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Coast FIRE calculator
How much do you need invested today so that compounding alone — no more saving — reaches your retirement target on time? Drag the sliders; the math updates as you move.
Assumptions (return, withdrawal rate)
Everything is in today’s dollars: the 5% return is after inflation, and the target is your spending divided by the 4% withdrawal rate. Simplified on purpose — see “What this calculator skips” below.
What is Coast FIRE?
Most retirement planning focuses on saving enough to cover all your expenses forever. Coast FIRE flips the script: you save until your investments are big enough that compounding finishes the job by your target retirement age. From that point you only need to cover your living expenses — no more mandatory saving. It’s especially powerful for younger savers, because more years of compounding mean a smaller coast target.
Want the full picture — where the idea comes from, the trade-offs, and worked examples? Read our Coast FIRE guide.
How the math works
Two steps. First, your retirement target (your FIRE number) is your annual retirement spending divided by your withdrawal rate — at the classic 4% rule, $60,000 a year needs $1.5 million. Second, the coast target discounts that number back to today:
Coast target = FIRE number / (1 + real return)years to retirement
Everything stays in today’s dollars — the return you set is a real (after-inflation) return, so the answer doesn’t quietly inflate itself. If you haven’t reached the target yet, the calculator also projects your balance forward with your monthly saving and finds the first year it crosses the (rising) threshold — the age you could stop saving.
What this calculator skips
This is a deliberately simple model: one flat real return, annual compounding, and no taxes, Social Security, market sequence risk, or partner. Those aren’t rounding errors — Social Security alone can move a coast plan by years, and the account your money sits in (401(k), Roth, taxable) changes what “spending $60K” actually costs.
The full Calcifir planner models all of it — real market history, taxes in all 50 states, Social Security timing, and your household — and it’s free to start. Build your real plan.
Common questions
Does reaching Coast FIRE mean I can retire?
No — it means you could stop saving and still retire on time, as long as your income covers your living expenses until then. Retiring outright needs the full FIRE number.
What return should I assume?
The default is 5% real (after inflation) — in the neighborhood of long-run diversified stock-heavy portfolios, net of inflation. Lower it to stress-test: a coast plan leans entirely on compounding, so it’s more sensitive to the return assumption than a normal savings plan.
Why does my coast number rise as I get older?
Fewer years to retirement means fewer doublings left, so more of the final number has to already be in the account. At retirement itself the coast target equals the full FIRE number.
More free tools: FIRE number calculator · 4% rule calculator · quick retirement check