Debt, investing and retirement

Coast FIRE Calculator: Could You Stop Saving for Retirement Today?

KOLLAZ · Updated October 1, 2026

Updated September 2026. This article is for educational purposes only and is not financial advice.

What if the hardest part of saving for retirement is already behind you? Not “am I rich enough to retire”, a smaller, sharper question: is the money you have already invested enough that compound growth alone will carry it to your retirement number, with zero further contributions? A coast FIRE calculator answers exactly that.

The math is almost rude in its simplicity. Your retirement target is 25 times your annual spend. Discount that number back to today at your expected real return. If what is invested now sits at or above the discounted number, you are coasting, you can stop saving for retirement and just cover today's bills.

What is Coast FIRE, actually?

Coast FIRE is the point where your invested savings are large enough that compound growth alone will carry them to your full retirement number by your target retirement age, with zero further contributions. Once you hit it, you only need to earn enough to cover today's bills; the aggressive saving phase is over.

It is not retirement, you still work. And it is not a promise, the growth is assumed, not guaranteed.

How does the Coast FIRE calculator formula work?

Four steps:

1. Set your FI number. Multiply your target annual retirement spend by 25. That is the 4% rule flipped around: $50,000 a year means a $1,250,000 target.
2. Discount it to today. Divide that target by (1 + your expected real return) raised to the years until retirement. Seven percent real over 25 years shrinks $1,250,000 to $230,311.
3. Compare it to what is invested now. At or above the coast number: you are coasting. Below it: the gap is the exact dollar shortfall, measured today.
4. Read the verdict. “You're already coasting” or “Not coasting yet, you need $X more invested today.” Plus your crossover age: when your current balance reaches the full number untouched.

What does a worked example show with $50,000, 7%, and $230,311?

Take Maya. She is 35, has $150,000 invested, wants to spend $50,000 a year starting at 60, and assumes a 7% real return.

Her FI number is $1,250,000 (50,000 × 25). She has 25 years of compounding ahead. Discounted: 1,250,000 ÷ 1.07²⁵ = $230,311. That is her coast number: left alone at 7% real, it becomes $1,250,000 exactly when she turns 60.

She has $150,000. Progress: 150,000 ÷ 230,311 = 65.1%. Gap: $80,311. Verdict: not coasting yet, she needs $80,311 more invested today. Every extra dollar she invests now is worth about $5.40 at 60, because 1.07²⁵ ≈ 5.43. That is the part most people miss: at 35, each dollar added buys five dollars of retirement.

Her crossover age is 66: if she never saves another dollar, her $150,000 still reaches $1,250,000, six years after her target. That gap is exactly what the $80,311 closes.

One more honest note: the number depends on years of runway, not birthdays. A 40-year-old targeting 65 with the same spend and return gets the same $230,311. Time does the discounting, not age.

I built the Coast FIRE Calculator to run exactly this math: five inputs in, coast number, progress, gap, crossover age, and a 40-year projection with the coast line drawn on it. It is an Excel spreadsheet with no macros, US$6.99, and it does one job.

What is the 4% rule behind the math?

The 25× target comes from the 4% rule. Financial planner William Bengen published the original research in the October 1994 issue of the Journal of Financial Planning: testing every rolling 30-year period from 1926 onward, a 4% initial withdrawal (adjusted for inflation each year) survived even the worst historical starting year. He called it the SAFEMAX rate. Four years later, three Trinity University professors extended the analysis in what became the Trinity Study: on a 50/50 stock-and-bond mix, 4% worked about 95% of the time over 30-year periods.

“About 95%” is doing real work in that sentence. The studies assume a 30-year horizon, U.S. historical returns, no taxes, and no fees. Early retirees face 40- to 50-year horizons, where most research puts the conservative number closer to 3.25–3.5%. And the 2024–25 updates genuinely diverge: Bengen himself raised his number to 4.7% with broader diversification, while Morningstar's forward-looking models cut theirs to 3.7% for new retirees. The coast number is a planning lens, not a promise, which is why the calculator lets you set your own return instead of hard-coding optimism.

What does “Coasting” get wrong?

Three traps worth naming.

“I can quit my job.” No. Coasting stops the saving, not the earning. You still need to cover today's bills in full. What changes is the pressure: the retirement account no longer needs feeding, so a lower-paying job you actually like stops being a financial crime. If the number is close but not quite there, the fix is partly saving and partly income, running the real math on a second income is the natural next calculation.

“The return is guaranteed.” The whole number pivots on one assumption. At 6% real, Maya's coast number jumps to about $291,000; at 8%, it falls to about $183,000. One percentage point moves the answer by roughly $50,000. Run it at 6% if you want a cushion. The honest version of this math always shows its sensitivity.

“Closer to retirement means an easier number.” It is the reverse. Fewer years of compounding means a bigger lump sum today. A 25-year-old targeting $1 million at 65 needs only about $67,000 invested; a 40-year-old needs about $184,000. Starting early is the entire trick, which is also why the milestone feels reachable. The number is small when you are young.

The same one-number thinking works closer to home: an emergency fund calculator turns savings divided by essentials into months of runway, Coast FIRE is that idea on a decades-long clock.

About Kollaz, Kollaz makes smart financial tools for modern money. This article's companion is the Coast FIRE Calculator spreadsheet: find your coast number, your progress, your gap, and your crossover age from five inputs.

Sources

• Wikipedia, William Bengen (opened and read September 2026): Bengen's October 1994 Journal of Financial Planning paper “Determining Withdrawal Rates Using Historical Data”; the 4% SAFEMAX rate as a worst-case historical finding; the Trinity Study (1998) popularizing it. https://en.wikipedia.org/wiki/William_Bengen
• ChooseFI, “Am I Coast FIRE? How to Calculate When You Can Stop Saving (2026)” (opened and read September 2026): Coast FIRE definition as the point where invested savings grow to the full FI number without further contributions; the discount formula FI ÷ (1+r)^n; 7% real as the standard assumption; worked examples by age. https://choosefi.com/financial-independence/coast-fire
• CompoundLadder, “4% Rule 2026: Bengen's 4.7% Safe Withdrawal Update” (opened and read September 2026): Trinity Study success table (4% ≈ 95% on a 50/50 mix over 30 years); the 2024–25 revisions (Bengen 4.7%, Morningstar 3.7%, Vanguard 3.5–4.5%); the rule's assumptions, no taxes, no fees, 30-year horizon. https://www.compoundladder.com/guides/4-percent-rule