The Coast FIRE calculator takes your age, the age you want to retire, your yearly spending and a few rates, and tells you how much you need invested today. This page shows every step, so you can check the answer yourself.
Step 1: the target
target = yearly spending in retirement ÷ withdrawal rate (a monthly figure is multiplied by 12 first)
40,000 a year (about 3,333 a month) at a 4% withdrawal rate needs 1,000,000. The 4% comes from a 1994 study of historical US stock and bond returns (William Bengen, "Determining Withdrawal Rates Using Historical Data", Journal of Financial Planning), which found that taking 4% in the first year of retirement, then raising that amount with inflation, lasted at least 30 years in every period it tested.
Step 2: the return after inflation
real return = (1 + return) ÷ (1 + inflation) − 1
Everything is worked out in today's money, so the spending you enter keeps its meaning. A 10% return with 3% inflation is (1.10 ÷ 1.03) − 1 = 6.80% a year in today's money. Subtracting 3 from 10 would give 7% and slightly understate it.
Step 3: back to today
Coast FIRE number = target ÷ (1 + real return) ^ years to retirement
With 30 years to go, 1,000,000 ÷ (1 + 6.80%)^30 = 139,103. That much invested today reaches the target by 65 with nothing more added. Growth is compounded monthly at the same yearly rate, which gives the same result over whole years.
Step 4: how soon monthly saving gets you there
Someone in that position with 60,000 invested is 79,103 short today. Adding 700 a month, the calculator checks each month in turn for the first one from which the pot, left alone, still reaches the target by 65. Here that is 14 years and 10 months, at 49. The monthly amount is assumed to rise with prices, like everything else in today's money. When the amount entered can never get there in time, the calculator shows the monthly amount that reaches the full target by the retirement age instead.
The return averages
The buttons under the return field fill in these long-run averages, before inflation:
| Average | Yearly return | Period | Source |
|---|---|---|---|
| S&P 500, long-run average | 10.7% | January 1957 to August 2026, dividends reinvested | Robert Shiller's dataset, compiled by officialdata.org |
| Global stocks (MSCI World) | 9.1% | December 1987 to August 2026, gross return in US dollars | MSCI World Index factsheet |
| US bonds (Bloomberg US Aggregate) | 6.6% | 1976 to 2023, total return | Bogleheads wiki, citing the index publisher |
| High-yield savings, typical in 2026 | 4% | a typical top online savings rate in September 2026, not a long-run average | FDIC national rates and published bank rate tables |
Past averages are not a promise. Try a lower return as well and see how much the answer moves. Fees and taxes are not included: take fees off the return you enter.