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Case Study: Coast FIRE

Persona

Jamie, age 35, has accumulated $480,000 across three accounts and wants to "coast" -- stop contributing entirely and let compound growth carry the portfolio to a traditional retirement at 60.

Parameter Value
Current age 35
Retirement age 60
Planning horizon to 95
Monthly spending $4,000
Annual savings $0 (coasting)
Current portfolio $480,000
Allocation 70/30 stocks/bonds 
from monteplan.config.defaults import coast_fire_plan
plan = coast_fire_plan()

The Coast FIRE Concept

Coast FIRE means having enough invested that, even with zero additional contributions, compound growth is expected to build a sufficient retirement portfolio by a traditional retirement age. The key variable is: is $480K at age 35 enough to coast to 60?

Analysis

Growth Phase (35-60)

With 25 years of compound growth and no contributions:

  • At 7% nominal stock returns and 70/30 allocation, the expected portfolio at age 60 is substantial
  • But the range of outcomes is wide: good markets could double the expected value, while poor markets might barely grow the portfolio

Spending Phase (60-95)

The 35-year retirement is shorter than the FIRE case but still longer than traditional. Key factors:

  • $4,000/month spending requirement
  • No guaranteed income (no Social Security modeled in the base case)
  • 35 years of inflation eroding purchasing power

Sensitivity Results

The most impactful parameters for Jamie's plan:

  1. Stock returns -- The plan is entirely dependent on market performance since there are no contributions
  2. Monthly spending -- Each $500/month reduction significantly improves success
  3. Inflation -- 25+ years of compounding inflation matters enormously
  4. Retirement age -- Delaying to 62 or 63 adds meaningful safety margin

What If Jamie Contributes a Little?

Coast FIRE is black-and-white (contribute $0), but a "barista FIRE" approach -- contributing even $500/month from part-time work -- can dramatically improve outcomes:

from monteplan.config.schema import AccountConfig

# Modify one account to add small contributions
plan_barista = plan.model_copy(update={
    "accounts": [
        AccountConfig(account_type="taxable", balance=100_000, annual_contribution=6_000),
        AccountConfig(account_type="traditional", balance=300_000, annual_contribution=0),
        AccountConfig(account_type="roth", balance=80_000, annual_contribution=0),
    ],
})

Takeaways

  1. Coast FIRE works best with a large starting portfolio -- $480K at 35 is near the edge; $600K+ provides more margin
  2. The plan is equity-dependent -- with zero contributions, returns must do all the work
  3. Consider adding Social Security -- even modest benefits at 67 significantly improve the post-60 phase
  4. Small contributions make a big difference -- "barista FIRE" with even $6K/year in contributions is much more robust
  5. Inflation is the silent risk -- 60 years of inflation (age 35-95) requires real returns to do heavy lifting