FIRE · United States · Updated 2026

How to Calculate Your Coast FIRE Number

Your Coast FIRE number is the amount you need invested today so that compound growth alone reaches your full FIRE target by retirement — with zero further contributions. The whole calculation is one formula:

Below is how to get each piece, three worked US examples, and the mistakes that throw the number off. Prefer to skip the arithmetic? Jump straight to the free Coast FIRE calculator — it runs exactly this formula on your own inputs.

Step 1 — Find your full FIRE number

Your FIRE number is the portfolio you need at retirement to live off withdrawals indefinitely. Divide your desired annual retirement spending by your safe withdrawal rate. At the widely-used 4% rule, that is simply annual expenses × 25:

Example:if you expect to spend $50,000 a year, your FIRE number is $50,000 ÷ 0.04 = $1,250,000. Keep this figure in today’s dollars— Step 2 handles inflation for you. A more cautious 3.5% withdrawal rate raises the target (× 28.6); an aggressive 4.5% lowers it.

Step 2 — Choose a real return rate and years to retirement

Two inputs drive the compounding:

• Real return rate. Use an inflation-adjustedreturn so you can leave your expenses in today’s dollars. A common long-run US stock-index assumption is 7% real (roughly a ~9–10% nominal return minus ~2–3% inflation). Conservative planners use 5–6%.
• Years to retirement.Your target retirement age minus your current age. If you’re 35 and aiming for 65, that’s 30 years of compounding.

The single most common Coast FIRE error is inflating your future expenses andusing a nominal return. That double-counts inflation. Pick one lane — a real return with today’s-dollar expenses is the cleaner one.

Step 3 — Discount your FIRE number back to today

Divide your FIRE number by (1 + real return) raised to the number of years to retirement. The result is your Coast FIRE number:

This is just the future-value formula run backwards. It answers: “What lump sum, growing at r for n years, lands exactly on my FIRE number?”

Step 4 — Compare it to what you have saved

If your current invested savings already meet or beat your Coast FIRE number, congratulations — you’ve hit Coast FIRE and can stop contributing (you still need to cover day-to-day expenses, but your retirement is on autopilot). If not, the difference is your remaining savings gap.

Three worked examples

All three assume a 7% real return and a 4% withdrawal rate, with expenses in today’s dollars.

Example 1 — Start early (age 30)

• Annual expenses: $50,000 → FIRE number = $1,250,000
• Retire at 65 → 35 years of compounding
• 1.07³⁵ ≈ 10.68
• Coast FIRE number = $1,250,000 ÷ 10.68 ≈ $117,000

Sanity check: $117,000 growing at 7% for 35 years ≈ $1.25M. Starting this early, roughly $117K invested and never topped up carries you to full FIRE.

Example 2 — Mid-career (age 40)

• Annual expenses: $60,000 → FIRE number = $1,500,000
• Retire at 65 → 25 years of compounding
• 1.07²⁵ ≈ 5.43
• Coast FIRE number = $1,500,000 ÷ 5.43 ≈ $276,000

Example 3 — Late start (age 50)

• Annual expenses: $70,000 → FIRE number = $1,750,000
• Retire at 67 → 17 years of compounding
• 1.07¹⁷ ≈ 3.16
• Coast FIRE number = $1,750,000 ÷ 3.16 ≈ $554,000

Notice the pattern: the same 7% return, but the Coast FIRE number climbs steeply the later you start — from ~$117K at 30 to ~$554K at 50 — because compounding has fewer years to work. Time is the lever, not the return assumption.

US-specific note: accessing money before 59½

Reaching Coast FIRE means you stop saving — not that you start withdrawing. If you also plan to retire before 59½, remember that pulling from a 401(k) or Traditional IRA early generally triggers a 10% penalty on top of income tax. The usual bridges are a Roth conversion ladder, a taxable brokerage account, or 72(t) SEPP withdrawals. Coast FIRE gets your total number to the finish line; account structure decides how you reach it before the penalty age.

Common questions

What return rate should I use?

7% real is the common baseline from long-run US stock-index history; 5–6% is more conservative. A lower assumed return produces a largerCoast FIRE number. These are projections, not guarantees — markets don’t deliver a smooth 7% every year.

Should I count Social Security?

Many people leave it out for a conservative plan. If you do include it, it lowers the annual spending your portfolio has to cover, which lowers both your FIRE number and your Coast FIRE number.

What if I can’t reach my Coast FIRE number yet?

The gap closes by saving more now, pushing back your retirement age (more compounding years, a smaller Coast number), or trimming planned retirement expenses (a smaller FIRE number). The calculator shows how each lever moves the number.

Related

• Coast FIRE calculator — this formula, on sliders
• FIRE calculator — full 4% rule + bridge-portfolio math
• Lean FIRE and Barista FIRE calculators
• Average net worth by age (US) — how your savings compare