Most retirement math asks how much you must save. The Coast Calculator flips the question: what if you never saved another dollar? It takes the money you have already invested, grows it forward at your expected return, and checks whether that growth alone reaches your retirement target.

The calculator above gives you three things: a straight yes-or-no coast verdict, the projected value of your savings at retirement, and the exact lump sum you would need today to close the gap if you fall short. This guide explains the math, walks through four worked examples with the tool's real numbers, and shows how to read the result honestly.

What Does the Coast Calculator Do?

You enter four numbers: your current invested amount, your expected annual return, your target retirement amount, and the years until you retire. The calculator compounds your savings forward and compares the result with your target.

If the projected value meets or beats the target, you can coast — no further contributions are needed. If it falls short, the calculator tells you exactly how far short and what lump sum invested today would fix it.

How to Use the Coast Calculator

Start with your current invested amount: the total in retirement accounts, brokerage, or any portfolio earmarked for retirement. Use invested money only — cash in a checking account earns nothing here.

Pick an expected annual return. A realistic long-run average after inflation is 5 to 7 percent for a stock-heavy portfolio. Then enter your target retirement amount and the years left, press Calculate, and read the headline verdict first, then the rows.

What "Coasting" Actually Means

Coasting means your money does the remaining work. Picture a cyclist who has pedaled up to speed and can now glide: the pedaling was the saving, and the glide is compound growth carrying you the rest of the way.

It does not mean quitting your job. Most people who coast keep working; they simply stop directing new money into retirement accounts and redirect it toward today — travel, family, a calmer schedule, or a career they actually enjoy.

The Compound Growth Formula Behind the Verdict

The calculator's entire engine is one formula: compound growth with zero contributions. The formula is:

Future value = P x (1 + r)^t

Here P is your current invested amount, r is your expected annual return as a decimal, and t is the years until retirement. Plug in $80,000, 7 percent, and 20 years: $80,000 x 1.07^20 equals $309,574.76. The calculator compares that number against your target and issues the verdict.

Worked Example: $80,000 Today, 20 Years at 7%

A 45-year-old has $80,000 invested, expects 7 percent returns, wants $300,000 by age 65, and has 20 years left. Growing the savings: $80,000 x 1.07^20 equals $309,574.76.

Comparing with the target: $309,574.76 beats $300,000, so the verdict is Yes, you can coast. The surplus is $9,574.76, and the required lump sum today was only $77,525.70 — she already holds more than that. She can stop contributing today and still land her number at 7 percent returns.

Worked Example: Shortfall of $158,536 With 12 Years Left

A 53-year-old has $120,000 invested, expects 6 percent returns, targets $400,000, and retires in 12 years. Growing the savings: $120,000 x 1.06^12 equals $241,463.58.

Comparing with the target: $241,463.58 falls short of $400,000, so the verdict is Not coasting yet. The shortfall is $158,536.42, and the required lump sum today is $198,787.75. Continued contributions or a higher-return portfolio are the realistic fixes.

Worked Example: Can a $50,000 Windfall Make You Coast?

A 40-year-old inherits $50,000, invests it all, expects 8 percent returns, targets $500,000, and has 25 years to retirement. Growing the windfall: $50,000 x 1.08^25 equals $342,423.76.

Comparing with the target: $342,423.76 is below $500,000, so the verdict is Not coasting yet. The shortfall is $157,576.24, and the required lump sum today would have been $73,008.95. The windfall covers more than two-thirds of what was needed — modest additional saving bridges the rest.

Why the Required Lump Sum Today Matters

The most underused row is the required lump sum today. It translates your whole retirement goal into one present-day number: invest this much now, at your assumed return, and you are done.

Compare your actual invested amount against it directly. In the first example the required lump sum was $77,525.70 against an actual $80,000 — coasting confirmed in one glance. In the windfall example it was $73,008.95 against $50,000 — a clear, quantified miss.

Coasting vs. Traditional Retirement Saving

Traditional planning asks for a monthly contribution, year after year. Coasting asks for a balance — one number, sitting invested, growing on its own. The strategies suit different life stages.

Coasting favors the early saver. A dollar invested at 35 has 30 years to compound; the same dollar at 55 has 10. Early savers reach their coast number while young, then enjoy decades of optional contributions.

The Return Rate: The Number That Changes Everything

Run the first worked example at 5 percent instead of 7, and $80,000 grows to only $212,263.82 over 20 years — suddenly the $300,000 target is $87,736.18 short and the verdict flips to no. Two percentage points reversed the entire answer.

Use a return you would still believe after a decade of mediocre markets, then run the numbers again two points lower. If you coast at both, your margin is real. If only the optimistic run says yes, you are not coasting — you are hoping.

Common Coast Calculator Mistakes

The biggest mistake is entering total net worth instead of invested assets. Home equity, cars, and emergency funds are not compounding at 7 percent. Only count money that is actually invested for retirement.

The second mistake is using a pre-inflation return against a target stated in today's dollars. If your target is $300,000 in today's spending power, your return must also be stated after inflation — otherwise the comparison mixes two different currencies of value.

Where Coasting Is Useful

Coasting is most useful at decision points. Considering a pay cut for a better job? Check whether you coast first — the answer changes the risk. Planning a sabbatical? The verdict tells you whether the portfolio can survive the pause.

Parents use it to justify stepping back during expensive childcare years, when contributions are painful and compound growth still has decades to run. The coast number defines "enough," and enough is the beginning of choice.

How to Interpret Your Result

Read the verdict first, but do not stop there. A "yes" with a $9,574.76 surplus is fragile; a "yes" with a $200,000 surplus is robust. The size of the margin is the real message.

A "not yet" verdict comes with two numbers that matter: the shortfall, which tells you the scale of the miss, and the required lump sum today, which tells you the one-time fix. Compare the required lump sum with what you actually have — the ratio shows how far along the journey you are.

Worked Example: Late Starter at 50 With $150,000

A 50-year-old has $150,000 invested, expects 6 percent returns, targets $600,000, and retires at 65 — 15 years of growth. Growing the savings: $150,000 x 1.06^15 equals $359,483.73.

Comparing with the target: $359,483.73 is well below $600,000, so the verdict is Not coasting yet. The shortfall is $240,516.27, and the required lump sum today was $250,359.04 — she holds about 60 percent of what her goal demands. Time is the constraint here; continued contributions or a slightly later retirement are the honest remedies.

What to Do If the Verdict Says "Not Yet"

You have four levers: save more now, earn a higher return, lower the target, or give compounding more time. The calculator's shortfall row tells you exactly how much each lever needs to move.

Saving more now is the lever you control completely — every extra dollar compounds for the full remaining period. Chasing higher returns is the riskiest lever; extending the timeline by two or three working years lets compounding do heavy lifting. Most real plans mix all four.

Frequently Asked Questions

1. What is a coast number?

Your coast number is the amount you need invested today to grow to your retirement target without further contributions. In the first example it was $77,525.70: invest that at 7 percent for 20 years and it becomes $300,000. Once your balance exceeds your coast number, you can coast.

2. How is this different from the 4% rule?

The 4% rule answers how much you need at retirement to fund spending — roughly 25 times annual expenses. The Coast Calculator answers a different question: whether the money you have now will grow into that number by itself. Use the 4% rule to set the target, then this calculator to check whether you can coast to it.

3. Does coasting mean I stop working?

No. Coasting means you can stop contributing to retirement savings; most people who coast keep working and earning. Your paycheck no longer needs to fund retirement accounts — the invested balance is already doing that job through compound growth.

4. What return rate should I enter?

Use a realistic long-run real return: 5 to 7 percent for a stock-heavy portfolio, 3 to 4 percent for a conservative one. Then test two points lower. If you still coast at the lower rate, your verdict is solid; if not, treat the "yes" as fragile and keep saving.

5. Should I use pre-tax or post-tax amounts?

Be consistent: if your invested amount is pre-tax (like a 401(k) balance), your target should also be pre-tax, because withdrawals will be taxed. Mixing a pre-tax balance with an after-tax spending target understates what you really need.

6. Does inflation affect the result?

Yes. The calculator compares your projected value with your target, so both must be in the same dollars. Enter a real (after-inflation) return and a target in today's dollars. A 7 percent nominal return with 3 percent inflation is a 4 percent real return — a very different verdict.

7. Can I coast if I am already retired?

Not cleanly. The question changes from "will my savings grow to the target" to "how much can I safely withdraw." Once you start drawing down, the calculator's no-contribution assumption breaks, since withdrawals actively shrink the balance.

8. What if my investments earn an uneven return?

Real returns bounce around the average, and a bad sequence early on hurts more than the average suggests. Pad your target by 10 to 20 percent, or test at a lower rate, to build a buffer for sequence-of-returns risk.

9. Is a "yes" verdict a guarantee?

No. It is a projection based on assumptions that will not hold exactly. Treat it as a strong signal, not a contract. Re-check every year or two, especially after big market moves, and keep a safety margin between your projected value and your target.

10. How do taxes change the math?

Taxes reduce your effective return and your spendable balance. A 7 percent pre-tax return in a taxable account might be closer to 5 percent after taxes and fees. If your savings sit mostly in taxable accounts, enter a return one to two points below the headline figure.

11. What counts as my "current invested amount"?

Only money invested for retirement: 401(k)s, IRAs, brokerage accounts, pensions with a cash value. Exclude your home, cars, emergency fund, and money you plan to spend before retirement. Overstating this input is the most common way to manufacture a false "yes."

12. Can a windfall make me coast?

Sometimes — but not automatically. Invest the windfall, enter the new total as your current amount, and re-run. The required-lump-sum row tells you exactly how big it needed to be: in the example, $73,008.95 would have sealed it.

13. How often should I re-check my coast status?

Once a year is plenty, plus after any major event: a market crash, a big bonus, a job change, or a revised target. Coasting is a status, not a one-time achievement, and the verdict can flip in either direction as balances and targets move.

14. Does coasting work for couples?

Yes, but run the numbers jointly: combine invested balances, use a shared target based on combined retirement spending, and use the later retirement date. Running two separate calculations confuses the target instead of clarifying it.

15. What is the difference between coasting and Coast FIRE?

Coasting is the math: your current savings grow to your target with no more contributions. Coast FIRE is the lifestyle strategy built on it — reaching the coast number, then working only to cover today's expenses while the portfolio handles retirement. This calculator tests the math; Coast FIRE is what you do with a "yes."