What coast means here
Coast FIRE, in this educational model, is a checkpoint for invested assets. It is the balance that could grow to a chosen retirement portfolio target by a chosen age if the real return you type happened every year and you added no further retirement contributions. It is not a date to retire, and it is not permission to stop paying living expenses.
The form opens with an example: age 35, retirement age 65, $80,000 invested, $40,000 of annual retirement spending, $12,000 of annual contributions, a 5.00% real return, and a 4.00% withdrawal rate. Reset restores that example and clears the estimate. Those figures are placeholders so the fields are not empty. They are not a recommendation.
The arithmetic
Amounts stay in today's dollars. The return is a real return, meaning after inflation. There is no second inflation path and no currency conversion. USD is the only unit.
Retirement portfolio target = annual retirement spend ÷ withdrawal rate Coast number today = retirement portfolio target ÷ (1 + real return) ^ years until retirement Portfolio after n whole years = assets × (1 + r) ^ n + contribution × ((1 + r) ^ n − 1) ÷ r When r is 0, portfolio after n years = assets + n × contribution
Contributions are added at the end of each year. Year 0 is the balance you have now, with no new contribution included. The coast threshold in a later year is the retirement target discounted over the years still left:
Coast threshold in year n = retirement portfolio target ÷ (1 + r) ^ (years remaining)
The calculator looks for the earliest whole year from 0 through the age gap where the projected portfolio is at least that year's coast threshold. The comparison uses exact rational values. Dollars on screen are rounded half-up to the cent and are not used for that comparison. A displayed gap of $0.00 can still be a shortfall of less than one cent.
If that year is found, later years in the chart and table stop adding contributions. The portfolio then grows only at the assumed return. If the year is 0, you are already at the coast checkpoint under these assumptions, so the projection includes no new retirement contributions. If no year qualifies, the status is exactly: Not reached by your target retirement age under these assumptions.
With no new contributions, the portfolio and the coast threshold are scaled by the same return. Starting short of today's coast number therefore stays short. Waiting does not close that gap inside this model, including when the real return is zero or negative.
Bounds
The limits below are input limits for this page. They are not views about markets.
| Input | Accepted | Precision |
|---|---|---|
| Current age | 18 to 80 | Whole years |
| Retirement age | 18 to 100, and not earlier than current age | Whole years. Equal ages are valid. |
| Invested assets | 0 to 100,000,000.00 USD | At most two decimal places |
| Annual spend | 0 to 10,000,000.00 USD | At most two decimal places. Zero is valid. |
| Annual contributions | 0 to 2,000,000.00 USD | Required. Use 0 if none. At most two decimal places. |
| Real return | -99.99% to 12.00% | Up to two decimal places. Must be greater than -100%. |
| Withdrawal rate | 0.01% to 10.00% | Up to two decimal places. Zero is rejected. |
What is left out
Taxes, fees, and fund expenses are ignored unless the return you type is already net of them. The model has no Social Security, pension, annuity, or other income. It has no debts. It uses one return every year, so it does not show sequence-of-returns risk. It does not model lifespan or a change in spending. It does not name or recommend an investment product.
A negative real return is allowed inside the bounds. Powers use the integer number of years, so the sign of the return does not switch the formula to a guess. Equal current and retirement ages are allowed: only the current balance is tested, because a same-year contribution is not added to the starting point.