Coast FIRE is personal-finance shorthand, not an official term, for the point where an already-saved balance, left untouched and grown only by an assumed annual return, is modeled by the compound-interest formula to reach a stated future dollar target by a chosen date without further contributions; $150,000 growing at an assumed 6% for 25 years models to about $643,780.61.
What coast fire calculation means
A Coast FIRE calculation applies the same compound-interest formula used by Investor.gov's Compound Interest Calculator - future value equals present value times (1 plus the assumed rate) raised to the number of years - to a current balance alone, testing whether that balance, without new deposits, reaches a target under a chosen rate and horizon. This tests a lump sum alone; this site's own retirement-savings and 401(k) projections instead model a starting balance plus a stream of ongoing contributions, which is a different calculation once new deposits are added back in.
| Starting balance | $150,000 |
|---|---|
| Assumed annual return | 6% |
| Horizon | 25 years |
| Modeled balance | $643,780.61 |
| Illustrated target | $650,000 |
What is a Coast FIRE calculation, and how does it use compound growth?: worked numbers
Starting from $150,000 with no further contributions, an assumed 6% annual return, and a 25-year horizon: $150,000 x 1.06^25 equals about $643,780.61. If the modeled target for a later retirement date is $650,000, this scenario falls about $6,219 short at 6%; a slightly higher rate assumption or a longer horizon would change the projected balance, illustrating how sensitive the model is to the chosen rate and timeframe.
How to calculate coast fire calculation
Future value = present value x (1 + assumed annual rate) ^ number of years. Choose the present value from an actual current balance, choose the rate and horizon as explicit assumptions rather than promised figures, then compare the result with a separately stated dollar target for a chosen future date.
Use related Calculatort tools when the inputs are known: retirement savings projection; FIRE number calculator; fund-fee twenty-year impact.
Common mistakes
Do not present the assumed annual return as a historical average or a promised outcome - it is a chosen input, and a real portfolio's annual returns vary rather than compounding smoothly at one flat rate every year. Do not compare a nominal future-dollar target with today's dollars without separately adjusting for inflation.
Where the calculation stops
This model excludes market volatility and sequence-of-returns risk, investment fees and expense ratios, taxes on gains or withdrawals, and the possibility that the saver adds or removes money before the target date. It is a sensitivity scenario for one assumed rate and horizon, not a projection of any specific investment's future performance.
coast fire calculation: source and verification
Investor.gov, the SEC's investor-education site, hosts the Compound Interest Calculator that computes growth from a starting amount, an assumed rate, and a chosen time horizon, the same formula this scenario applies to a single lump sum. Read the named source. This source names the transaction-specific starting balance and the conditions that qualify it.
Use the result as a dated scenario
Recalculate the coast fire calculation case when its listed input changes.
A second scenario: a higher starting balance over a shorter horizon
Suppose a saver has $300,000 already invested, assumes a 7% annual return, and wants the modeled balance in 15 years with no further contributions. Applying the same formula, future value equals present value times (1 plus rate) to the number of years: $300,000 times 1.07^15. Since 1.07^15 is about 2.7590, the modeled balance is about $300,000 times 2.7590, or $827,700.60 - reached with a shorter horizon and a higher assumed rate than the $150,000-over-25-years example above, showing how sensitive the same formula is to both inputs at once.
What changes when the rate assumption moves by one point
| Assumed annual return | $150,000 after 25 years, no new contributions |
|---|---|
| 4% | about $399,875.40 |
| 5% | about $507,953.25 |
| 6% | about $643,780.61 |
| 7% | about $814,114.95 |
| 8% | about $1,027,271.25 |
The same $150,000 starting balance and 25-year horizon produce a range spanning more than $627,000 depending only on which annual return is assumed - the single most sensitive input in the whole calculation, and the one the SEC's Compound Interest Calculator lets a user change freely because it is a chosen assumption, not a published rate.
Solving the formula in reverse: what rate reaches a target
The same equation can be solved for the rate instead of the balance. If a saver wants $150,000 to reach exactly $650,000 in 25 years, divide the target by the starting balance - $650,000 divided by $150,000 is about 4.3333 - then take the 25th root: 4.3333^(1/25) minus 1 is about 6.03%. That reverse-solved rate can be checked by plugging it back into the forward formula: $150,000 times 1.0603^25 should land close to $650,000, the same verification method used to confirm any compound-interest scenario runs both directions consistently.
The dispute this guide resolves: adding a contribution stops being Coast FIRE
A frequent mix-up is running a Coast FIRE case that still includes a monthly contribution, the way the SEC's Compound Interest Calculator allows once a contribution field is filled in. Once a contribution is added, the calculation is no longer testing whether the current balance alone reaches the target; it becomes a combined lump-sum-plus-contributions projection, a different scenario using the same lump-sum-plus-annuity math this site's own official-calculators guide works through separately. The zero-contribution condition is not a detail of Coast FIRE - it is the definition being tested.
The dispute this guide resolves: nominal dollars are not today's purchasing power
A modeled balance such as $643,780.61 is stated in nominal future dollars, not adjusted for inflation, and the compound-interest formula used to produce it makes no claim about purchasing power. Comparing that nominal figure against a spending target expressed in today's dollars - without separately discounting the target or the result for assumed inflation over the same 25 years - compares two numbers on different bases, a distinct error from choosing the wrong rate or horizon in the first place.
Using a real (inflation-adjusted) rate instead of a nominal one
The same compound-interest formula can be run with an inflation-adjusted real rate instead of a nominal one, which changes the modeled balance's meaning rather than its arithmetic. If a saver assumes a 6% nominal return and a 3% long-run inflation assumption, the Fisher relation gives a real rate of (1.06 divided by 1.03) minus 1, about 2.91%. Running $150,000 at a 2.91% real rate for 25 years gives $150,000 times 1.0291^25, or roughly $307,000 in today's purchasing power - a materially smaller number than the $643,780.61 nominal figure, because the real-rate version already subtracts the inflation the nominal version leaves for the reader to subtract separately.
What changes if the target date moves five years earlier
Suppose the same $150,000 balance and 6% assumed return need to reach $650,000 not in 25 years but in 20. Applying future value equals present value times (1 plus rate) to the number of years: $150,000 times 1.06^20. Since 1.06^20 is about 3.2071, the modeled balance is about $150,000 times 3.2071, or $481,065 - short of the $650,000 target by roughly $168,935, showing that shortening the horizon by five years at the same assumed rate has a much larger effect than the one-point rate changes shown in the sensitivity table above.