Coast FI Calculator
Find the age you could stop contributing to retirement accounts and still reach your number by retirement, growth alone. Enter your savings, your monthly contribution, and a target portfolio to see it.
Assumed average annual return
A single flat rate for every year, not a real portfolio's glide path. It also assumes returns arrive as a smooth average every year, which real markets don't do. Treat the age below as a rough estimate, not a guarantee.
Current age
45Retirement age
65Current retirement savings
$250,000Monthly contribution, all accounts
$1,500401(k), IRA, HSA, taxable brokerage, everything you're putting toward retirement each month, including any employer match.
Your retirement number (target portfolio)
$1,200,000Not sure what yours is? Use the Retirement Number Calculator first, then bring that number here.
This assumes one flat return every year and doesn't account for taxes, inflation, Social Security, or a market downturn hitting right after you stop contributing. ThunderHarbor's full Coast FI Age uses your real accounts and tax treatment, then stress-tests that age against 1,000 simulated market sequences in Risk Analysis, not just the average case.
What is a coast FI age?
Coast FI, short for coast financial independence, is the point where your current retirement savings, left alone to grow, would reach your target portfolio by the time you plan to retire, with no further contributions needed. Once you hit it, every dollar you would have contributed is free to spend, save for something else, or redirect toward paying down debt instead.
It is not the same as being fully financially independent today. You still need your investments to keep growing for however many years remain until retirement, you are only relieved of the need to keep adding new money to them.
How the calculation works
Starting from your current age, this tool checks each age between now and retirement: if you kept contributing normally up to that age, then stopped entirely, would the resulting balance, grown at your assumed return with no further deposits, reach your target portfolio by retirement? The youngest age where that holds is your coast FI age.
If even contributing all the way to retirement does not reach your target, there is no coast age to find. That is a signal to revisit your contribution rate, your target, or your retirement age, not a bug in the math.
An estimate, not a promise
Every coast FI age, this one included, is built on an assumed average return applied evenly across every year between now and retirement. Real markets do not work that way. A downturn that hits after you stop contributing, particularly in the years closest to retirement, has no new deposits coming in to cushion it.
A coast FI age is a genuinely useful planning estimate. It is not a settled fact, and treating it as one is exactly how a confident-looking number turns into a real shortfall a decade later. Before you act on it, it is worth checking how that same age holds up against real historical market sequences, not just the smooth average case.
Frequently asked questions
What is a coast FI age?
The age at which you could stop contributing to retirement accounts entirely and still reach your retirement number by your planned retirement age, relying on investment growth alone from that point forward.
How is coast FI age calculated?
Starting from your current savings and monthly contribution, this calculator finds the youngest age at which your projected balance, grown at your assumed return with no further contributions, would still reach your target portfolio by retirement.
Is coast FI age a guarantee?
No. It is an estimate based on an assumed average annual return. Real markets do not move in a smooth average line, so a coast FI age that looks safe on paper can still leave you short if a downturn hits after you stop contributing.
What happens if I stop contributing and the market drops?
A downturn after you stop contributing, especially close to retirement, can leave your portfolio short of your target with no new contributions to offset it. Worth stress-testing against historical sequences before acting on it.
Does this calculator account for taxes or Social Security?
No. This is a simplified, single-rate estimate. It does not account for taxes, account type, inflation, or guaranteed income like Social Security and pensions.