Can you walk me through a Coast FIRE calculation step by step?

Coast FIRE is the point where your current savings, left alone to grow, will reach your retirement number by your target age without you adding another dollar. The calculation itself is four numbers and one formula.

What four numbers do I need?
Your current invested assets, the real annual return you assume, the age you plan to retire, and the portfolio you want at that age. Everything else is derived from these.

What is the formula?
Project your assets forward with compound growth, then compare against your target. Future value = current assets × (1 + annual return) ^ years. If that future value is at or above your target, you have reached Coast FIRE.

How do I pick a return assumption?
Use a real return (after inflation) between 5% and 7% for a diversified stock portfolio. The lower you set it, the more conservative the answer — and the safer it is to believe. Do not use the 10% nominal number and then forget that inflation eats part of it.

How do I estimate my retirement target?
Multiply your expected yearly spending in retirement by 25 (the classic 4% rule). If you spend $40,000 a year, target = $1,000,000. If your spending is lumpy, model the years with big expenses separately instead of one flat number.

Walk me through a real example.
Say you are 35 with $80,000 invested, assume 6% real return, and want $1,200,000 at 60. Years to grow: 25. Future value = 80,000 × 1.06^25 ≈ $343,000. That is well short of $1,200,000, so you are not coasting yet — you still need to save for a while. Now raise the starting pot to $280,000: 280,000 × 1.06^25 ≈ $1,202,000, and you are there.

What about taxes and accounts?
The formula assumes all money is investable and ignores taxes on withdrawal. If part of your pot sits in a taxable account, haircut the effective return by your expected tax drag — usually 0.5–1% — before running the math.

What does the calculator actually change as I tweak inputs?
Each input moves the line: a higher return assumption lowers the required starting pot; an earlier retirement age raises it; and the spending number dominates everything because it sets the target. Play with those three and you will see which one you actually control.

If you want to run these numbers without a spreadsheet, Coast FIRE Calculator (https://thecoastfirecalculator.com) does the compounding math for you and shows the gap between where you are and where you need to be.