Coast Fire Calculator — Your Coast Number, Funding Gap, and Timing

I’m Selina Marquez, finance consultant and investment specialist. This tool answers a focused question: how much you need invested today so compounding alone can cover future expenses by your target retirement age—without further investing after you hit that “coast” number.

Quick start: enter a few inputs to see if you’ve coasted yet

Provide your Current age, Target retirement age, Current annual expenses, Safe withdrawal rate (%), Expected portfolio return (%), Expected inflation (%), Current portfolio, and Annual contributions. You’ll get:

• Years to retirement
• Required portfolio at retirement (in today’s dollars and inflated forward)
• Your Coast number (what you need now)
• Projected portfolio at retirement (based on current balance and contributions)
• Status: whether you’ve already “coasted”

How the Coast FIRE math works and what moves the result

The calculator converts nominal return and inflation into a constant real return: (1+return)/(1+inflation)-1. It grows your current portfolio by this real rate to retirement, adds the future value of your Annual contributions, and compares that to what you’ll need at retirement to support expenses using the Safe withdrawal rate (SWR).

• Biggest levers: Current annual expenses, SWR, Expected portfolio return, and Expected inflation.
• Lower expenses or a higher SWR reduces the required portfolio at retirement. Higher real returns lower your Coast number.

Formula snapshots: compact and decision-ready

Core equations

• Years = max(0, Target retirement age − Current age)
• Real return r = (1 + nominal return) / (1 + inflation) − 1
• Required at retirement = (Current annual expenses × (1+inflation)^Years) / SWR
• Coast number (today) = Required at retirement / (1 + r)^Years
• Projected at retirement = Current portfolio × (1 + r)^Years + FV of contributions

Variable glossary

• Nominal return, inflation in % per year; SWR as a % (e.g., 4%).
• Contributions assumed annual at year-end; results are rounded to whole dollars.

Worked example with realistic numbers and clean rounding

Inputs

• Current age: 30; Target retirement age: 60
• Current annual expenses: $40,000 (today)
• SWR: 4%; Expected portfolio return: 7% (nominal)
• Expected inflation: 2.5%; Current portfolio: $50,000
• Annual contributions: $12,000

Outputs

• Years to retirement: 30
• Required portfolio at retirement: $1,739,222
• Coast number (today): $680,545
• Projected portfolio at retirement: $1,258,601
• Status: Not yet

Interpretation: At today’s $50,000, you have not “coasted.” If you reach about $680,545 today, compounding alone could carry you to the required $1.74 million by 60, assuming the inputs hold.

Scenario test: tweak two levers and see the difference fast

• Reduce expenses to $35,000: Required at retirement falls proportionally. Coast number declines, often by a meaningful margin because SWR scales off expenses.
• Increase inflation to 3.5% (same 7% nominal): Real return drops, raising both the Required at retirement (via higher expense inflation) and the Coast number (harder compounding hurdle).

Insight: Managing spending levels and inflation assumptions usually delivers more impact than small SWR tweaks.

Use cases: plan your glidepath, pause contributions, or reassess

• Pausing contributions: Check if your Current portfolio ≥ Coast number. If yes, you can “coast” and let time do the work.
• Early retirement target shift: Move the Target retirement age earlier. Your Years shrink; Coast number rises because growth time is shorter.
• Career break planning: Set Annual contributions to $0 to stress-test your current progress.

Trade-off: Aggressive return assumptions lower the Coast number but heighten sequence-of-returns and shortfall risk. Balance realism with resilience.

Limits, assumptions, and frequent mistakes to avoid

• Assumptions: Constant real returns, annual contributions at year-end, expenses grow with headline inflation, withdrawals at SWR from retirement onward.
• Range checks: Avoid negative ages or SWR ≤ 0. Nominal return should generally exceed inflation over long horizons.
• Common pitfalls: Using pre-tax expenses while the SWR implies after-tax spending; ignoring fees and taxes (both reduce real returns); choosing an SWR without considering asset mix and longevity.

Pro tips to calibrate your coast path with confidence

• Start with grounded inputs: 3–5% real return is often optimistic across cycles; stress-test at lower real returns.
• Revisit SWR: 3.5–4% is a common range for diversified portfolios; consider longevity, fees, and sequence risk.
• Tighten expenses: Each $1,000 reduction in annual spending lowers your required portfolio by roughly $25,000 at a 4% SWR.

When to use a FIRE number calculator versus this tool

If you’re actively saving for years, the Coast FIRE approach focuses on the sufficiency of today’s balance. A broader financial independence calculator targets a full “FI number” driven by long-run contributions and spending plans. Use both to cross-check your trajectory.

Related terms used by readers: Coast to FI calculator, Coast number estimator, FIRE coast number, financial independence coast, retire early coast plan, compounding-to-retire tool.