Why This Calculation Matters
Knowing how to calculate coast fire number accurately is the difference between a reliable financial milestone and a misleading one. Many people run the numbers with nominal returns, forget to subtract inflation, or include non-investable assets — all of which produce a coast fire number that is artificially low and creates false confidence.
This guide walks through the correct coast fire calculation — the same formula used in our coast fire calculator — with worked examples at each step. If you prefer to skip the maths, the free coast fi calculator does all of this automatically.
Step 1: Calculate Your Full FIRE Number
Your full FIRE number is the total portfolio you'd need at retirement to withdraw sustainably without depleting principal. It's derived from the 4% rule:
At 4% SWR: FIRE Number = Annual Spending × 25
$48,000/yr → $1,200,000 | $60,000/yr → $1,500,000 | $80,000/yr → $2,000,000
Important: Use your retirement spending estimate, not current spending. Account for healthcare, travel, hobbies, and any lifestyle changes you expect in retirement. A 10–15% buffer above your estimate is prudent.
For Social Security or a deferred pension, use the dedicated calculator so income is applied only from its actual start age and bridge years are included.
Step 2: Calculate Your Real Return Rate
The real return rate is what your investments actually earn after inflation and fees — the rate that matters for how to calculate coast fire accurately.
Real Return = (1 + Net Nominal) ÷ (1 + Inflation) − 1
Default: [(1 + 10%) × (1 − 0.18%) ÷ (1 + 3%)] − 1 = 6.60%
The fee is modeled as a proportional annual asset fee. Simple subtraction is only an approximation, not the exact formula used.
The S&P 500 has historically returned approximately 10% per year in nominal terms. After inflation (~3%) and typical index fund fees (0.03–0.20%), a real return of 6.5–7% is a standard coast fire planning assumption. For conservative planning, use 5–6% real — especially for very early retirees targeting 40+ year horizons.
This is why the coastfire calculator includes a separate inflation field and fees field: entering 10% growth without subtracting inflation and fees produces a real return that overstates your actual purchasing power growth, giving you a falsely low coast fire number.
Step 3: Calculate the Coast Fire Number
Now discount your FIRE number back to today using the present-value formula:
Where: Years = Retirement Age − Current Age
Worked examples at 7% real return:
| Age Now | Retire At | Years | FIRE Number | Coast Fire Number |
|---|---|---|---|---|
| 25 | 65 | 40 | $1,250,000 | $84,000 |
| 30 | 65 | 35 | $1,250,000 | $117,000 |
| 35 | 65 | 30 | $1,250,000 | $164,000 |
| 40 | 65 | 25 | $1,250,000 | $230,000 |
| 45 | 65 | 20 | $1,250,000 | $322,000 |
| 50 | 65 | 15 | $1,250,000 | $452,000 |
Notice how the coastfire number at 25 ($84,000) is dramatically smaller than at 50 ($452,000) — for the identical FIRE target. This is the compounding time advantage that makes early investment so powerful in coast fire calculation.
For detailed age-specific tables with different FIRE targets, see our guide on coast fire number by age.
Step 4: Compare to Your Current Invested Assets
Add up all liquid, market-invested assets:
- 401(k) and 403(b) balances
- Traditional and Roth IRA balances
- Taxable brokerage accounts
- HSA investment portion (not just cash balance)
Do not include: your primary home's equity, car value, cash savings, pension entitlement (unless liquid and invested), or RSU/stock options that haven't vested.
If your total invested assets ≥ your coast fire number — you've reached Coast FIRE. If not, the coast fire retirement calculator shows exactly how large the gap is and how long it will take to close at your current contribution rate.
Using the Free Coast Fire Calculator
If you'd rather not calculate manually, the free coastfire calculator handles all four steps automatically. Here's what each field means:
| Calculator Input | What to Enter | Default |
|---|---|---|
| Current Age | Your age today | — |
| Retirement Age | Target age to fully retire | — |
| Annual Spending | Expected yearly spend in retirement (today's dollars) | — |
| Current Invested Assets | Sum of all investment accounts | — |
| Monthly Contributions | What you add each month (set $0 to check if coasting) | — |
| Investment Growth Rate | Nominal annual return | 10% |
| Inflation Rate | Annual inflation assumption | 3% |
| Safe Withdrawal Rate | Retirement withdrawal percentage | 4% |
| Investment Fees | Annual expense ratio | 0.18% |
Running Sensitivity Analysis
Because the calculation is sensitive to the real-return assumption, comparing multiple scenarios can show the range of possible outputs:
| Scenario | Nominal Return | Real Return (~) | Coast Fire Number (35 yr, $1.25M target) |
|---|---|---|---|
| Conservative | 8% | 4.67% | $253,377 |
| Base Case | 10% | 6.60% | $133,307 |
| Optimistic | 12% | 8.54% | $70,952 |
A user may choose to compare a higher target as a buffer scenario, but the appropriate margin depends on risk tolerance, spending flexibility, other income, taxes, and personal circumstances.