Coast FIRE Calculator

Coast to Financial Independence

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Understanding Coast FIRE

What is Coast FIRE?

Coast FIRE answers one simple question: how much do I need invested today so that it grows to fully fund retirement — without contributing another dollar? Once you reach your Coast FIRE number, your existing investments are projected to compound into your full FIRE number by retirement age, all on their own.

Coast FIRE shifts the focus from "How much do I need to save?" to "How long can compounding do the work for me?" Once you cross that threshold, any additional income can go toward lifestyle upgrades, reduced work hours, a career change, or other goals — because your retirement is already handled.

Step 1: Calculate Your FIRE Number

First, determine how much you need at retirement. The standard FIRE formula is:

FIRE Number=Annual SpendingSafe Withdrawal Rate\text{FIRE Number} = \frac{\text{Annual Spending}}{\text{Safe Withdrawal Rate}}

For example, if Miguelito expects to spend $60,000 per year in retirement and uses a 4% SWR:

FIRE Number=$60,0000.04=$1,500,000\text{FIRE Number} = \frac{\$60{,}000}{0.04} = \$1{,}500{,}000

That means Miguelito would need $1.5 million invested at retirement to sustainably withdraw $60,000 per year. The commonly used 4% withdrawal rule comes from research known as the Trinity Study, which analyzed historical retirement outcomes using diversified portfolios.

Step 2: Work Backwards Using Compound Growth

Now we determine how much you need invested today for that amount to grow to your FIRE number by retirement. The compound growth formula is:

A=P×(1+r)tA = P \times (1 + r)^t

Where A is the final amount (your FIRE number), P is the present amount (what we're solving for), r is the annual rate of return, and t is the years until retirement. Rearranging to solve for P:

P=A(1+r)tP = \frac{A}{(1 + r)^t}

Substituting the FIRE number for A gives us the Coast FIRE formula:

Coast FIRE Number=Annual SpendingSWR×(1+r)t\text{Coast FIRE Number} = \frac{\text{Annual Spending}}{\text{SWR} \times (1 + r)^t}

This tells you the minimum amount you need invested today so that it compounds into your full FIRE number by retirement age. For Miguelito at age 30 with a 4% real return and 35 years to retirement:

Coast FIRE Number=$60,0000.04×(1.04)35=$60,0000.04×3.946≈$380,000\text{Coast FIRE Number} = \frac{\$60{,}000}{0.04 \times (1.04)^{35}} = \frac{\$60{,}000}{0.04 \times 3.946} \approx \$380{,}000

If Miguelito has $380,000 invested today, he never needs to save another dollar for retirement — compound growth will do the rest.

Accounting for Inflation

Inflation reduces purchasing power over time, so it must be included in long-term projections. Instead of inflating future expenses separately, the calculator adjusts your investment return by subtracting inflation:

rreal=rnominal−rinflationr_{\text{real}} = r_{\text{nominal}} - r_{\text{inflation}}

For example, with an expected return of 7% and inflation of 3%:

rreal=7%−3%=4%r_{\text{real}} = 7\% - 3\% = 4\%

All values are therefore expressed in today's dollars, which simplifies planning and keeps spending assumptions consistent. Assets like stocks and real estate tend to rise with inflation, while cash loses value — this is why having your money invested is crucially important for long-term wealth preservation. Historically, the S&P 500 has averaged around 10% annual returns before inflation, though future returns are never guaranteed.

What Reaching Coast FIRE Means

If your current invested assets are equal to or greater than your Coast FIRE number:

  • You no longer need to save for retirement
  • Your existing investments are projected to grow to your full FIRE number by retirement age
  • Any additional income can be used for lifestyle upgrades, reduced work hours, or other goals

For Miguelito, reaching Coast FIRE at 30 means he could switch to a lower-paying job he loves, go part-time, or take a sabbatical — all without jeopardizing his retirement. His invested assets are working for him in the background.

Using This Calculator

Fill in the inputs and click Calculate. The graph shows two lines:

  • Green line (Net Worth) — the projected growth of your current investments over time, including your monthly contributions
  • Blue line (Coast FIRE Number) — the amount you would need at each age to coast to retirement without further contributions. This line decreases as you get closer to retirement (because there's less time for growth, you need more already saved)

When the green line crosses above the blue line, you've reached Coast FIRE! Here's what each input means:

  • Current Age — your age today, the starting point for the projection
  • Retirement Age — when you plan to retire
  • Annual Spending in Retirement — how much you expect to spend per year in retirement (often less than current spending since mortgage may be paid off and Medicare covers healthcare)
  • Current Invested Assets — your total invested assets like stocks, bonds, and retirement accounts (exclude emergency fund and real estate unless you plan to sell)
  • Monthly Contribution — how much you save and invest each month
  • Investment Rate of Return — the average annual return you expect (default 7%, conservative; S&P 500 has historically averaged ~10%)
  • Inflation Rate — the expected annual inflation (U.S. historical average is about 3%)
  • Safe Withdrawal Rate — the percentage you plan to withdraw each year in retirement (4% is the most common recommendation from the Trinity Study)