Coast FIRE Calculator
Enter your monthly expenses and we calculate your retirement target automatically. Then find the exact moment you can stop contributing and let compound interest carry you the rest of the way.
Retirement Target
Enter monthly expenses or set a target directly.
In today's dollars · All inputs and results are inflation-adjusted
Calculator Inputs
📊 Multi-Phase Returns
Model de-risking before retirement for a more accurate coast number
Not yet on track for your retirement target
Your final portfolio is $780.1K short of $1.20M
Target
$1.20M
Required Coast Number
$452.3K
Need at coast age to hit target
Portfolio at Coast Age
$154.8K
After 15 yrs contributing
Final Value
$419.9K
After 35 total years
Total Invested
$100.0K
Your cash in
Total Gains
$319.9K
Compound growth
📬 One investing insight per week
Coast FIRE math and compound interest — free, every week.
Portfolio Growth Over Time
Amber = your contributions · Teal = compound growth
You invest $500/mo for 15 years, reaching a portfolio of $154.8K (required coast number: $452.3K).
You stop contributing and let compound interest grow $154.8K to $419.9K over 20 more years — $780.1K below your target.
For educational purposes only. Past market performance does not guarantee future results. Full disclaimer →
How This Calculator Works
① Enter today's numbers
All inputs — monthly expenses, contributions, initial investment — are in today's dollars. No need to guess future values.
② Inflation is subtracted
Your inflation rate is subtracted from your expected return to give a real return. At 8% return and 3% inflation, the calculator uses 5% — so all results stay in today's purchasing power.
③ Two-phase projection
The calculator models a contribution phase (you invest monthly) and a coast phase (you stop contributing and compound growth takes over). The coast number is what you need at the end of phase one.
The maths
Retirement target = Monthly expenses × 12 ÷ withdrawal rate. At $4,000/mo and 4% withdrawal rate, your target is $1,200,000 in today's dollars.
Required coast number = Retirement target ÷ (1 + real return)^coast years. This is what you need invested at coast age so compound growth alone carries you to your target.
Real return = Nominal return − inflation rate. Using real return keeps every number in today's purchasing power, so results are directly comparable to your current expenses.
This approach matches the methodology used by most Coast FIRE calculators. Setting inflation to 0% gives you nominal projections identical to tools that don't account for inflation at all.
Frequently Asked Questions
What is a Coast FIRE number? +
Your Coast FIRE number is the amount you need invested today so that — without any additional contributions — your portfolio will grow to your target retirement amount by the time you want to retire. Once you hit your coast number, you can "coast" to retirement purely on compound growth.
What is the difference between contribution years and invested years? +
Contribution years are the years you actively invest money each month. Invested years are the additional years your money continues to grow after you stop contributing. This distinction is the core insight of Coast FIRE — your money keeps compounding even after you stop adding to it.
What annual return rate should I use? +
Historically, the US stock market (S&P 500) has returned approximately 10% annually before inflation, or roughly 7% adjusted for inflation. For conservative projections, many Coast FIRE planners use 6–7%. For nominal (non-inflation-adjusted) projections, 8–9% is common. Always use a rate you are comfortable with.
Can I stop contributing entirely after reaching my coast number? +
In theory, yes — once you reach your coast number, your investments should grow to your retirement target without additional contributions. In practice, most people continue to contribute at a lower rate for comfort and to account for uncertainty in market returns.
How is this different from a regular compound interest calculator? +
Standard compound interest calculators treat your investment timeline as a single continuous phase. CoastVest's calculator lets you model two distinct phases: a contribution phase (where you invest monthly) and a coast phase (where contributions stop but money keeps growing). This reveals the dramatic impact of starting early and stopping contributions well before retirement.
How do I calculate my Coast FIRE number? +
Coast FIRE number = Retirement target ÷ (1 + real return)^coast years. Your retirement target is annual spending ÷ withdrawal rate. For example: $50,000 annual spending at 4% SWR = $1,250,000 target. If you have 25 years to coast at 5% real return, your coast number is $1,250,000 ÷ (1.05^25) ≈ $369,000. The calculator above handles this automatically.
What is a good Coast FIRE number? +
A good Coast FIRE number depends on your target retirement spending and how many years you have left until retirement. As a rough guide: if you want to spend $40k/year in retirement with 25 coast years at 5% real return, your coast number is around $295,000. With 20 coast years it rises to $376,000. The earlier you reach your coast number, the less you need — because more time means more compounding.
Can you Coast FIRE at 40? +
Yes — Coast FIRE at 40 is one of the most common goals. If you retire fully at 65, coasting from 40 gives you 25 years of compound growth. At 5% real return, $1 invested at 40 grows to $3.39 by 65. So if your retirement target is $1,250,000, your coast number at 40 is roughly $369,000. Many people reach this by their late 30s with consistent investing.
What is the difference between Coast FIRE and Lean FIRE? +
Coast FIRE is about reaching a portfolio size that will grow to your target without further contributions — you still work but stop investing. Lean FIRE is about actually retiring on a minimal budget, typically under $40,000 per year. They address different questions: Coast FIRE asks 'when can I stop contributing?' while Lean FIRE asks 'what is the minimum I need to retire now?'