How to Calculate Your Coast FIRE Number (Step-by-Step with Examples)

A complete guide to calculating your Coast FIRE number with worked examples, a reference table by age, adjustments for Social Security and pensions, and a couples calculation walkthrough.

Coast FIRE is the point where your investments are large enough that, without adding another dollar, they will grow to fund your retirement on their own. Once you hit your Coast FIRE number, you only need to earn enough to cover current expenses. The portfolio handles everything else.

The formula looks straightforward at first. Two inputs, one result. But getting it right requires understanding which assumptions drive the number and how to adjust for your situation: individual, couple, or a household expecting Social Security or a pension. This guide walks through the full calculation with worked examples, a reference table, and common mistakes to avoid.

What Coast FIRE Actually Means

Standard FIRE (Financial Independence, Retire Early) requires saving enough to cover all living expenses indefinitely from investments, typically 25 times annual expenses using the 4% withdrawal rule.

Coast FIRE sets a lower bar. You need enough invested today so that compound growth alone, with no additional contributions, will reach that full FIRE number by your target retirement age. You still work, but only to cover current expenses. The portfolio is on autopilot.

The name comes from sailing. Once you have enough momentum (investment growth), you stop rowing (contributing) and coast to the destination.

This is different from just hitting a "good" savings balance. Coast FIRE is a specific, calculable threshold. Either your portfolio is large enough to reach retirement on its own, or it is not. The Coast FIRE Calculator gives you that number instantly, but understanding the math behind it helps you make better decisions about when to hit the gas and when to ease off.

The Coast FIRE Formula

The formula comes from basic compound interest, worked backwards:

Coast FIRE Number = Full FIRE Number / (1 + Annual Real Return)^Years Until Retirement

Where:

• Full FIRE Number = Annual retirement expenses x 25
• Annual Real Return = expected return after inflation (typically 5-7%)
• Years Until Retirement = target retirement age minus your current age

Example:

• Annual retirement expenses: $60,000
• Full FIRE number: $60,000 x 25 = $1,500,000
• Current age: 35, target retirement age: 60
• Years to grow: 25
• Expected real return: 6%

Coast FIRE Number = $1,500,000 / (1.06)^25
                 = $1,500,000 / 4.292
                 = $349,500

If you have $349,500 invested today at age 35, you have already reached Coast FIRE. That balance will grow to $1.5 million by age 60 at 6% real annual returns without a single additional contribution.

The Coast FIRE Calculator runs this calculation in seconds so you can test different return rates, retirement ages, and expense levels side by side.

Step-by-Step: How to Calculate Your Coast FIRE Number

Step 1: Estimate your annual retirement expenses Work from your current spending and adjust for what will change in retirement. Remove commuting costs and work-related expenses. Add healthcare if you retire before Medicare at 65. Most planners use 80-100% of current spending as a starting point.

Step 2: Calculate your full FIRE number Multiply annual retirement expenses by 25. This is the portfolio size that would fund permanent retirement using the 4% withdrawal rate.

Step 3: Set your target retirement age This does not have to be 65. Many Coast FIRE planners target 55 or 60. The longer the growth window, the smaller the Coast FIRE number you need today.

Step 4: Choose your expected real return Use a real (after inflation) return, not a nominal one. For a diversified stock-and-bond portfolio, 5% real is conservative. For a heavily equity-weighted portfolio, 6-7% real is commonly used. Using real returns keeps all figures in today's dollars.

Step 5: Apply the formula Divide your full FIRE number by (1 + r)^n, where r is your real return and n is years until retirement.

Step 6: Compare to your current portfolio If your invested balance equals or exceeds the Coast FIRE number, you have coasted. If not, the gap tells you exactly how much more you need to accumulate before contributions become optional.

Coast FIRE Reference Table

The table below shows the Coast FIRE number for a $1,500,000 full FIRE target at different starting ages, assuming 6% real returns and a retirement age of 60. All figures are in today's dollars.

Time is the most powerful variable here, not the return rate. At age 25, you need $147,000. Wait until 40 and you need $467,500 for the same outcome. A five-year delay at age 25 costs you about $114,000 in required savings, far more than any improvement in investment returns could offset.

How to Calculate Coast FIRE for Couples

Couples have two incomes, two portfolio balances, and sometimes different retirement timelines. There are two approaches depending on your situation.

Option 1: Combined household calculation Add both partners' investment balances together. Compare the total against a single Coast FIRE number based on combined household expenses.

Example:

• Combined retirement expenses: $80,000/year
• Full FIRE number: $80,000 x 25 = $2,000,000
• Both partners aged 38, targeting retirement at 60 (22 years)
• 6% real return
• Combined portfolio: $420,000

Coast FIRE Number = $2,000,000 / (1.06)^22 = $588,000

They have $420,000 against a $588,000 target. Not there yet, but $168,000 away. They know exactly what to aim for.

Option 2: Individual calculations with separate timelines If one partner wants to retire at 57 and the other at 63, calculate each person's number separately using their own timeline and current balance. Each person has their own Coast FIRE threshold to hit.

Why Coast FIRE changes the couple dynamic: Once Coast FIRE is reached, both partners only need to earn enough to cover current expenses. One partner can shift to part-time or a lower-paying job without derailing the retirement plan. The portfolio carries the long-term weight.

Adjusting for Social Security and Pension Income

The standard 25x formula assumes your investment portfolio funds 100% of retirement expenses. If you expect Social Security benefits or a pension, your portfolio needs to cover less, which lowers both your full FIRE number and your Coast FIRE number.

Social Security adjustment:

Estimate your annual Social Security benefit at your target retirement age. Subtract it from annual retirement expenses before calculating the full FIRE number.

Example:

• Annual retirement expenses: $60,000
• Expected Social Security: $18,000/year
• Portfolio must cover: $42,000/year
• Adjusted full FIRE number: $42,000 x 25 = $1,050,000
• Coast FIRE number at age 35, retiring at 60: $1,050,000 / (1.06)^25 = $244,600

Social Security alone drops the Coast FIRE target from $349,500 to $244,600 in this example. The later you retire, the larger your benefit (claiming at 70 vs 62 can increase monthly payments by 76%), and the more your Coast FIRE number shrinks.

Pension adjustment:

Treat a guaranteed pension the same way. Subtract the annual pension income from retirement expenses before applying the 25x rule. A pension paying $25,000/year against $60,000 in expenses means the portfolio only needs to cover $35,000, cutting the full FIRE target to $875,000 and the Coast FIRE number proportionally.

For people still in the accumulation phase figuring out where to direct contributions, the Roth IRA Contribution Calculator shows how much you can add each year based on your income and filing status.

How to Calculate Coast FIRE in Excel

For those who prefer a spreadsheet:

1. Cell A1: annual retirement expenses (e.g., 60000)
2. Cell A2: full FIRE number: =A1*25
3. Cell A3: years until retirement (e.g., 25)
4. Cell A4: real return rate (e.g., 0.06)
5. Cell A5: Coast FIRE number: =A2/(1+A4)^A3

Change any cell and A5 updates immediately. To build a sensitivity table, put different current ages in a column and let Excel calculate the remaining years automatically. This is the same calculation the online coast fire number calculator runs behind the scenes.

What Comes After Reaching Coast FIRE

Reaching your Coast FIRE number does not mean stopping work entirely. It means the work you do is now optional for retirement funding. That distinction changes everything.

Shift careers or go part-time. The pressure to maximize income and contributions lifts. Many Coast FIRE households move one partner to part-time or switch to lower-paying work with better hours or meaning.

Geographic arbitrage. Since the portfolio handles retirement, covering current expenses on a lower income is the only requirement. Living in a lower-cost city or country reduces the income needed from work significantly.

Take on more risk in your career. Starting a business, going freelance, or taking a lower-paying role at a mission-driven organization is far less financially risky when retirement savings are already on track.

Stop optimizing, start living. A lot of FIRE planning involves deferring enjoyment. Coast FIRE is a logical stopping point for intense optimization. The portfolio coasts; so can you.

If full retirement without any income is the goal, the full FIRE number remains the target. If a low-stress job for benefits and spending money sounds appealing alongside a growing portfolio, Barista FIRE is a related framework that formalizes exactly that approach.

Common Mistakes When Calculating Your Coast FIRE Number

Using nominal rather than real returns. If you use an 8% nominal return but inflation runs at 3%, your real return is roughly 5%. Use real returns so the Coast FIRE number and your full FIRE target stay in the same (today's) dollar terms.

Not adjusting for healthcare before Medicare. Retiring at 55 means 10 years of private health insurance before Medicare eligibility at 65. Healthcare costs for a family can exceed $20,000/year. Build this into annual retirement expense estimates or the full FIRE number is understated.

Only counting retirement accounts. Taxable brokerage accounts, after-tax savings, and other investment balances all count toward your Coast FIRE number. The formula does not distinguish by account type.

Recalculating too rarely. A Coast FIRE number calculated at 30 based on $55,000 in projected expenses may not reflect reality at 38 after lifestyle changes, kids, or a new city. Recalculate once a year with updated expense estimates.

Confusing Coast FIRE with Lean FIRE. Coast FIRE is about portfolio self-sufficiency, not about minimizing expenses. You can Coast FIRE on a $5,000/month lifestyle. The number just scales with the expenses.

Frequently Asked Questions