Coast FIRE Calculator (Financial Independence)

Understanding Coast FIRE and financial independence

FIRE stands for Financial Independence, Retire Early. The idea is to save hard and invest the difference until work stops being mandatory. Most people aim for a FIRE number, which comes out of the 4% rule (sometimes called the 25x rule). You want roughly 25 times your yearly expenses invested, since pulling out 4% a year is generally thought to be sustainable. Where does the 4% come from? The Trinity Study (1998) tested a 50/50 stock-bond portfolio and found it lasted through 30-year retirements about 95% of the time when withdrawals were held at 4% and adjusted for inflation.

There are a few flavors of FIRE. Lean FIRE means a frugal life on a smaller portfolio, Regular FIRE assumes a middle-class budget, and Fat FIRE covers a comfortable, higher-spending lifestyle that needs a much bigger nest egg. Coast FIRE works on a different logic. It's the smallest invested net worth that, with no further contributions, grows on its own through compounding to hit your FIRE number by the time you retire. After you reach Coast FIRE you keep working to pay today's bills, but the saving-for-retirement part is done.

How the calculation works

The Coast FIRE number is just the present value of your FIRE number, discounted by your expected return across the years you have left before retirement: CoastNumber = FIRENumber / (1 + r)^years. r is the expected annual return and years is the time to retirement. Stick to a real return (after inflation, usually somewhere around 5%-7%) and pair it with a FIRE number stated in today's money, so the two match up. Combine a nominal return with today's expenses and you'll overstate how much things grow.

Worked example

Say your yearly expenses run $40,000, which puts your FIRE number at 25 × 40,000 = $1,000,000. You're 35, you want to retire at 65 (so 30 years to go), and you pencil in a 5% real return. That gives CoastNumber = 1,000,000 / (1.05)^30 ≈ 1,000,000 / 4.322 ≈ $231,400. Already sitting on roughly $231k invested? You could stop contributing today and still land at $1M by 65 on growth alone.

Frequently asked questions

Is the 4% rule still safe? As a planning baseline, yes. But people retiring early with a 40-50 year runway tend to dial it down to a more cautious 3.25%-3.5% to soften sequence-of-returns risk.

What return should I assume? A stock-heavy portfolio is often modeled with a 5%-7% real return. Assume less and your Coast FIRE number comes out higher, which is the safer side to err on, so leaning conservative usually pays off.