Coast FIRE Number = Full FIRE Target ÷ (1 + real return)years to retirement
Full FIRE Target = Annual Retirement Spending ÷ Withdrawal Rate
Where all figures are evaluated under your chosen assumptions and denominated in today's dollars.
The Formula, Step by Step
Calculating your Coast FIRE number involves two sequential calculations using standard present-value financial mathematics:
Step 1: Determine Your Full FIRE Target
Your Full FIRE target is the total portfolio value required on the day you retire to fund your lifestyle through investment withdrawals.
Full FIRE Target = Annual Retirement Spending ÷ Withdrawal Rate
- Annual retirement spending: The estimated annual living expenses you anticipate spending in retirement, expressed in today's purchasing power (today's dollars).
- Withdrawal rate: The annual percentage of your portfolio you plan to withdraw during retirement as a planning assumption (for example, a 4% planning convention, or 0.04, which equates to 25× annual spending).
See how the Coast and Full FIRE milestones differ →
Step 2: Discount the Target Back to Today
Once you know the future portfolio target, calculate how much money you need invested today so compound growth alone can cover the remaining distance over your accumulation runway.
Coast FIRE Number = Full FIRE Target ÷ (1 + r)n
- r (real return): The expected annual compound investment return after adjusting for inflation (for example, 7% or 0.07).
- n (years to retirement): The number of years between your current age and your planned retirement age (
Target Retirement Age − Current Age).
Why use real returns and today's dollars? Pairing an inflation-adjusted (real) return with spending stated in today's dollars keeps all figures denominated in constant purchasing power. You do not need to forecast decades of cumulative inflation or guess future nominal prices; the inflation adjustment is already embedded directly in the real return rate. To review baseline assumptions and return conventions, see our methodology section.
Worked Example: Age 30
To demonstrate the calculation in practice, consider a deterministic, step-by-step example for an investor at age 30:
Deterministic Baseline Inputs
- Current age: 30
- Target retirement age: 65
- Years to retirement (n): 65 − 30 = 35 years
- Annual retirement spending: $50,000 (in today's dollars)
- Withdrawal rate: 4% (0.04)
- Assumed real return: 7% (0.07)
1. Calculate Full FIRE Target
$50,000 ÷ 0.04 = $1,250,000
At a 4% withdrawal rate, funding $50,000 of annual living costs requires a $1,250,000 nest egg at age 65 (25× annual spending).
2. Calculate Coast FIRE Number
$1,250,000 ÷ (1 + 0.07)35 = $1,250,000 ÷ (1.07)35
Computing the compounding factor over 35 years: (1.07)35 ≈ 10.676582. Dividing the target by this growth multiple yields:
$1,250,000 ÷ 10.676582 ≈ $117,079
Under these assumptions, an individual at age 30 with approximately $117k ($117,079) invested in a diversified portfolio could stop making additional retirement contributions, and compound growth at a 7% real return would expand that balance to $1.25M by age 65.
Open this age 30 scenario in the calculator →Why the Return Assumption Matters
Because the formula applies compounding exponentially over multi-decade periods, small variations in your assumed real return produce dramatic shifts in your required starting capital. Neither historical averages nor optimistic estimates are guaranteed.
The table below shows how the required Coast FIRE number changes for the exact same age-30 baseline ($50,000 annual spending, 4% withdrawal rate, $1,250,000 target, 35 years to age 65) across different real annual return rates:
| Assumed Real Return | 35-Year Compounding Factor (1 + r)35 | Exact Coast FIRE Number | Rounded Estimate |
|---|---|---|---|
| 4.0% real return | 3.946× | $316,769 | ~$317k |
| 5.0% real return | 5.516× | $226,613 | ~$227k |
| 6.0% real return | 7.686× | $162,632 | ~$163k |
| 7.0% real return | 10.677× | $117,079 | ~$117k |
Notice the spread: dropping the assumed real return from 7% to 4% nearly triples the upfront savings required today—from ~$117k to ~$317k. No single rate should be considered "best" or "recommended." Modeling conservative scenarios (such as 4% to 5% real returns) alongside moderate baselines helps safeguard plans against lower-than-average market eras.
Why Retirement Age Matters
The exponent in the denominator is years to retirement. Every additional year between today and your target retirement date gives your capital more time to compound, which exponentially decreases the starting sum required today, all else equal.
Conversely, targeting an early retirement age (such as age 50 or 55 instead of 65) significantly compresses your compounding runway. Shortening the runway means the portfolio has fewer doubling cycles, requiring a substantially higher Coast balance today. To compare milestone targets across different age brackets, explore our Coast FIRE by age benchmark guide.
Why the Withdrawal Rate Matters
Your withdrawal rate determines the size of the initial Full FIRE target before any compounding is applied:
- 4.0% withdrawal rate: Multiplies spending by 25× ($50,000 ÷ 0.04 = $1,250,000).
- 3.5% withdrawal rate: Multiplies spending by ~28.6× ($50,000 ÷ 0.035 ≈ $1,428,571).
Selecting a lower withdrawal rate creates a larger target nest egg to provide extra margin against longevity or market drawdowns, but it also increases the Coast FIRE amount needed today. Neither withdrawal rate is guaranteed to succeed across every retirement horizon or portfolio structure. See our methodology sources for historical withdrawal-rate research.
Common Calculation Mistakes
When applying the Coast FIRE formula manually, watch out for these frequent planning pitfalls:
- Mixing nominal returns with today's-dollar spending: Using a nominal return (such as 10%) with spending stated in today's purchasing power causes severe under-saving because inflation will erode the true purchasing power of the future balance. Always pair today's-dollar expenses with inflation-adjusted (real) returns.
- Forgetting that fees and taxes are not modeled: Investment management fees, fund expense ratios, and income taxes on withdrawals reduce take-home wealth. If taxes or fees apply, your pre-tax target must be adjusted upward.
- Treating the assumed return as guaranteed: Real markets experience prolonged downturns, sequence-of-returns volatility, and periods of below-average returns. Compounding in reality is never a smooth, guaranteed curve.
- Using years until current age instead of years until retirement: The compounding exponent
nmust strictly represent the remaining accumulation years until retirement (Retirement Age − Current Age), not your current age. - Confusing Coast FIRE with Full FIRE: Reaching your Coast FIRE number means your existing balance can compound to full retirement later. It does not mean you have reached full financial independence today.
- Assuming reaching Coast FIRE means you can stop working now: Because your invested assets must stay in the market to compound, you cannot withdraw from them today. You still need earned income or cash flow to cover immediate living expenses.
Formula vs. Calculator
Understanding the manual Coast FIRE formula gives you complete mathematical transparency into how your retirement targets are derived and how compounding functions behind the scenes. However, doing manual calculations for multiple retirement ages, fluctuating return scenarios, or varying contribution plans can be time-consuming.
An interactive calculator allows you to stress-test scenarios instantly, visualize your growth trajectory, and evaluate how ongoing contributions alter your timeline.
Test Scenarios with the Interactive Calculator
Adjust your retirement age, annual spending, savings rate, and expected return in real time to see your customized Coast FIRE number and compounding timeline.
Open Coast FIRE Calculator →Continue exploring our educational FIRE guides:
- What Is Coast FIRE? Meaning, Math, and How It Works — An overview of the Coast FIRE lifestyle, work flexibility, and financial implications.
- Coast FIRE by Age Benchmark Guide — Detailed benchmark savings tables across ages 20 through 50 and spending levels from $40k to $100k.
- Calculator Methodology & Assumptions — Detailed explanations of discounting logic, inflation adjustment conventions, and source research.
Sources & Background
- William P. Bengen, "Determining Withdrawal Rates Using Historical Data," Journal of Financial Planning (1994). Bengen analyzed historical retirement periods and found that initial withdrawal rates near 4% were historically sustainable across the scenarios he studied.
- Philip L. Cooley, Carl M. Hubbard, Daniel T. Walz, "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable," AAII Journal (1998). Known as the Trinity Study, analyzing portfolio survival rates across different asset allocations and withdrawal rates.
These foundational studies inform common withdrawal-rate planning context; they do not establish the Coast formula or any particular withdrawal rate as a guaranteed outcome under future economic conditions.
Last updated: September 27, 2026
Educational Disclaimer: This guide is provided solely for educational and illustrative purposes, not personalized financial, investment, or tax advice. Calculations rely on mathematical models and hypothetical assumptions that do not guarantee future performance. Consult a qualified fiduciary financial professional before making retirement planning decisions.