Coast FIRE Calculator: How Much Do You Need to Invest Today to Retire Early?

Most people think about FIRE as a single destination — the number you need in your portfolio to quit working forever.

Coast FIRE reframes the entire question. Instead of asking "when can I stop working?", it asks: "when can I stop trying so hard?"

It's one of the most liberating milestones in personal finance, and most people hit it far earlier than they expect.

What is Coast FIRE?

Coast FIRE is the point at which you have enough invested that — even if you never contribute another penny — compound growth alone will carry your portfolio to your full FIRE number by traditional retirement age.

Once you've hit your Coast number, you only need to earn enough to cover your current living expenses. You've already taken care of future you. You can "coast" to retirement.

The name comes from the image of cycling up a steep hill, then reaching the crest and coasting down the other side — no more pedalling required.

The Coast FIRE Formula

Your Coast FIRE number depends on three things:

1. Your full FIRE target — annual expenses × 25
2. Your expected annual investment return — typically 6–7% real (inflation-adjusted)
3. Years until traditional retirement — usually to age 60 or 65

Coast FIRE Number = FIRE Target ÷ (1 + annual return)^years to retirement

This is just discounted present value — asking: "what lump sum, invested today at 7% per year, will grow to my FIRE target by retirement?"

Real Examples at Different Ages

Let's use a FIRE target of £1,000,000 (annual expenses of £40,000 × 25) and a 7% annual return.

If you start investing at 25

With 40 years until age 65, compound growth does extraordinary work.

• Coast FIRE Number = £1,000,000 ÷ (1.07)^40 = £66,780

Invest £66,780 by age 25 — roughly £22,000/year for 3 years with a decent salary — and you never need to invest another pound for retirement. Everything after that is just covering today's expenses.

If you reach Coast at 30

35 years of compounding.

• Coast FIRE Number = £1,000,000 ÷ (1.07)^35 = £94,561

Save aggressively in your mid-to-late 20s, hit this number, and the retirement question is settled.

If you reach Coast at 35

30 years of compounding.

• Coast FIRE Number = £1,000,000 ÷ (1.07)^30 = £131,367

Still achievable with a few years of high savings rates — and far less than the full £1 million.

If you reach Coast at 40

25 years of compounding.

• Coast FIRE Number = £1,000,000 ÷ (1.07)^25 = £184,249

The pattern is clear: every year you delay roughly adds £15,000–20,000 to your Coast number. Time is the most expensive resource in investing.

Why Coast FIRE Is So Powerful

It decouples saving from earning

Most financial advice says: earn more, save more, invest more. Coast FIRE creates a finish line for the saving phase. Once you've hit it, you can take a lower-paying job you love, go part-time, start a business with uncertain income — because your retirement is already funded.

It makes career risk feel smaller

Many people stay in jobs they hate because of retirement anxiety. Coast FIRE eliminates that specific fear. You've already solved the retirement problem; now you're just choosing how to spend the next 30 years.

It's achievable for most people

Unlike full FIRE — which often requires a decade or more of high savings rates — Coast FIRE can be reached in 3–5 years with focused effort in your 20s or early 30s.

How to Calculate Your Coast FIRE Number

1. Calculate your FIRE target: Take your expected annual expenses in retirement × 25
2. Choose your retirement age: Typically 60 or 65
3. Count the years: Years between now and your target retirement age
4. Apply the formula: FIRE Target ÷ (1.07)^years

SpendGuard's FIRE Planner does this automatically. It uses your real monthly expenses from your transaction history to calculate your FIRE target, then projects your Coast FIRE milestone based on your current portfolio and contribution rate.

When your portfolio hits the Coast threshold, SpendGuard marks it as a milestone — one of the more satisfying notifications in personal finance.

The Caveat Worth Knowing

Coast FIRE assumes your investments grow at a steady rate. Real markets don't behave that way. A 30% drop in your early 40s could push your Coast number back significantly.

Most Coast FIRE practitioners use a conservative 6% real return (rather than 7%) to build in margin. SpendGuard's Monte Carlo simulation runs 1,000 scenarios to show you the probability range — not just the optimistic single path.

The destination is real. The math is sound. The earlier you start, the less you need. That's the whole point.