Penny Runs the NumbersA 30-year-old with $50,000 invested who saves $1,000 a month can stop saving at 39 and still reach a $1,000,000 FIRE number ($40,000 a year at a 4% withdrawal rate) by 65, assuming a 6% real return. At a 5% real return the coast age is 46; at 4% it is 58.
What the calculator shows
Your FIRE number is the portfolio that pays your yearly spending at your withdrawal rate: $40,000 a year at 4% means $1,000,000, or 25 times spending. Your coast number at any age is the amount that, with no more deposits, grows into the FIRE number by your retirement age.
The chart draws both: the red line is the coast number for each age, the blue line is your balance if you keep saving until you reach it and then stop. Where the lines cross is your coast age. The dashed line shows what happens if you never stop saving.
Why the return assumption matters so much
Everything is in today's dollars, so the return is a real, after-inflation return. US stocks returned about 6.8% a year after inflation from 1928 to 2025 (Damodaran's S&P 500 series deflated by the BLS CPI-U); 10-year Treasury bonds returned about 1.5%. The default 6% sits below the all-stock history.
Small changes compound over decades. With the default inputs, a 6% return gives a coast age of 39, 5% gives 46 and 4% gives 58: one point of return is worth about seven years of saving.
Retiring earlier, saving more or less
An earlier retirement leaves less time for growth. Retiring at 60 instead of 65 moves the coast age from 39 to 46, and at 55 the lines never cross: you would have to keep saving the whole way.
How much you save matters, but not linearly. At $500 a month the coast age is 56; at $2,000 a month it is 34. Halving your saving costs 17 years, doubling it buys only 5.
Questions
What is Coast FIRE?
Coast FIRE is the point where your invested savings, left alone, are enough to grow into your full retirement number by your planned retirement age. After that you only need to earn enough to cover current expenses; further retirement saving becomes optional.
How do you calculate your Coast FIRE number?
Divide your FIRE number by (1 + real return) raised to the number of years until retirement. With a $1,000,000 FIRE number, a 6% real return and 35 years to go, the coast number is $1,000,000 / 1.06^35 = $130,105.
What return should I use for Coast FIRE?
Use a real (after-inflation) return. US stocks returned about 6.8% a year after inflation from 1928 to 2025; bonds much less. The calculator defaults to 6% as an assumption, and the slider lets you test 4% or 5%, which push the coast age much later.
What does this calculator leave out?
It assumes a constant return every year (no market crashes before retirement), no taxes or fees, contributions that rise with inflation, and spending paid entirely by the portfolio. Social Security or a pension would lower the spending you need to cover.
Sources and assumptions
- ASSUMPTION: long-run real return of an invested portfolio, mostly stocks: assumption, anchored on historical data (Damodaran; BLS CPI-U) (checked 2026-11-11)
- CPI-U, U.S. city average, all items, not seasonally adjusted, December 2025 (December 1927: 17.3); used to turn nominal returns into real returns: U.S. Bureau of Labor Statistics, CPI-U series CUUR0000SA0 (checked 2026-11-11)
- ASSUMPTION: the '4% rule' withdrawal rate. FIRE number = yearly spending / 4% = 25x spending: assumption based on Bengen (1994), 'Determining Withdrawal Rates Using Historical Data', Journal of Financial Planning, Oct 1994 (FPA reprint) (checked 2026-11-11)
- ASSUMPTION: yearly spending paid FROM YOUR PORTFOLIO in retirement (not total spending): assumption; context: BLS Consumer Expenditure Survey, households age 65+, 2025 average annual expenditures $64,445 (series CXUTOTALEXPLB0407M) (checked 2026-11-11)
- All amounts are in today's dollars; the return is a real (after-inflation) return, so contributions are assumed to rise with inflation.
- Constant return every year: no sequence-of-returns risk before retirement. Real markets swing; treat the coast age as an estimate, not a date.
- Monthly contributions at month end; the annual return is converted to an equivalent monthly rate.
- Coast age = first birthday at which your balance is at least the amount that grows, untouched, to the FIRE number by your retirement age.
- No taxes, fees, Social Security or pensions are modelled; spending means spending paid by the portfolio.
Last updated 2026-11-11.