The FIRE number is a big, sexy number. But what about coastFI, the invested assets benchmark for early-career go-getters that lets you lighten up on saving later in life? (And maybe go enjoy a cocktail by the pool while your money compounds? 🍹)
What's that number?
Coast FIRE (technically coastFI, for my fellow grammar nerds) is the point at which your current invested assets would compound the rest of the way to your FIRE number by your chosen retirement age without additional contributions.
This lets you take the money you were shoveling into investments or index funds and put it toward other things instead.
- CoastFI (coast FIRE) offers more near-term flexibility than regular FIRE.
- With coastFI, you save and invest aggressively in the early part of your career to build up your retirement nest egg.
- Once you hit your coastFI number, you can scale back to just covering your expenses, knowing your existing investments will grow to fund a traditional retirement timeline.
- This lets you pursue lower-paying passions, switch to part-time work, or take mini-retirements much earlier in life.
The coastFI (coast FIRE) calculator math
To measure your coastFI number, you’ll estimate what will be enough money for you to become work-optional (your FIRE number), then work backward using typical compound interest formulas.
First, there's the FIRE number. Here's a very simplified version of the formula:
- Write down your expected annual expenses in retirement.
- Then, multiply this annual spending by 25 (if using the 4% rule) to get your FIRE number:
FIRE number = E x 25
Where E is your expected annual expenses.
Why 25?:The 25-year figure comes from the 4% rule for retirement, which states that withdrawing 4% of investments per year will very likely last for 30 years without running out of money. It wasn’t tested past 30 years, though, so if you’re planning to be work-optional for longer than that, you might need to factor in additional inflation.
From here, we can reverse-engineer our coastFI number. To determine when the coast could start, we'd need to know:
- Our end goal (the FIRE number) (F)
- Our expected annual real return on our investments (r)
- How many years those investments would passively compound at that expected rate of return (t)
That formula would look something like this:
coastFI number = F / (1 + r) ^ t
Where F is your FIRE number, r is the annual return (expressed as a decimal, so 6% would be 0.06), and t is the number of years you have until you want to stop working.
The big idea behind CoastFI is that you’re hustling harder in your early years to create more flexibility in midlife. Once you’ve reached the CoastFI benchmark, you know that, based on historical returns, your money would grow the rest of the way to your FIRE number, which is traditionally built on the 4% rule and therefore has inflation factored in.
Example
Retire at 60, expecting $50,000/yr expenses ($1.25M FIRE number), 6% return:
Coast FIRE by age 30 → about $217,600 invested (30 years to compound)
Coast FIRE by age 35 → about $291,200 (25 years)
Coast FIRE by age 40 → about $389,800 (20 years)
These amounts would compound to $1.25M by age 60 at a 6% annual real return, without any additional contributions, if reached on time.
How much do I save to reach my coastFI goal?
It depends on several factors. These include your current age, desired retirement age, expected annual retirement expenses, and projected investment growth rate.
Online calculators (like this one!) crunch numbers to give you a personalized coastFI target to work towards. They can also show how small changes, like retiring a few years later or living on less, impact your coastFI number.
Ultimately, coastFI calculators illustrate the power of compound growth over time. By frontloading your retirement savings in your 20s and 30s, you give your money decades to snowball before you actually need it.
The origins of coastFI
The FIRE movement traces back to the 1992 book Your Money or Your Life by Vicki Robin and Joe Dominguez. The concepts behind coastFI started appearing on FIRE forums and blogs in the 2000s under names like Barista FIRE and RV FIRE. The actual term “coast FIRE” seems to come from a 2016 ChooseFI podcast episode with entrepreneur Sam Dogen.
Over the last several years, coastFI has become much more visible. It's particularly appealing to young adults who have the time and energy to stack cash now, but don’t want to do that for decades. Since young and midlife adults have a lot of compounding time on their side, their coastFI number is much lower.
CoastFI gives you nuance
CoastFI offers a more balanced, flexible approach to financial freedom. It also gets you hyped about the true power of compounding, and we love a lil' extra motivation mojo around here.
For ambitious young professionals, coastFI creates the option to downshift into a lower-paying but more fulfilling career after reaching their coastFI number. Those who like their career can keep working, but also save less aggressively, and still have peace of mind that they’re on track.
If you're looking for a strategy that doesn't require decades of grind, calculating your coastFI number is a great place to start. ⬥
Coast FIRE calculator FAQs
What is a Coast FIRE number?+
It's the amount you need invested today so that, without adding another dollar, compound growth alone reaches your full FIRE number by retirement age.
How do you calculate Coast FIRE?+
Coast FIRE number = FIRE number ÷ (1 + r)^t, where r is your expected annual return (as a decimal) and t is the years until your target retirement age. Your FIRE number is your annual expenses × 25.
What return rate should I use?+
Many calculators default to ~7% (the long-run real return of the S&P 500); a conservative choice is 5–6%. Use a real, inflation-adjusted rate so your today's-dollars target stays consistent.
Can you Coast FIRE at any age?+
Yes, but the younger you start, the lower your number, because compounding has more time to work. As you age, you'll need a larger amount already invested.
