How the Coast FIRE calculator works
FIRE stands for financial independence, retire early. Your FIRE number is the portfolio that could pay your yearly spending at a chosen withdrawal rate. Coast FIRE is the earlier milestone where the money already invested is large enough that compounding alone could carry it to that FIRE number by your target retirement age. After that point you still need income to pay today’s bills, but in this simple model you no longer have to add to retirement savings.
The calculator runs three steps. First it works out the spending your portfolio has to cover: annual retirement spending minus any other income you expect, such as a pension or Social Security. Second it divides that by your withdrawal rate to get the FIRE number — at 4% that is 25 times spending. Third it discounts the FIRE number back to today using your real return for the years remaining. That discounted figure is your Coast FIRE number.
Why we use today’s dollars and a real return
Retirement spending decades away is easier to think about in today’s money. If you enter spending in today’s dollars, the return must be a real return — the growth left after inflation. Mixing a nominal return such as 8% with today’s-dollar spending overstates how far your money goes. As a rough illustration only, a 7% nominal return with 2.5% inflation is about a 4.4% real return (1.07 ÷ 1.025 − 1). Our guide to real versus nominal returns explains the conversion.
Reading your result
If your invested balance is at or above the Coast FIRE number, the tool says you appear to have reached Coast FIRE and shows how large the balance would grow by retirement with no new money. If not, it shows the gap and, using the monthly contribution you enter, the first month at which your balance would meet the coast target for the time then remaining. That month is earlier than the date you would reach the full FIRE number, which is the whole appeal of the idea.
The return assumption dominates the answer. With a $1,000,000 FIRE number and 30 years to go, the Coast FIRE number is about $411,987 at a 3% real return, $231,377 at 5% and $131,367 at 7%. Try a lower return than you expect to see how sensitive your plan is; there is no single correct figure, and the order in which returns arrive also matters once you begin withdrawals.
What the model leaves out
- Markets do not deliver a constant return. A deep fall shortly before or after you stop contributing can push your timeline back.
- Taxes on withdrawals, investment fees and future health-insurance costs are only included if you build them into spending or lower the return.
- Rules on reaching retirement accounts before age 59½ are covered in our early-access guide, with links to the IRS.
- Social Security estimates should come from your own record at SSA; the “other income” box lets you include them in today’s dollars.
Use the result as a planning sketch. Revisit it every year with your real balance, and consider a fee-only fiduciary adviser for a full plan.