A Coast FIRE calculation works backwards from a future portfolio target. It estimates the balance that would grow to that target without further contributions if a stated annual return occurred for a stated number of years.
The result is conditional arithmetic. It does not establish that a person's retirement is funded or that they can stop saving. A calculator cannot turn an assumed future return into an agreed payment from an investment.
Define the future target first
FinTarget's calculator divides an annual spending input by a withdrawal-rate input to form a future portfolio target. In a fictional US dollar example, $40,000 divided by 0.04 produces $1,000,000.
The $40,000 and 4% are example inputs, not a recommended spending level or withdrawal rate. The calculation does not prove that a $1,000,000 portfolio will support the spending for a particular retirement length.
The target is also not the amount needed today. Coast FIRE discounts that target back through the assumed accumulation period. The word discount here describes dividing a future amount by an assumed growth factor.
Work backwards through 25 years
Suppose the fictional target is 25 years away and the calculation assumes a fixed 5% annual return. The present balance is $1,000,000 divided by 1.05 raised to the power of 25.
The result is approximately $295,303. In the mathematical model, that balance grows to $1,000,000 after 25 equal annual growth periods with no contributions, withdrawals, fees or taxes.
Keeping the target and time unchanged but using 3% produces approximately $477,606. Using 7% produces approximately $184,249. These are three calculations, not forecasts or probabilities.
The large difference comes from repeating the assumed return over many years. It does not establish that a higher return is available merely because a smaller present balance is preferred.
Change the date or the spending input
At the same 5% assumed return, allowing only 20 years raises the present balance for the $1,000,000 target to approximately $376,889. There are five fewer growth periods.
If the annual spending input becomes $48,000 and the example withdrawal rate remains 4%, the target becomes $1,200,000. With 25 years and a 5% assumed return, the present balance becomes approximately $354,363.
The example shows why a number copied from someone else's calculation has little meaning without the inputs. Spending, the chosen date and both percentage assumptions are part of the result.
A fixed return omits the path
Real investments change in value, and their annual results are uncertain. A fixed-return formula draws a smooth mathematical path. It does not describe the order of actual gains and losses or the effect of selling investments during a decline.
Inflation creates another measurement issue. A future dollar amount and an amount expressed in today's purchasing power are different targets. The example has not automatically increased spending for inflation. Labeling an input as a real or nominal amount does not add an inflation model to the calculator.
Fees, tax, account access restrictions, Social Security, healthcare costs and changing spending also affect an actual retirement picture. FinTarget's simple Coast FIRE tool does not model those US-specific circumstances or test the chance of funds lasting through retirement.
Record the assumptions with the result
The Coast FIRE calculator accepts a currency, ages, spending, savings and percentage assumptions. Selecting USD changes the currency used to display amounts. It does not apply US tax or retirement account rules.
A saved scenario therefore needs its input date, the spending basis, time horizon, assumed return and withdrawal input beside the calculated amount. Those details make the fictional $295,303 reproducible without presenting it as a personal saving instruction.