Coast FIRE Number Calculator and Formula

Your Coast FIRE number is the amount already invested that could grow to your retirement target without any further contributions. Enter your target retirement balance, years until retirement, and assumed annual return to estimate the amount you would need invested today; the result is a projection, not a guarantee.

Formula

The calculator uses the future-value relationship rearranged to find the present amount:

Coast FIRE number = Retirement target ÷ (1 + annual return)^years

• Retirement target is the estimated investment balance needed at retirement, in dollars.
• Annual return is the assumed yearly investment growth rate, entered as a percentage. The formula converts it to a decimal by dividing by 100.
• Years is the number of years before the planned retirement date.
• Coast FIRE number is the estimated balance required now, in dollars, if it compounds at the assumed rate and receives no further contributions.

This version assumes one constant annual return and annual compounding. It does not account for contributions, withdrawals, taxes, fees, inflation, or changing investment performance.

Worked Example

Using the calculator’s default values: a $1,000,000 retirement target, 20 years, and a 7% annual return:

1. Convert the percentage to a decimal: 7 ÷ 100 = 0.07.
2. Calculate the growth factor: (1 + 0.07)²⁰ = 1.07²⁰ ≈ 3.86968.
3. Divide the target by that factor: $1,000,000 ÷ 3.86968 ≈ $258,419.92.

The estimated Coast FIRE number is $258,419.92, rounded to two decimal places. In this simplified projection, that balance would grow to approximately $1,000,000 over 20 years if it earned 7% each year and no money were added or taken out. The growth factor shown by the calculator is 3.87x after rounding.

What Coast FIRE Means

Coast FIRE is a personal-finance concept describing a point at which an invested retirement balance is projected to grow to a chosen retirement goal without additional retirement contributions. The word “coast” refers to relying on compounding for future growth; it does not mean that someone has stopped working, stopped saving for other goals, or can necessarily retire immediately.

For example, someone who reaches a Coast FIRE estimate might choose a lower-paying job, reduce retirement contributions, or focus on current expenses. Whether that choice is practical depends on income, debt, emergency savings, household needs, and the assumptions behind the calculation. The number is a planning benchmark rather than a personal recommendation.

How to Choose Your Inputs

• Set a retirement target. Estimate the amount you want invested at the planned retirement date. A target should reflect the lifestyle and retirement income you are planning for, not merely an arbitrary round number.
• Choose the time horizon. Count the years from now until the intended retirement date. A longer horizon lowers the present amount required in this formula, all else equal.
• Use a considered return assumption. Returns differ across investments and time periods, and actual results vary. Rather than treating one optimistic estimate as certain, compare several plausible assumptions. Keep the rate consistent with whether your target is stated in today’s dollars or future dollars.
• Keep units consistent. This calculator uses dollars for the balance and whole years for the time period. It does not convert currencies or accept a monthly return.

If you want a target expressed in today’s purchasing power, inflation matters. One approach is to estimate a target in future dollars that includes expected inflation and use a nominal return assumption. Another is to use a target and return both expressed in inflation-adjusted, or “real,” terms. Mixing a real target with a nominal return can make the estimate misleading.

How to Read the Result and Its Limits

The result answers a narrow question: how much would need to be invested now for a specified target to be reached under the chosen constant-return assumption? It is not the amount needed to cover every financial goal, and it does not calculate an appropriate retirement target for you.

Investment returns are not smooth or guaranteed. A single average rate hides year-to-year rises and falls, while fees and taxes can reduce what remains invested. Inflation also changes what a fixed dollar amount can buy. A real-world projection may therefore end above or below the calculator’s result.

Use the output as a scenario, not a promise. Try different rates and timelines, and consider whether you would still be comfortable if growth were lower than assumed. If you continue making contributions, those additions can reduce how much must be invested today, but this calculator deliberately assumes no future contributions.

Common Calculation Mistakes

• Entering 7 instead of 7% conceptually. The input is a percentage, and the formula divides it by 100. Do not enter 0.07 when you mean 7%; that would be interpreted as 0.07%.
• Confusing the Coast FIRE number with the retirement target. The calculator’s first result is the estimated balance needed now, not the projected balance at retirement.
• Assuming the return is guaranteed. The rate is an assumption for a compound-growth estimate, not a forecast of a specific account’s result.
• Rounding too early. Keep full precision through the calculation and round only the displayed answer. Small rounding differences can appear if intermediate values are rounded.
• Ignoring inflation or fees. Confirm that the target and return assumptions are expressed on a consistent basis, and remember that costs can reduce net growth.

Frequently Asked Questions

What is a Coast FIRE number?
It is an estimate of the investment balance needed today to grow to a chosen retirement target without additional contributions, given a selected return and timeline.

How do I calculate my Coast FIRE number?
Divide your retirement target by (1 + annual return)^years. Convert a percentage return to a decimal first; for example, 7% becomes 0.07.

Does reaching Coast FIRE mean I can retire?
No. The calculation assumes the invested balance can grow until the target retirement date. It does not establish that you can stop working now or pay current expenses.

What rate of return should I use?
There is no universally correct rate. Choose an assumption suited to your planning horizon and investment mix, and compare outcomes under lower and higher assumptions rather than relying on a single figure.

Does the calculation include inflation?
Not automatically. You must use a target and return that are consistent: either account for inflation in the future-dollar target and nominal return, or use inflation-adjusted values for both.

Can I still contribute after reaching my Coast FIRE number?
Yes. The calculation simply models no additional contributions. Continuing to invest could increase the eventual balance or provide more room if returns are lower than assumed.

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in target | Retirement target | $ | 1000000 | 0
in years | Years until retirement | years | 20 | 0
in rate | Assumed annual return | % | 7 | 0
out coast | Coast FIRE number | $ | target / (1 + rate / 100) ^ years | 2
out growth | Growth factor | x | (1 + rate / 100) ^ years | 2