs, before coasting becomes possible.

The coast point is not retirement itself. You still need income to live on between now and retirement age. What changes is that this income no longer has to stretch to cover large retirement contributions as well as everyday spending.

Why the Return Assumption Matters So Much

Small differences in the annual return create enormous differences over decades. At 5 percent, money roughly doubles every 14 years. At 8 percent, it doubles in about 9 years. Over a 25 or 30 year horizon, that gap compounds into hundreds of thousands of pounds or dollars.

Long-run stock market returns have often been quoted around 7 to 10 percent a year before inflation, but past performance never guarantees future results, and fees, taxes, and cash holdings all drag the real figure down. A cautious planner tests several rates rather than trusting one.

Try the calculator at 5, 7, and 9 percent and look at the spread of outcomes. Planning around the lower end of the range, and treating anything above it as a bonus, is the approach most likely to survive contact with real markets.

How to Use the Calculator

You need three numbers: what you have invested today, a realistic annual return, and the years until you plan to retire. Then follow these steps.

1. Enter your current savings or portfolio value in the first box, using only money invested for retirement.
2. Enter the expected annual return as a percentage in the second box, for example 7 for 7 percent.
3. Enter the number of years the money will grow, usually your retirement age minus your current age, in the third box.
4. Press Calculate to see the projected value, the total growth, and the average growth figure.
5. Compare the projected value with your retirement target, then repeat with a lower return to see a cautious version of the picture.

Worked Example 1: Thirty Thousand Over Eighteen Years

A saver with $30,000 wants to see what it becomes in 18 years at a 7 percent annual return, with no further additions.

Step 1: Starting point. The current pot is $30,000, growing at 7 percent a year for 18 years.

Step 2: Growth factor. 1 plus 7 percent, raised to the power of 18, gives about 3.38.

Step 3: Projected value. $30,000 multiplied by 3.38 equals about $101,398.

Step 4: Total growth. $101,398 minus $30,000 equals about $71,398 earned purely from growth.

Step 5: Average growth. $71,398 spread over the period gives roughly $3,967 per year.

The pot crosses six figures purely through growth, with $71,398 of the final value earned rather than contributed. That is the coast idea in its simplest form.

Worked Example 2: Twelve Thousand Starting Much Earlier

A younger saver has only $12,000 but gives it 30 years at 7 percent, testing whether time can beat a bigger starting pot.

Step 1: Starting point. The current pot is $12,000, growing at 7 percent a year for 30 years.

Step 2: Growth factor. 1 plus 7 percent, raised to the power of 30, gives about 7.61.

Step 3: Projected value. $12,000 multiplied by 7.61 equals about $91,337.

Step 4: Total growth. $91,337 minus $12,000 equals about $79,337 earned purely from growth.

Step 5: Average growth. $79,337 spread over the period gives roughly $2,645 per year.

Starting with far less but adding twelve extra years lands close to the first example. In coast planning, an early start is one of the strongest advantages there is.

Inflation: The Quiet Adjustment

The projected values in this calculator are nominal: they show the number on the screen in future money, not what that money will buy. Inflation erodes buying power every year, so a pot that looks huge in 25 years will feel smaller when it arrives.

A simple way to adjust is to run the calculation with a real return: your expected return minus inflation. If you expect 8 percent growth and 3 percent inflation, test the calculation at 5 percent. The result then reads roughly in today's buying power, which is far easier to compare with your current spending.

Your retirement target needs the same treatment. A target based on today's spending should be compared against an inflation-adjusted projection, or the target itself should be grown by inflation. Mixing a today's-money target with a future-money projection is the most common Coast FIRE mistake.

What to Do Before and After Your Coast Point

Before the coast point, the priority is building the pot as early as possible, because early money gets the most years of growth. Capturing employer pension contributions, keeping fees low, and avoiding long breaks in investing all matter more than chasing the highest possible return.

After the coast point, the decision becomes about lifestyle rather than survival maths. Some people keep saving and retire earlier or richer. Others deliberately trade income for time: fewer days, different work, or a business of their own, knowing retirement is already funded in the background.

Revisit the calculation every year or two. Markets move, spending changes, and retirement ages shift. A plan that was comfortably coasting can drift off course, and equally, a good few years in the market can bring your coast point forward sooner than expected.

Risks and Honest Limitations

Real returns do not arrive evenly. Markets fall as well as rise, and a crash just before retirement hurts more than the same crash twenty years earlier. The smooth projection here cannot show that sequence risk, so treat the output as a planning guide, not a promise.

The calculation also assumes you add nothing and withdraw nothing for the whole period, and that the return stays constant after any fees and taxes. Real life includes contributions, withdrawals, tax wrappers, and charges, all of which move the real outcome away from the simple projection.

None of this makes the tool useless. It makes it a starting point. Use it to understand the power of your timeframe, test cautious assumptions, and frame better questions, ideally with a regulated financial adviser before making major life decisions based on the numbers.

Reading Your Three Results as a Plan, Not a Prediction

The projected value is best read as a direction of travel. It tells you what today's decisions are worth in the future under one set of assumptions, and its real value is in comparisons: this pot versus that pot, this timeframe versus a longer one, this return versus a cautious one.

The total growth figure often changes behaviour more than the headline value. Seeing that the majority of a future pot comes from growth rather than contributions reframes saving: the job of new contributions is mainly to give growth more material and more time to work with.

The average growth figure helps with intuition about pace, but remember the real path is back-loaded. Early years contribute small amounts of growth and later years contribute large ones, so a plan can look unimpressive at year five and powerful at year twenty without anything changing.

Behaviour Traps on the Road to Coasting

The first trap is return chasing: raising the assumed return in the calculator until the answer looks good, then building a life decision on it. The assumption is not a lever you control. Savings rate, timeframe, and fees are the levers that actually respond to your choices.

The second trap is lifestyle creep after reaching the coast point. Coasting works because spending stays covered by income while investments stay untouched. Quietly withdrawing from the pot, or letting spending rise beyond income, breaks the maths that made coasting possible.

The third trap is never rechecking. A coast plan made at 35 rests on guesses about returns, spending, health, and retirement age that will all move. A short annual review, updating the three inputs with real figures, keeps the plan connected to reality.

Tips for Better Results

1. Test a cautious return too. Run the numbers at a lower rate, such as 5 percent, so your plan does not depend on everything going well.
2. Use invested money only. Include pensions and investment accounts meant for retirement, not your everyday bank balance or emergency fund.
3. Adjust for inflation. Subtract expected inflation from your return to see results in today's buying power, which is easier to judge.
4. Compare against a clear target. The projection means most when you hold it against a specific retirement pot size based on your expected spending.
5. Recheck every year or two. Market moves and life changes shift your coast point, so refresh the calculation rather than trusting an old result.
6. Count fees in your return. Platform and fund charges reduce real growth, so a slightly lower return assumption is usually more honest.
7. Remember coasting is not retiring. You still need income for daily life until retirement age; coasting only removes the need to keep adding savings.
8. Get advice for big decisions. Changing jobs or hours based on a projection is a major step, so a conversation with a regulated adviser is worthwhile.

Frequently Asked Questions

1. What is Coast FIRE?

Coast FIRE is the point where your invested savings are large enough that growth alone should carry them to your retirement target.

After that point you only need to earn enough to cover current spending, not to keep adding large savings.

2. What does this calculator show?

It projects what your current savings become after a set number of years at an assumed annual return, with no further contributions.

It also shows the total growth earned and an average growth figure for the period.

3. What annual return should I enter?

Many planners test a range, such as 5 to 8 percent, rather than relying on one figure.

Lower assumptions give a more cautious picture, and past market performance never guarantees future returns.

4. Does the calculator include inflation?

No. The results are in future money, not adjusted buying power.

To approximate today's value, enter your expected return minus inflation, for example 8 minus 3 equals 5 percent.

5. How do I know my retirement target?

A common starting point is 25 times your expected annual retirement spending.

Compare the projected value here against that target to see whether you are past your coast point.

6. What if the projected value is below my target?

You have not reached your coast point yet for that target and timeframe.

Adding more savings now, extending the timeframe, or revisiting the target are the three levers that close the gap.

7. What if it is above my target?

On these assumptions, your current pot should reach the target without further contributions.

Many people still keep saving to retire earlier, build a safety margin, or increase their future spending options.

8. Why does the average growth figure change between years?

Growth compounds, so later years add far more money than early years at the same percentage rate.

The average simply spreads the total growth evenly to give a feel for the pace.

9. Should I include my home in current savings?

Most people exclude their home, because it does not produce retirement income unless you plan to sell or downsize.

Include pensions and investment accounts that are genuinely set aside for retirement.

10. Does coasting mean I can stop working?

No. Coasting means you can stop adding to retirement savings, but you still need income for everyday life until retirement.

Many people use it to reduce hours or change to work they enjoy more.

11. How often should I recalculate?

Once a year or two is sensible, and after big changes such as a market fall, a new job, or a changed retirement age.

Regular small reviews keep the plan honest without constant worry.

12. Are the projections guaranteed?

No projection is guaranteed. Markets vary, and the calculation assumes a smooth, constant return.

Treat the figures as a planning guide and test cautious assumptions before making decisions.

13. What is the biggest mistake people make with Coast FIRE?

Comparing a target based on today's spending with a projection in future, inflation-inflated money.

Adjusting one side or the other for inflation keeps the comparison fair.

14. Can I coast earlier by taking more risk?

Higher expected returns come with higher risk and no certainty, and losses near retirement are especially damaging.

Extending the timeframe or increasing savings are more controllable ways to reach the coast point sooner.

15. Is Coast FIRE right for everyone?

It suits people who value flexibility at work before full retirement and can build a pot early.

Anyone with unstable income or no emergency fund should secure those foundations before relying on a coasting plan.

CONCLUSION

The Cost Fire Calculator shows the quiet power of money already invested: given enough years, growth does the work that saving used to do. Knowing your projected value, your total growth, and how they compare with your target turns Coast FIRE from an idea into a measurable milestone.

Use cautious assumptions, adjust for inflation, and review the numbers as life changes. Reaching your coast point does not end financial planning, but it does change the question from how much more you must save to how you want to spend your time.