Coast FI, explained: the 4 assumptions behind the number
This guide is for general education and is not personalized investment advice. Your situation is unique. Consider talking with a qualified professional before making financial decisions.
There's a retirement formula all over your feed that says, once you hit a certain number, you might be able to stop saving for retirement. It's a genuinely useful idea. It's also four guesses in a trench coat. This guide walks through what Coast FI is, the four assumptions hiding inside it, a sandbox to test your own what-ifs, and a worksheet to write your guesses down.
wait, what is it?
What Coast FI actually is
Coast FI ("coast financial independence") is an offshoot of the FIRE movement (Financial Independence, Retire Early). Instead of saving aggressively so you can quit work early, the idea is to save enough early on that, in theory, your investments could grow to your retirement goal by themselves, even if you never added another dollar. From that point, you'd "coast": you'd still work to pay today's bills, but you might not need to keep adding to retirement savings.
Think of riding a bike up a hill and then coasting down the other side. The catch: the formula assumes the road is smooth and that you know exactly where the bottom of the hill is. Real roads have bumps, and nobody knows exactly where their hill ends.
the formula, in plain words
How the number gets calculated
Most Coast FI calculators do one thing: they take your retirement goal and work backward to how much you'd need invested today, using an assumed yearly return and the number of years until you retire. Written with letters:
| Coast number | = Goal ÷ (1 + return)years |
|---|---|
| Goal | How much you think you'll need at retirement |
| Return | The yearly growth rate the calculator assumes |
| Years | Years from now until you plan to retire |
That's it. Simple math. But every letter in it is a guess about the future, and one of the biggest guesses (inflation) doesn't even show up as its own letter. That's what the rest of this guide is about.
why it's everywhere
Why people like it
Google searches for Coast FI are up about 50% from a year earlier, and a TIAA survey of about 1,000 Americans found 15% said they were actively pursuing it (as reported by The Wall Street Journal, September 2026). It's easy to see the appeal:
- It's a motivating checkpoint. "Retire with enough" is vague. A specific milestone gives you something to aim at.
- It shows why starting early matters. The "years" in the formula does a lot of work. The more time money has to stay invested, the less you'd need set aside today to reach the same goal, in theory.
- It talks about life now, too. A lot of retirement content is all sacrifice. Coast FI asks what flexibility you might want before 65, which is a fair question.
None of that is wrong. The issue isn't the idea. It's treating the output of one formula as a finish line.
what's it really assuming?
The 4 assumptions hiding inside the number
For each one: what the formula assumes, what could change, and one question to write down (the worksheet below collects them).
how much is "enough"?
1. The goal
What it assumes: that you know what you'll spend each year in retirement, maybe 30 or 40 years from now. Many calculators turn yearly spending into a total goal using a withdrawal rule of thumb, and that rule is its own assumption with its own debates.
What could change: where you live, whether you rent or own, health care costs, whether you support family, what you want retirement to look like. Your 25-year-old self and your 55-year-old self may want very different things.
Write down: where did my goal come from, and what is it based on?
markets don't move in a straight line
2. The return
What it assumes: one steady growth rate, every single year. That's a hypothetical, not a prediction.
What could change: real markets don't move in a straight line. Returns vary from year to year and can be negative, sometimes for several years in a row. The order of good and bad years can matter too, especially near the time you start withdrawing. And because the formula compounds that rate over decades, a small change in the assumed rate can change the answer a lot. Most calculators also leave out fees and taxes.
Write down: what return did the calculator use, where did that number come from, and does it account for fees?
the letter that isn't there
3. Inflation
What it assumes: often nothing out loud. Some calculators quietly use a return that already subtracts inflation. Others expect you to enter your goal in future dollars. If the goal is in today's dollars and the return isn't adjusted for inflation (or the other way around), the result can be off by a lot.
What could change: what a dollar buys in 30 years is one of the biggest unknowns in all of personal finance. And not every cost rises at the same pace, so the things you'll spend the most on later in life may not track the average.
Write down: is my goal in today's dollars or future dollars, and is the return before or after inflation?
life happens
4. Your life
What it assumes: that you retire at exactly the age you typed in, that you never need to touch the money before then, and that you keep earning enough to cover every bill along the way without dipping into it.
What could change: kids, a house, aging parents, a health event, a layoff, a career change, a sabbatical. Any of these can change how much you need, when you need it, or how long you work.
Write down: what could happen in the next 10 years that might change my timeline or my goal?
okay, let me try it
The Coast FI sandbox
Now that you know what's inside the formula, play with it. Every box starts empty on purpose: the return, inflation, and withdrawal rate are your guesses, not mine. Never thought about these before? The cheat sheet below gives you a starting point for each one.
how to use it
Four steps, about two minutes
- Enter your ages. How old you are now and the age you picture retiring.
- Estimate yearly spending in today's dollars. Start with what life costs you now, then adjust for what might change (a paid-off mortgage, more travel, more health care). Don't try to guess future prices here; the inflation box handles that.
- Fill in your guesses for withdrawal rate, return, and inflation. Use the cheat sheet as a reference point, not an answer.
- Read the what-ifs. Each tile changes one guess. Then try it again: a hopeful version, a middle version, and a cautious version. The gap between them is the real lesson.
never thought about this before?
Cheat sheet: reference points for each box
These are published averages and research figures, so you have somewhere to start. They are not recommendations, and your numbers may look very different.
| Box | A reference point | Source |
|---|---|---|
| Retirement age | Retirees reported retiring at 61 on average; people still working expected to retire at 66. | Gallup, April 2022 |
| Yearly spending | Households headed by someone 65 or older spent about $61,400 in 2024, on average. | U.S. Bureau of Labor Statistics, Consumer Expenditure Survey, 2024 |
| Other income | The average Social Security retirement benefit is about $2,071 a month (January 2026). That's per person, while the spending figure above is per household. Get your own estimate with a free account at ssa.gov. | Social Security Administration, 2026 |
| Withdrawal rate | The well-known "4% rule" comes from 1994 research on historical data. Morningstar's most recent estimate is 3.9%, for spending that rises with inflation over a 30-year retirement, with a 90% chance the money lasts. | William Bengen, Journal of Financial Planning, 1994; Morningstar, Dec. 2025 |
| Yearly return | No reference point here, on purpose. Returns depend heavily on the time period and the mix of investments, they vary year to year, and past returns don't predict future ones. Try a few different numbers, including a low one, and watch the first what-if tile. | None |
| Inflation | The Federal Reserve's long-run goal is 2%. Actual consumer prices rose about 2.5% a year over the last 30 years (1995 to 2025) and about 3% a year since 1926. | Federal Reserve; Minneapolis Fed CPI table (averages calculated from it) |
| Yearly fees | The average expense ratio was about 0.14% for index stock ETFs and 0.40% for stock mutual funds (asset-weighted). Your fund's exact number is on its fact sheet. | Investment Company Institute, 2025 |
One simplification to know about: the sandbox treats your other income as starting the day you retire. In real life, Social Security can start anywhere from 62 to 70, and the age you claim changes the amount (see ssa.gov).
This sandbox is an educational tool. Results are hypothetical, depend entirely on the assumptions you enter, and are not a prediction or a recommendation to save, stop saving, or invest in any way. It uses one steady return every year, which real markets don't do, and it leaves out taxes, changes in your plans, and the order of good and bad years. It isn't connected to StratiCo or any of its strategies. Nothing you type is saved or sent anywhere.
the part most people miss
Coasting isn't the same as stopping
- You still have to work. Coast FI covers retirement, not today. Rent, groceries, and everything else still need an income.
- The match may stop with you. If your employer matches 401(k) contributions, the match usually depends on you contributing. Check your plan's rules (your summary plan description or HR) so you know how it works before you change anything.
- It's a checkpoint, not a finish line. Because all four assumptions can change, the number can change too. A lot of people find it more useful as something to recheck every year than as a one-time "I'm done."
your turn
Worksheet: write your assumptions down
No math here. The point is to see exactly what you'd be assuming, in your own words. Copy it into your notes app, fill it in, and redo it once a year or after any big life change.
| Assumption | Question to answer | My answer |
|---|---|---|
| Goal | What yearly spending is my goal based on, and where did that come from? | |
| Return | What return did the calculator assume? Where did that number come from? Does it include fees? | |
| Inflation | Is my goal in today's dollars or future dollars? Is the return before or after inflation? | |
| Years | What retirement age did I enter, and how sure am I about it? | |
| Life | What could happen in the next 10 years that might change my goal or timeline? | |
| Match | Does my employer match contributions? What does my plan say happens if I stop? | |
| Recheck | When will I look at this again? |
bring this with you
Questions to ask a professional
Your worksheet is a great starting point for a conversation. Questions worth asking before you change how much you contribute:
- Looking at my worksheet, which of these assumptions has the biggest effect on the number?
- How would you think about the return and inflation assumptions for someone with my timeline?
- What happens to my employer match, and anything else, if I lower or stop contributions?
- What would you want to see in place (cash cushion, insurance, other goals) before someone considers coasting?
- How often should something like this be revisited?
- How are you compensated?
- Will you act as a fiduciary for this advice at all times?
If taxes or account rules come up, confirm the details at IRS.gov or with a tax professional.
the most important part
Keep it in perspective
Coast FI is a useful checkpoint and a great reason to start early. It's a formula, not a forecast.
- Every input is a guess. Goal, return, inflation, and your life can all change over decades.
- Recheck it. Treat the number as something to revisit, not a one-time finish line.
- Get a second set of eyes before you stop. A calculator gives you information. A professional can help with direction and next steps.
Sources: Oyin Adedoyin, "Young People Are Obsessed With This Simple Retirement-Savings Formula," The Wall Street Journal, September 7, 2026 (Google search interest and TIAA survey figures). Yahoo Finance, "Young Americans Are Embracing This Retirement Hack to Gain Financial Freedom Years Earlier," 2026 (TIAA survey size). AAII, "The Coast FI Retirement Formula and Its Shortcomings," 2026. Gallup, "More in U.S. Retiring, or Planning to Retire, Later," 2022 (survey April 1-19, 2022). U.S. Bureau of Labor Statistics, Consumer Expenditure Surveys, average annual expenditures, reference person age 65 or over, 2024 (via FRED series CXUTOTALEXPLB0407M). Social Security Administration, "What is the average monthly benefit for a retired worker?" and 2026 COLA Fact Sheet. William P. Bengen, "Determining Withdrawal Rates Using Historical Data," Journal of Financial Planning, October 1994. Amy C. Arnott, Christine Benz, and Jason Kephart, "What's a Safe Retirement Withdrawal Rate for 2026?," Morningstar, December 3, 2025. Federal Reserve, Statement on Longer-Run Goals and Monetary Policy Strategy (2% inflation goal, reaffirmed August 2025). Federal Reserve Bank of Minneapolis, Consumer Price Index, 1913- (annual averages; 1995 to 2025 and 1926 to 2025 compound averages calculated from the table). Investment Company Institute, "Trends in the Expenses and Fees of Funds, 2025."