Coast FIRE Calculator: Find Out If You Can Stop Saving Today

What Is a Coast FIRE Number and How Is It Calculated?

Your Coast FIRE number is the amount you need invested right now so that compound growth — with zero additional contributions — reaches your full retirement target by your chosen retirement age.

The formula is:

**Coast FIRE Number = Retirement Target ÷ (1 + r)^n**

Where r is your expected annual real return (after inflation) and n is the number of years until retirement.

For example: if you need $2,000,000 at age 65 and you're 35 today, that's 30 years. Assuming a 5% real return: $2,000,000 ÷ (1.05)^30 = **$462,000**. If you have $462,000 invested today, you're done — compound interest handles the rest.

The retirement target itself comes from the 4% rule: multiply your desired annual spending by 25. Spending $80,000/year in retirement? Your target is $2,000,000. This rule, rooted in Bengen's 1994 research and validated by the Trinity Study, gives a portfolio a historically high probability of lasting 30+ years.

Two variables matter most: your assumed real return and your years to retirement. A higher return lowers your Coast number dramatically. Going from 4% to 6% real returns cuts your required Coast number by roughly 30% on a 30-year horizon. That's why getting the inputs right is critical — which is exactly what our calculator forces you to do.

How to Use the Coast FIRE Calculator — Step by Step

Open the Coast FIRE calculator and enter four numbers. That's it.

**Step 1 — Current portfolio value.** This is your total invested assets: 401(k), IRA, brokerage, HSA if you'll use it for retirement. Exclude your emergency fund and home equity.

**Step 2 — Annual retirement spending.** What do you actually need in today's dollars? Be honest — most people underestimate by 15–20%. The calculator multiplies this by 25 to get your retirement target.

**Step 3 — Current age and target retirement age.** The gap between these two numbers is your compounding runway. A 32-year-old targeting age 60 has 28 years. A 45-year-old targeting 60 has only 15. That difference is enormous: $300,000 at 6% real grows to $1.44M over 28 years but only $719k over 15.

**Step 4 — Expected real return.** We recommend 5–6% for a diversified equity-heavy portfolio (the S&P 500's historical real return since 1926 is approximately 7%, but factoring in fees and sequence risk, 5–6% is a conservative and defensible assumption).

The calculator then shows you two things: your Coast FIRE number, and the gap (positive or negative) between that number and what you have today. A positive gap means you're not there yet. A negative number means you've already coasted — you could stop contributing today.

For a broader retirement picture beyond the Coast number alone, use the full plan editor to layer in Social Security, part-time income, and spending phases.

Worked Examples: Three Scenarios with Real Numbers

**Scenario 1 — The 30-Year-Old Who's Already There** Marcus is 30, has $220,000 invested, wants to retire at 60, and plans to spend $60,000/year. His retirement target: $1,500,000. Real return assumption: 6%. Coast FIRE number: $1,500,000 ÷ (1.06)^30 = **$261,000**. Marcus has $220,000 — he's $41,000 short. At his current savings rate of $1,500/month, he hits his Coast number in about 2 years. After that, he can redirect contributions entirely.

**Scenario 2 — The 42-Year-Old With a Modest Portfolio** Preeti is 42, has $180,000 invested, wants to retire at 65, and plans to spend $70,000/year. Retirement target: $1,750,000. Real return: 5%. Coast number: $1,750,000 ÷ (1.05)^23 = **$611,000**. Preeti is $431,000 short — she has significant saving still to do. But this number is clear and actionable: once she crosses $611,000, she can coast.

**Scenario 3 — The 50-Year-Old Who Coasted and Won** Derek is 50, stopped contributing at 38 when he had $310,000, and is now checking in. His target retirement age is 67, spending $75,000/year → $1,875,000 target. At 5% real return over 29 years (from age 38), $310,000 grows to $310,000 × (1.05)^29 = **$1,198,000**. He's not fully coasted after all — but he has a clear gap of $677,000 at retirement, which the full plan editor can help him address with Social Security credits and part-time income projections.

The takeaway: small differences in assumptions produce large differences in outcomes. Run your own scenario, then stress-test it at 4% and 6% returns to see the range.

The Inputs That Move the Needle Most on Your Coast FIRE Number

Not all inputs are equally sensitive. Here's how each one affects your Coast number:

**Years to retirement** has exponential impact. Each additional year of compounding roughly multiplies your money by (1 + r). Ten extra years at 6% more than doubles your portfolio — which means your required Coast number is cut in half. Retiring at 67 instead of 57 doesn't require twice the savings to coast; it requires about half.

**Real return assumption** is the most dangerous variable because it feels abstract. The difference between 4% and 7% real over 30 years isn't 3 percentage points — it's the difference between tripling your money and multiplying it by 7.6×. We suggest running the calculator at both 4% and 6% to bound your uncertainty.

**Annual spending target** doubles as your primary retirement lever. Cutting planned retirement spending from $80,000 to $60,000/year reduces your retirement target by $500,000 — which reduces your Coast number by roughly $115,000 if you're 35 years from retirement at 6% real. Geographic arbitrage, paid-off mortgage, or reduced lifestyle costs in retirement have a compounding-level impact on how soon you can coast.

**Current portfolio value** is the one number you control today. It's also the benchmark. Every time your statement crosses your Coast FIRE number, you've crossed the finish line — even if you don't realize it.

What Happens After You Hit Your Coast FIRE Number

Hitting your Coast FIRE number doesn't mean retiring — it means retiring from mandatory saving. You still need to cover your living expenses between now and retirement, which is where most people get tripped up.

The Coast FIRE strategy works best when paired with a lifestyle that doesn't require aggressive saving to sustain. If you're spending 95% of your take-home pay to cover rent, food, and childcare, hitting your Coast number buys you peace of mind — but not flexibility. If you're at 50–60% spending, you suddenly have a lot of runway to reduce hours, take a career risk, or work in a lower-paying field you actually enjoy.

A few practical moves once you've coasted:

- **Redirect retirement contributions to a taxable brokerage** for liquidity before age 59½ — useful if you plan to retire early. - **Maintain your employer match** if you have one. A 50% match is a guaranteed 50% return. Even a coaster should capture free money. - **Don't stop tracking.** Market downturns can push your portfolio below your Coast number temporarily. Check annually and stay calibrated.

For a complete picture of what your financial life looks like between now and retirement, the full plan editor at FinAI models your spending, savings, and investment trajectory across every decade. It's the difference between knowing your Coast number and actually having a plan.

Coast FIRE vs. Lean FIRE vs. Barista FIRE: Which Path Fits You?

Coast FIRE is one of several FIRE variants — and choosing the right one matters because they imply completely different financial targets and timelines.

**Traditional FIRE** means you've fully funded retirement and can stop working entirely. Target: 25× annual spending, fully accumulated. A $80,000/year lifestyle requires $2,000,000 invested *now*.

**Coast FIRE** means you've invested enough that compound growth handles the rest — but you still need income to cover current living expenses until retirement. Your portfolio does the heavy lifting; you just need to not touch it.

**Barista FIRE** is similar to Coast but explicitly assumes part-time or flexible work provides supplemental income and, crucially, employer health insurance. Popular among people in their 40s who want to slow down but can't lose benefits.

**Lean FIRE** means achieving traditional FIRE on a very low spending target — typically under $40,000/year. This requires a $1,000,000 portfolio and demands extreme frugality in retirement.

Coast FIRE tends to appeal to people who actually like their work — or at least find it tolerable — but want to remove the anxiety of mandatory saving. If that's you, it's worth running your numbers right now. You might already be there. Use the Coast FIRE calculator to find out in under 60 seconds.

Common Mistakes That Give You a False Coast Number

The math behind Coast FIRE is clean. The inputs people use are often not.

**Mistake 1 — Using nominal returns instead of real returns.** If you plug in 10% (the S&P 500's approximate historical nominal return), you're ignoring inflation. At 3% annual inflation, that 10% becomes roughly 6.8% real. Using nominal returns makes your Coast number look smaller than it is — a dangerous illusion.

**Mistake 2 — Including home equity.** Your primary residence is not a retirement asset unless you plan to sell it or reverse-mortgage it. Count only liquid, invested assets.

**Mistake 3 — Underestimating retirement spending.** Healthcare is the most common culprit. Fidelity's 2025 estimate for average retiree healthcare costs is approximately $165,000 per person over retirement. If you're planning to retire before Medicare eligibility at 65, budget $800–$1,500/month for private coverage in today's dollars.

**Mistake 4 — Ignoring sequence of returns risk.** If a market crash hits in the first five years of retirement, a mathematically sound Coast plan can still fail. The calculator's output is a central estimate — not a guarantee. Stress-test your plan with a lower return assumption, and consult a fee-only financial advisor for personalized guidance.

**Mistake 5 — Not revisiting annually.** Your Coast number isn't static. Spending targets change, expected returns shift, and your portfolio fluctuates. Build an annual check-in into your calendar — 15 minutes with the calculator every January is enough to stay calibrated.

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