What Makes a Good Coast Fire Calculator?
Not every coast fire calculator is built equally. A coastfire calculator that doesn’t account for inflation, fees, or a proper safe withdrawal rate will produce results that look reassuringly low — but are dangerously optimistic. The best coast fire calculator includes all the variables that matter in a real financial plan:
- Separate inflation rate and investment fee fields (not just a nominal growth rate)
- Adjustable safe withdrawal rate (not locked to 4%)
- A visual coast fire chart showing portfolio trajectory over time
- Both the coast fire number and the full FIRE number as outputs
- A clear pass/fail: have you already reached Coast FIRE?
- Full transparency on the formula — or a link to methodology documentation
Our free coast fire retirement calculator includes all of these. Read the full methodology to verify the formula.
Every Input Explained
Current Age & Retirement Age
The gap between these two is your compounding window. A larger gap = more time for compound interest to grow your investment = lower coast fire number required. Moving your target retirement age from 65 to 68 can reduce your coast fire number by 15–20%.
Annual Spending
Your expected annual spending in retirement, in today’s dollars. Do not use current spending directly — adjust for expected retirement lifestyle differences. Add a 10–15% buffer for healthcare and unexpected costs. For a coast fire calculator with social security: subtract expected annual SS benefit from this figure before entering.
Current Invested Assets
The total value of all liquid, market-invested accounts: 401(k), 403(b), Roth IRA, traditional IRA, taxable brokerage, HSA investment portion. Do not include: primary home equity, savings accounts, pension entitlements, or unvested RSUs.
Monthly Contributions
Your current monthly investment amount. Set this to $0 and run the calculation to check whether you’ve already reached Coast FIRE with your current invested assets — no further contributions needed. This is the most important test in the coast fi calculator.
Investment Growth Rate
The nominal annual growth rate expected from your portfolio. Default is 10% (approximate historical S&P 500 average). This is before subtracting inflation and fees — the coastfire calculator does that automatically in the real return calculation.
Inflation Rate
Annual inflation assumption. Default is 3% (US long-run average). For a coast fire calculator UK, use 2.5%. For coast fire calculator India, use 5–6%. For coast fire calculator Canada or Australia, 2–2.5% is appropriate.
Safe Withdrawal Rate
The annual retirement withdrawal percentage. Default 4% (from the Trinity Study). For early retirees targeting a 40–50 year horizon, consider 3.5%. Lower SWR = larger FIRE number = larger coast fire number.
Investment Fees
Annual expense ratio. Default 0.18% (typical for low-cost index funds). The difference between 0.05% (Vanguard VTSAX) and 1.00% (active managed fund) on a $300,000 portfolio over 30 years exceeds $400,000 in lost growth — directly increasing your required coast fire number by years. See our index fund guide.
Understanding the Output
Coast FIRE Achieved (Green)
Your current invested assets, compounding at your real return rate, will reach your full FIRE number by your retirement age without any further contributions. You are officially coasting. No mandatory retirement saving required.
Not Yet Coasting (Amber)
The calculator shows the gap between your current assets and your coast fire number, and how long it will take to close at your current contribution rate. This is the most actionable output — it tells you exactly how much more you need and when you’ll get there.
The Coast Fire Chart
Three projected lines: portfolio with contributions (gold), coast path/no contributions (dashed gold), full FIRE target (green dashed). Reading the chart: where the “With Contributions” line first crosses the FIRE target = your full FIRE date. Where the “No Contributions” line crosses the FIRE target = when your current portfolio would coast there alone.
Six Output Metrics
- Coast Fire Number — Today’s target
- Full FIRE Number — Annual Spending ÷ SWR
- Projected at Retirement — With ongoing contributions
- Estimated Annual Income — Projected balance × SWR
- Years to Retirement — Simple countdown
- Real Return (Net) — What the calculator actually uses after inflation and fees
Calculator Variants
Coast Fire Calculator for Couples
Run the coast fire calculator for couples by entering combined invested assets and a shared annual retirement spending figure. For more precision, run two separate calculations — one per partner with each person’s individual assets — then compare both to the shared FIRE target. See our full couples guide.
Coast Fire Calculator with Social Security
Subtract expected annual SS benefit from Annual Spending. $60,000 spending − $14,000 SS = $46,000 effective spending. Enter $46,000 as Annual Spending in the coast fire calculator with social security approach. This reduces both your FIRE number and coast fire number proportionally.
Coast Fire Calculator with Pension
Same approach as SS: subtract expected annual pension income from your retirement spending before entering it. For the coast fire calculator with pension: only enter liquid invested assets in Current Assets. Pension is an income offset to spending, not an invested asset.
5 Common Calculator Errors
- Using nominal return without noting that inflation/fees are subtracted separately — Our coastfire calculator handles this automatically. Manual calculators often miss this and produce artificially low coast fire numbers.
- Including home equity — Your primary home’s equity is not an invested asset that compounds toward retirement. Only include liquid investment accounts.
- Forgetting to set contributions to $0 for the coast test — The most important test: set contributions to $0 and rerun. If it shows green, you’ve already reached Coast FIRE.
- Using current spending instead of retirement spending — These can differ substantially. Retirement typically brings lower commuting/work costs but higher healthcare and leisure costs.
- Running once and never updating — Your coast fire number changes as your portfolio grows and life circumstances evolve. Recalculate annually with the coast fire retirement calculator.