Coast FIRE age calculator

Coast FIRE — the moment saving becomes optional

Coast FIRE is a specific milestone in the financial independence journey: the point where your existing portfolio will grow to your retirement number by traditional retirement age even if you never contribute another dollar. You still need a job to pay today's bills, but you no longer have to save. That freedom opens doors: lower-paying meaningful work, starting a business, sabbaticals, reduced hours.

The math of coasting

Coast FIRE works backward from a traditional FIRE number. Steps:

1. Figure out your FIRE number — typically 25× annual expenses (4% rule) or 30× (3.3% rule).
2. Figure out your target retirement age.
3. Discount the FIRE number back to today using your expected real return: coast number = FIRE number ÷ (1 + r)years.
4. If current portfolio ≥ coast number, you've hit Coast FIRE. Stop (or continue if you want to retire earlier).

Example: $60k/yr expenses → $1.5M FIRE number at 4%. Target retire age 65, current age 33. 32 years to grow at 5% real: (1.05)³² = 4.765. Coast number = 1.5M / 4.765 = $315k. If you have $315k at 33, you can coast to $1.5M by 65 with no further saving.

The age at which compound interest takes over

Coast FIRE is really about finding when compound growth becomes bigger than your contributions. For a disciplined saver contributing 20% of income, compound interest typically exceeds contributions around year 12–15 of the savings journey. By year 20, growth is 3–4× new contributions.

This is why early career contributions matter so much — and why late-career doubling-down often can't catch up. Someone who saves 20% of $80k ($16k/yr) from 25 to 40 has about $490k at 40 (at 7% growth). If they stop and let it compound at 5% real to 65, it becomes $1.65M. Someone who saves nothing 25–40 and then tries to catch up from 40 with $32k/yr (20% of $160k) has only $1.40M at 65.

Coast FIRE vs other FIRE flavors

• Lean FIRE — minimalist, $25k–$40k/yr retirement spending. Coastable in your 30s for disciplined savers.
• Fat FIRE — upscale, $150k+/yr retirement spending. Requires $4M+ and typically can't be coasted before 40s.
• Coast FIRE — the milestone between nothing saved and full FIRE. You can stop retirement saving but still need income for current expenses.
• Barista FIRE — part-time income covers current expenses while investments compound untouched. Middle ground between Coast and full FIRE.
• Full FIRE — portfolio covers current expenses; no job required.

What changes at Coast FIRE

Hitting Coast FIRE changes the psychology of work more than the math:

• Pay doesn't matter as much. If you don't need to save, a 25% pay cut to do work you enjoy is more palatable.
• Retirement benefits don't matter as much. If you're not contributing, a job without 401(k) match or pension isn't a dealbreaker.
• Geographic flexibility. Coasting income needs are lower, so high-cost metros become optional.
• Starting a business becomes feasible. The opportunity cost of no salary for 1–2 years while you build something doesn't torpedo retirement.

Risks to coasting

Coast FIRE is mathematically appealing but has real risks:

• Sequence-of-returns. Your portfolio may not compound at 5% real. The 2000–2010 decade produced 0% real return for US stocks. If you stopped saving at the start of a bad decade, your coast number wasn't actually a coast number.
• Lifestyle creep. If spending grows faster than inflation, your coast number was calculated on outdated expense assumptions.
• Health insurance. Pre-Medicare (65), coasting in the US typically requires ACA marketplace coverage. This runs $500–$1,200/mo per adult, which can bust a low-income coast plan.
• Long-term care. The 4% rule doesn't cover nursing home years at $100k+/yr. Plan for LTC insurance or additional buffer.

Common Coast FIRE mistakes

1. Ignoring inflation. If your FIRE number is in today's dollars, you need to use REAL (inflation-adjusted) return rate. Nominal 7% minus 3% inflation = 4% real.
2. Overly-optimistic returns. Using 8% real when long-run history is 5–7% for 60/40 leads to under-saving.
3. Ignoring taxes. Coast FIRE math is usually done on traditional-IRA or 401(k) balances, which will be taxed. $1.5M pretax is closer to $1.15M after tax at 22% bracket.
4. Forgetting to revisit. Coast FIRE works backward from assumptions about 20–30 years from now. Every 3–5 years you should rerun the math with updated figures.