How to calculate Coast FIRE

1. Set the retirement target and time available

Let F be the retirement portfolio goal in today’s dollars. Enter it directly or estimate it from annual portfolio-funded spending S and an initial withdrawal-rate assumption w:

F = S / w

Let N be retirement age minus current age, measured in years.

Enter your own spending, withdrawal assumption, ages and return in the calculator. The formulas above use those values; no fixed dollar demo is shown here.

2. Put returns and the target on the same basis

Let n be the nominal annual return and i annual inflation, both as decimals. The exact inflation-adjusted rate r is:

r = (1 + n) / (1 + i) − 1

Subtracting inflation from nominal return is only an approximation. Illustrative conversion: 7% nominal return and 3% inflation give about 3.8835% real return, not exactly 4%.

If you enter a real return directly, do not deduct inflation a second time. In this model, “real” means inflation-adjusted; taxes are not automatically deducted. Use a return assumption after investment fees and make a separate allowance for relevant taxes.

3. Discount the target to today

Coast FIRE number today = F / (1 + r)^N

Apply the same real-return formula to your own goal and years remaining. The withdrawal-rate assumption and the investment-growth assumption are separate inputs.

Actual results can be lower than the model, including a loss of capital. Change only the real-return assumption in the calculator to see how today’s coast number moves for your goal and time horizon.

4. Find the coast year if you keep contributing

Let P₀ be the current invested balance, C an end-of-year contribution in today’s dollars, and t the years elapsed. Before coasting, the projected balance is:

P(t) = P₀(1 + r)^t + C × ((1 + r)^t − 1) / r

When r = 0, use P(t) = P₀ + C × t instead. At each year, compare the balance with the coast threshold for the remaining time:

Threshold at year t = F / (1 + r)^(N − t)

The first year with P(t) ≥ threshold is the modeled coast point. After that point, the chart stops contributions. The calculation assumes no withdrawals before retirement and constant real contributions while saving.

If you keep contributing, use the calculator’s year-by-year path with your own balance, contribution and return. The first whole year your projected balance meets the age-specific threshold is the modeled coast age.

At zero real growth, the coast threshold equals the retirement goal. With negative real growth, the earlier threshold can exceed the final goal. If the plan never meets the threshold by retirement, report that directly rather than inventing a later coast date.

Try your own assumptions in the Coast FIRE calculator. The SEC’s guide to performance claims explains why hypothetical projections and guarantees are different.