Table of Contents
Key takeaways
- A coast FIRE calculator shows how much you need invested today so that growth alone can reach your FIRE number by retirement.
- The formula is simple: Coast FIRE number = FIRE number ÷ (1 + real return)^years to retirement.
- With a $1.5 million FIRE number and 30 years to go, your coast number is about $261,000 at a 6% real return, but about $347,000 at 5%.
- Coasting does not mean you stop working. You still need to cover your living costs until retirement.
- Every result rests on assumptions. Treat it as a planning estimate, not a promise.
A coast FIRE calculator works out how much you need invested today so that growth alone, with no further contributions, can carry your portfolio to your retirement target. TheGlobalNext, a personal finance site covering FIRE, saving and investing, wrote this guide to show you the formula, run the numbers by age and flag the mistakes that distort the answer.
Coast FIRE is a lighter version of the FIRE (Financial Independence, Retire Early) movement. Scripps News describes it as investing heavily early, then letting your investments keep growing while you carry on working. A growing number of younger investors are turning to the approach, according to Benzinga.
You can find your full FIRE number with our FIRE calculator, then follow the three steps below to turn it into your coast number.
What a Coast FIRE Calculator Does
A coast FIRE calculator tells you whether your current investments are already big enough to grow into your retirement target on their own. It takes your age, retirement age, expected spending and an assumed return, then compares the answer with what you have saved.
In full FIRE, you keep saving until your portfolio can fund your life. In Coast FIRE, you build a large pot early, then ease off saving while compounding does the rest. The catch, as Scripps News points out, is that you still need an income to pay today’s bills.
That trade-off is why the idea appeals to people who want a less demanding job, a career change or more to spend now, without giving up on retirement.
The Coast FIRE Calculator Formula
You need two inputs: your FIRE number and the number of years you have left. Everything else is arithmetic.
Step 1: Find Your FIRE Number
Your FIRE number is the portfolio size that can fund your retirement spending. The common shortcut is yearly retirement spending × 25, which is the same as dividing by a 4% withdrawal rate. If you expect to spend $60,000 a year, your FIRE number is $1.5 million.
Our FIRE calculator does this for you and also shows your projected portfolio, years to FIRE and monthly passive income. If you are unsure what you will spend, start by tracking your spending. Scripps News cites a NerdWallet starting point of needing roughly 70% to 90% of your current income in retirement.
Step 2: Discount It Back to Today
This is the coast step. Divide your FIRE number by (1 + real return) raised to the number of years until retirement:
Coast FIRE number = FIRE number ÷ (1 + real return)^years
It is compound growth in reverse. It answers one question: what lump sum, growing at this rate for this long, lands exactly on my target? Of Dollars And Data works an example with a $2.5 million target, a 4% return and 30 years, which gives about $770,797. We checked that arithmetic and it holds.
Step 3: Compare It With Your Balance
If your invested balance is above the coast number, you have reached Coast FIRE. If it is below, the gap is how much more you need to invest. To track progress, divide your balance by your coast number and multiply by 100.
Coast FIRE Number Examples by Age
The table below assumes a $1.5 million FIRE number ($60,000 of yearly spending at a 4% withdrawal rate) and retirement at 65. These are our own illustrations, rounded to the nearest $100.
| Your age | Years to 65 | Coast number at 5% real | At 6% real | At 7% real |
|---|---|---|---|---|
| 25 | 40 | $213,100 | $145,800 | $100,200 |
| 35 | 30 | $347,100 | $261,200 | $197,100 |
| 45 | 20 | $565,300 | $467,700 | $387,600 |
| 55 | 10 | $920,900 | $837,600 | $762,500 |
Two lessons stand out. First, time does more work than return: at 7%, the coast number is about $100,200 at 25 but about $762,500 at 55. Second, the return you assume matters a lot. At 35, moving from 7% to 5% adds roughly $150,000 to the number you need.
What a Coast FIRE Calculator Needs From You
A coast calculator is only as good as the assumptions you feed it. Two choices matter most.
Real vs Nominal Returns
Use a real return, meaning the return after inflation, and keep your spending in today’s money. Using a nominal return with today’s-money spending ignores inflation, flatters your growth and makes your coast number look smaller than it should be. Pick one basis and stay consistent.
Many planners test somewhere between 5% and 7% real, and a lower figure is the more cautious choice. No one knows what markets will deliver, so run several rates instead of trusting one.
The 4% Rule vs Current Research
The 4% rule comes from William Bengen’s 1994 study. Morningstar notes that a fixed 4% start, adjusted for inflation each year, would not have failed over any 30-year period since 1926.
Morningstar’s forward-looking research is more cautious. Kiplinger reports that it puts the highest safe starting rate for US retirees in 2026 at 3.9%. For UK retirees, Morningstar’s UK research gives 4.1% as of 31 March 2026.
The withdrawal rate feeds straight into your result. At 3.9% instead of 4%, $60,000 of spending needs about $1.54 million rather than $1.5 million, roughly 2.6% more, and your coast number rises by the same proportion.
How to Read Your Result
If your balance is above your coast number, your model says growth can carry you to retirement without more contributions. You could lower your savings rate, but you still need income to cover your living costs.
If your balance is below it, the gap is your target. You can close it by investing more, pushing back your retirement age or planning to spend less in retirement.
Markets do not grow in a straight line. Morningstar’s UK research found that nearly 65% of its failed scenarios involved losses in the first five years of retirement, so it is worth stress-testing lower returns. Our guides on long-term investing explain how risk and return interact.
Coast FIRE Calculator Mistakes to Avoid
- Mixing real and nominal figures. Use one basis throughout.
- Assuming one smooth return. A constant rate is a simplification. Test a lower rate as well.
- Underestimating retirement costs. Moneywise points out that no one can predict exactly what retirement will cost, so build in a margin.
- Using today’s spending only. Base your FIRE number on the lifestyle you want at retirement, not just what you spend now.
- Treating the coast number as a quit-work number. It covers retirement, not the years in between.
- Ignoring the withdrawal rate. A small change moves your FIRE number and your coast number.
Coast FIRE Calculator FAQs
How accurate is a coast FIRE calculator?
A coast FIRE calculator is only as accurate as its assumptions. It uses a constant return, while real markets rise and fall, so treat the result as a planning range. Test several returns, such as 4%, 5% and 6% real, and see how far the number moves.
What is the difference between a coast calculator and a FIRE calculator?
A FIRE calculator finds the full portfolio you need to retire, while a coast calculator discounts that figure back to today to show what you need invested now. Use both: your FIRE number first, then your coast number.
Can I stop saving once I reach my coast FIRE number?
In the model, yes, but only if your assumptions hold and you can pay your living costs from earnings. Many people keep saving a smaller amount as a safety buffer.
Is Coast FIRE realistic?
It can be, especially if you start early, because time does much of the work. Its realism depends on your return, inflation and retirement spending, none of which can be known in advance. It is most realistic when you stress-test your numbers.
Conclusion
A coast FIRE calculator turns a vague goal into a number you can track: your FIRE number divided by growth over the years you have left. Start with the FIRE target, apply the formula at a few different returns, then compare the result with your balance.
Ready to begin? Open our FIRE calculator to find your FIRE number, then use the formula above to work out your coast number. For more, browse our FIRE and retirement guides.
This article is for general information and education only. It is not personal financial, tax or legal advice. Your situation is different, so consider speaking to a qualified adviser before you act.
