Coast FIRE Calculator

The formula

years of compounding = target age − current age
FI number   = annual expenses ÷ (withdrawal rate ÷ 100)
Coast number = FI number ÷ (1 + real return ÷ 100) ^ years of compounding

The coast number is your FI number discounted back to today. If your invested balance is at or above it, compounding alone can carry you to your target, and further contributions become optional rather than necessary.

Years until you reach the coast point

This is the part most Coast FIRE calculators get wrong, so it is worth being precise about. The coast number is not a fixed target. It rises every year. It is your FI number discounted over the years of compounding you have left, and each birthday takes one of those years away. On this page's default figures it climbs from about $331,000 at age 34 to about $1,429,000 at age 64. You are chasing a number that is walking away from you, just more slowly than you are moving.

So when you enter a current annual contribution, we step forward one year at a time and recompute the coast number for each year as we go. The balance earns the real return, then the contribution is added at year end. The count stops the first year your balance overtakes that year's coast number.

years left  = target age − current age
balance     = current net worth
year        = 0
coast(year) = FI number ÷ (1 + real return ÷ 100) ^ (years left − year)

while balance < coast(year) and year < years left:
    balance = balance × (1 + real return ÷ 100) + annual contribution
    year = year + 1

Holding the coast number fixed at today's figure instead, which is the simpler thing to do, makes the answer optimistic rather than cautious: it reports you coasting years before you actually are, and in some cases reports a year for something that never happens before your target age at all.

If your balance never overtakes the rising coast number before your target age, we say so rather than printing a number. A coast number is defined by that age, so clearing it afterwards is not coasting, it is just saving.

What Coast FIRE is and is not

• It is not stopping work. Coasting means your invested balance no longer needs new money. You still need income to cover your living costs between now and your target age.
• Reaching it is not irreversible. A long flat stretch in markets can put you back below the line. The number is a checkpoint, not a finish line.
• The return assumption does the heavy lifting. Over 30 years, moving from a 4% to a 6% real return cuts the coast number by more than 40%, and moving the other way raises it by about three quarters. Nobody knows which figure is right. Move the slider and watch.

Shared assumptions

• Returns are real, not nominal. Every return figure on this page is already inflation-adjusted, so all dollar amounts are in today's money. There is no separate inflation input to set.
• Returns are smooth. The same return is applied every year. Real markets are not smooth, and the order in which good and bad years arrive changes outcomes materially.
• No taxes, fees, or lumpy spending. Investment fees, taxes on withdrawals, one-off costs, and changes in your spending over time are not modelled.
• Nothing leaves your browser. The whole calculation runs client-side. We do not receive, log, or store anything you type.