Coast FIRE Calculator

Introduction

Coast FIRE is the sweet spot of the financial independence movement: the moment your current savings have grown enough that, left entirely untouched, they will reach your retirement target on their own by the time you stop working. From that point forward, you do not need to add another dollar to retire comfortably — you can "coast." Every paycheck after that can fund today's lifestyle rather than tomorrow's retirement, which is why the strategy is named after the idea of a car cresting a hill and coasting the rest of the way[nerdwallet-coastfire].

The idea is simple but the math is counterintuitive. Because compound growth is exponential, a modest lump sum saved in your twenties and thirties can rival contributions made throughout a career. Someone who accumulates $93,663 at age 30 and never contributes again — earning a real 7% annual return — reaches $1,000,000 by 65, the classic 4%-rule FIRE target for $40,000 of annual spending. The money does the work that saving would otherwise have to do[fioneers-coastfi].

This Coast FIRE Calculator turns that concept into a personal number. You enter your current age, your planned retirement age, your savings today, the annual spending you want to cover in retirement, the real return you expect, and the withdrawal rate you plan to live on. The tool instantly returns your Coast FI number — the amount you need in today's dollars — plus your FIRE target, your current savings' projected value, and, if you are not there yet, exactly how much to contribute monthly to reach Coast FIRE.

The distinction from full FIRE matters. Traditional FIRE requires you to keep saving aggressively until the day you quit. Coast FIRE lets you switch from "accumulating for retirement" to "coasting to retirement," which frees up cash flow for a career change, a smaller paycheck, or simply a less stressful life[investopedia-coast]. Before you trust the number, make sure the snapshot underneath it is accurate — run the Net Worth Calculator first to establish your real current savings, then use the Retirement Savings Gap Calculator to check how your projected income compares with what you will actually spend.

How to Use

The calculator runs automatically as you adjust any field. Start with your current age and your target retirement age — the number of years between them is the compounding window, and it drives everything. Next enter current savings: the total of your taxable investments, retirement accounts, cash, and any other money earmarked for retirement. Then enter annual spending, the amount you expect to withdraw from savings each year in retirement, denominated in today's dollars. The annual return is your expected real (after-inflation) investment return; 7% is the common historical assumption for a stock-heavy portfolio, while 4-5% suits a more conservative mix. Finally, set your withdrawal rate — 4% is the classic rule, 3% is more conservative, and higher rates require a larger FIRE number.

Three worked examples show how the tool behaves.

Example 1 — A 30-year-old with $0 saved. With retirement at 65, $40,000 of annual spending, a 7% real return, and a 4% withdrawal rate, the FIRE target is $40,000 ÷ 0.04 = $1,000,000. Over 35 years of growth, $1 grows to $10.68, so the Coast FI number is $1,000,000 ÷ 10.68 = $93,663. Our saver needs to accumulate roughly $93,700 by age 30 — and then stop. Saving $52 a month from now would still reach that Coast number by 65, because the tool accounts for the full window.

Example 2 — A 40-year-old who is already behind. Starting at 40 with $30,000 saved and the same $1,000,000 target, only 25 years of growth remain: $1 grows to $5.43, so the Coast number is $184,250. The gap is $154,250, and with a 7% annualized monthly return over 300 months, the saver needs about $270 a month to catch up to Coast FIRE by 65. Starting a decade later roughly doubles the monthly requirement — time is the multiplier Coast FIRE depends on.

Example 3 — Already coasting. A 45-year-old with $100,000 saved, spending $40,000, expecting 7%, retiring at 65: the Coast number is $64,605 per $10,000 of spending — $258,420 for $40,000 — but their current $100,000 grows to $386,968 by 65, well past the target. The status flips to Reached, and the monthly figure drops to $0. They have hit Coast FIRE and every future dollar is optional. To stress-test the growth assumption behind this, run the Future Value Calculator with different return rates and horizons.

The Formula

Coast FIRE inverts the standard future-value calculation. Instead of asking "what will my current savings grow to?", it asks "what must I have today so that my savings grow to my FIRE number by retirement?"

Here r is the annual real return (as a decimal, so 7% is 0.07) and n is the number of years from today to retirement. The denominator, (1 + r)^n, is the compound growth factor: how many times one dollar multiplies over the horizon. Dividing the future target by that factor converts tomorrow's dollars into today's.

If you are below your Coast number, the tool also computes the monthly contribution needed to close the gap:

This is the standard sinking-fund formula: it finds the monthly payment whose future value, at the monthly interest rate, equals the shortfall. The shorter your horizon or the lower your assumed return, the larger the payment. If your savings already exceed the Coast number, the shortfall is zero and the monthly requirement is $0 — you are coasting.

Because everything hinges on compound growth, the underlying assumptions deserve scrutiny. The SEC's compound interest calculator is a good place to visualize how sensitive the final figure is to both the rate and the horizon[investor-gov-compound]. Halving the horizon from 40 to 20 years roughly triples the Coast number at a 7% return; dropping the return assumption from 7% to 4% has a similar effect. The choice of withdrawal rate matters just as much: at 3% instead of 4%, the FIRE target for $40,000 rises from $1,000,000 to $1,333,333.

Reference Table

The table below shows the Coast FI number for $10,000 of annual spending at a 7% real return, retiring at 65, with a 4% withdrawal rate. Multiply the figure by your actual spending in tens of thousands. For $40,000 of spending, multiply by 4.

Read the table as a cost of waiting. At 25, $16,695 is enough to cover $10,000 of retirement spending forever; at 60, the same $10,000 of spending needs $178,247 — more than ten times as much. The relationship is exponential because it inverts the exponential growth of the portfolio. This is the single most important takeaway for young savers: the cheapest dollar you will ever save for retirement is the one you save this year, not the one you save in your fifties. The numbers assume a constant 7% real return, which matches the long-run real return of the U.S. stock market but is not guaranteed in any given decade.

The power of the concept is that the Coast number is an alternative to, not a replacement for, your FIRE target. It tells you the minimum savings that lock in retirement, so that every contribution beyond it shortens the path or increases the margin[smartasset-coastfire]. Whether the strategy suits you depends on your tolerance for relying on the market's long-run average, which the Compound Interest Calculator lets you explore at different rates and timeframes.

Practical Tips

• Establish your real savings before trusting the number. The Coast figure is only as good as the current-savings input. Use the Net Worth Calculator to inventory every investment and retirement account, and remember that this tool should count only money that will actually grow untouched until retirement.
• Use a real return, not a nominal one. Inflation is the silent tax on Coast FIRE. Modeling 7% nominal growth when the market returns 7% after 3% inflation leaves you 3% short of plan. Quit Like a Millionaire recommends planning on a conservative real return precisely because the whole strategy rests on it[quit-like-a-millionaire].
• Stress-test with 4% and 5% returns. Coast FIRE works spectacularly at 7% real returns and collapses at 2%. Run the calculator at several rates and plan for the worst case that still leaves you comfortable — the strategy has no second chance if your retirement window arrives short.
• Do not confuse "coasting" with "not saving." Reaching your Coast number means you no longer need to save for retirement, but every extra dollar you save still buys you earlier retirement or a larger cushion. Many coasters continue contributing modestly to retain the habit and the margin.
• Recheck the number every couple of years. Your spending, portfolio value, and horizon all drift. Re-running the tool every two years keeps the target honest, especially after salary changes that raise or lower the lifestyle you plan to fund.
• Plan the withdrawal rate deliberately. A 3% withdrawal rate gives a much larger FIRE number than 4% but leaves a bigger margin against market downturns and long retirements. The higher your spending in retirement, the more the choice matters.

Limitations

Coast FIRE rests on assumptions that deserve healthy skepticism. The 7% real return is the long-run average of the U.S. stock market, but it is not a guarantee — over a specific 35-year window, the realized return can be materially lower, especially if the early years coincide with a market crash (the "sequence of returns" problem). The strategy assumes you can and will leave the savings untouched from today until retirement, which is harder than it sounds when life presents cars, houses, children, or health emergencies. It also assumes your retirement spending in today's dollars stays constant; if your planned lifestyle grows faster than inflation, the Coast number is understated.

The withdrawal-rate choice is a model, not a law. The classic 4% rule was derived from U.S. data over roughly a century and may not survive different market conditions or a 40-year retirement. Quit Like a Millionaire argues the rule is too aggressive for many situations, which is why the calculator lets you change it[quit-like-a-millionaire]. The tool also ignores taxes: drawing $40,000 from a traditional 401(k) costs more than the headline number if withdrawals push you into a higher bracket, so plan in after-tax terms.

Finally, Coast FIRE is a savings benchmark, not a retirement plan. It does not model Social Security, pensions, part-time income, or health insurance costs, all of which change the real number you need. Pair the figure with the Retirement 401k Calculator to project actual account balances under different contribution and employer-match scenarios, and with the Present Value Calculator to translate any future lump-sum goals — a child's college, a second home — into today's dollars before you decide how aggressive to be.

Frequently Asked Questions

❓ What is Coast FIRE?

✅ Coast FIRE is the point at which your current savings, left untouched and growing at a projected real return, will reach your retirement target by the time you stop working. From that moment, you no longer need to contribute to retirement savings to retire comfortably.

❓ How is the Coast FI number calculated?

✅ Divide your annual retirement spending by your withdrawal rate to get your FIRE target, then divide that target by the compound growth factor (1 + return) raised to the number of years until retirement. The result is the savings you need today.

❓ What is a good Coast FIRE return assumption?

✅ Most planners use a 7% real (after-inflation) annual return, matching the long-run average of the U.S. stock market. Conservative planners use 4-5%. Because the number is very sensitive to this input, run the calculation at several rates.

❓ What is the difference between Coast FIRE and traditional FIRE?

✅ Traditional FIRE requires saving aggressively until you reach your full retirement number. Coast FIRE requires saving only until your portfolio can grow to that number on its own — after which you can stop contributing and still retire on schedule.

❓ Do I have to stop saving at Coast FIRE?

✅ No. Reaching your Coast number means further retirement contributions are optional, not forbidden. Additional savings buy an earlier retirement, a bigger buffer, or a more comfortable withdrawal rate. Many coasters keep saving small amounts for margin.

❓ Does Coast FIRE account for inflation?

✅ It should. Use a real (after-inflation) return rather than a nominal one so that your retirement spending stays expressed in today's dollars. Using a nominal return with today's spending overstates your progress.

❓ Is a 4% withdrawal rate safe?

✅ The 4% rule was derived from historical U.S. data and is widely used but not a guarantee. Conservative planners prefer 3%, especially for long retirements or uncertain markets. The calculator lets you adjust the rate and shows how much the target changes.

❓ What if my retirement age equals my current age?

✅ The calculator requires retirement age to be strictly greater than current age. With zero years of growth the Coast number would equal the full FIRE target, which defeats the purpose of the strategy — you would already need the complete sum today.

❓ How do I include Social Security?

✅ This tool does not model Social Security, pensions, or part-time income. If you expect to receive them, reduce the annual spending figure by the amount they will cover, then run the calculator on the remaining gap.

❓ Where does the concept come from?

✅ Coast FIRE emerged from the FIRE (Financial Independence, Retire Early) community in the late 2010s as a variant for people who enjoy their work but want the security of knowing retirement is locked in. Books like Quit Like a Millionaire and blogs like ChooseFI helped popularize it.