Coast FIRE Calculator

Coast FIRE is the point at which your existing investments are large enough to grow into your full retirement target, even if you never save another pound. You still need to cover your living costs from a job or other income, but you have stopped feeding the portfolio. The compounding does the rest of the work.

Our Coast FIRE calculator tells you how much money you need invested today to coast to financial independence by your chosen retirement age. Enter your current age, target age, expected return, and FI number, and the calculator gives you the smaller pot that compounds into the full thing.

Contents

• What Is Coast FIRE?
• How to Use the Coast FIRE Calculator
• The Coast FIRE Formula
• A Worked Example
• Why Coast FIRE Is the Most Useful Milestone
• Common Use Cases

What Is Coast FIRE?

Coast FIRE sits between two more familiar concepts. Full FIRE means you have enough invested to live off your portfolio indefinitely; you no longer need any earned income. Most people target a portfolio of 25 times their annual expenses, based on the 4% rule.

Coast FIRE is reached much earlier. It is the portfolio size that, left untouched and growing at your assumed rate of return, will reach your full FI number by the age you want to retire. You still need a job to cover your day-to-day expenses, but you no longer need to actively save.

A simple way to think about it: full FIRE means your portfolio works for both your present and your future. Coast FIRE means your portfolio works for your future, and you only have to work for your present.

How to Use the Coast FIRE Calculator

The calculator needs five inputs. Most people can complete it in a couple of minutes.

1. Current Age

Your age today. The calculator uses this as the starting point for compound growth.

2. Target FI Age

The age at which you want your portfolio to be large enough to cover your full living costs. This is when you would actually retire or stop needing earned income. Common targets are 50, 55, 60, or 65.

3. FI Number

Your total target portfolio at retirement. The calculator lets you either type a direct figure (if you have used the FI number calculator and know it) or derive one from annual expenses and a withdrawal rate.

If you go the annual expenses route, enter what you expect to spend each year in retirement. The default 4% withdrawal rate gives you a 25x multiplier. Conservative UK investors sometimes use 3% to 3.5%, which translates to 28x to 33x. Our Beyond the 4% Rule guide explains why UK investors often use a lower rate.

4. Expected Annual Return

The real (post-inflation) return you expect on your portfolio. A globally diversified equity portfolio has historically returned around 5% to 7% in real terms, though past performance is not a guide to future returns. As an illustration, you might try 5% for a conservative scenario, 7% for an optimistic one, 6% for the middle ground. The calculator is for information only and not a recommendation.

5. Current Portfolio Value

The total invested in stocks, ISAs, SIPPs, and any other long-term investments. Include your pension. Do not include your emergency fund or short-term cash. If you are not sure of the total, use the net worth tracker to get an up-to-date figure.

The Coast FIRE Formula

The maths is simpler than most FIRE calculations.

Coast FIRE Number = FI Number / (1 + r)^n

Where:

• r is the annual real return as a decimal (e.g. 0.06 for 6%)
• n is the number of years until your target FI age

If your FI number is £750,000 and you have 25 years of compounding at 6%, your Coast FIRE number is £750,000 / (1.06)^25, which is roughly £175,000. That £175,000 will grow to £750,000 by retirement on its own. After hitting £175,000, you have crossed Coast FIRE and can stop saving without derailing your plan.

A Worked Example

Sam is 34, planning to retire at 60, and expects to spend £30,000 a year in retirement. Using a 4% withdrawal rate, Sam's FI number is £750,000.

There are 26 years until age 60. At a 6% real return, the Coast FIRE number is:

£750,000 / (1.06)^26 = £164,500

So once Sam has £164,500 invested, the portfolio will compound to £750,000 by age 60 with no further contributions. Sam can take a lower-paid job, reduce hours, or take a career break, and the retirement maths still works out.

That same Sam, planning to retire at 50 instead, has only 16 years to compound. The Coast FIRE number jumps to £295,000. The earlier you want to retire, the larger the portfolio you need to coast on, because compounding has fewer years to do its work.

Why Coast FIRE Is the Most Useful Milestone

Full FIRE is a long way off for most people. If you are saving 20% of your income, full FI is typically 30 to 40 years away. That timeline is hard to stay motivated for, and life rarely cooperates with linear plans.

For households with a high savings rate, Coast FIRE can be reachable years earlier than full FI - the exact timeline depends on income, expenses, and market returns. Once you hit it, you may have options that did not exist before:

• Switch to lower-paid but more meaningful work
• Drop to four days a week without losing the retirement maths
• Take a year off to retrain, travel, or rest
• Stop stressing about job loss because the existing portfolio is doing the heavy lifting
• Start a business that initially earns less than your salary

The psychological shift is the real value. You go from "I have to save £X every month for the next 30 years" to "I can choose what kind of work feels right because the future is already funded."

Common Use Cases

Sanity-checking a career change - If you are weighing a pay cut for a job you would actually enjoy, run the Coast FIRE calculator with the new income. In many cases, a pay cut may affect how soon you reach full FI more than whether you reach it - but your own numbers will tell you for sure.

Planning a sabbatical - A year out of saving (or even out of working) costs less than you think if your portfolio is past Coast FIRE, because compounding continues whether you contribute or not.

Comparing retirement ages - The trade-off between retiring at 55 and 65 is much sharper at the Coast FIRE stage than at the full FI stage. The calculator makes the cost of an early retirement target clear.

Stress-testing your plan - Run the calculator at 5% and 7% returns and see how much the answer moves. If the spread is small, your plan is robust to return assumptions. If the spread is huge, your plan depends heavily on optimistic returns.

For a wider perspective on how Coast FIRE fits into the overall path to financial independence, see our piece on the boring middle, which covers staying motivated during the long stretch between starting and reaching full FI.

Further Reading:

Quit Like a Millionaire - Kristy Shen - A practical FIRE playbook with real-world tactics for hitting Coast FIRE and beyond, including how to think about returns, withdrawals, and life after the salary disappears. (Affiliate link - we may earn a small commission at no extra cost to you.)