Coast FIRE calculator: Figure out your coastFI number

Coast FIRE is a savings benchmark in which you’re pursuing a FIRE number, but want to save more now so you can have wiggle room later. By having more money earlier in your journey, you can “coast” the rest of the way to your goal. By doing this, you free up extra discretionary income in midlife.

Let’s say you’re a two-income household pursuing FIRE, but want to move down to one income later to start a family and have one spouse stay home. Or perhaps you’re in an intense job at a tech startup or a big-four-consulting firm, where the money is great, but you know you'll lose your mind if you work at this intensity for 30 more years.

By front loading your FIRE saving efforts, you give yourself more assets earlier in life, which will then have more time to compound. The saving rate you would have needed later is replaced by compounding.

There’s a bit of a trust fall feeling to this — “What will things look like in 20 or 30 years, really?” — but for households interested in FIRE that are also weighing other life milestones, Coast FIRE can be a useful mental model for career planning.

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What is the coastFI (Coast FIRE) formula?

Quick answer: Coast FIRE 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.

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.

The FIRE number

First, there's the FIRE number, which is the amount of money you need in invested assets to be able to live off the annual withdrawals alone. 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.

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 then be:

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.

Is it Coast FIRE or coast FI?

Both. Kind of. You're gonna get me in trouble here!

Coast FIRE is not financial independence. You have to keep working when you hit your Coast FIRE number. For those reasons, some people prefer coastFI because it lops off the “RE” part of the acronym.

But the whole idea of Coast FIRE is really an offshoot of the FIRE methodology. It's a checkpoint. This is a number we're trying to hit by a certain age, so that we can then pull back on saving, but still forecast hitting another, larger number later in life.

This is also not a FIRE-specific thing. Plenty of people have done something like this with their money management without ever being aware of FIRE culture. Maybe someone works their butt off in midlife to get their assets really strong. Or perhaps they sell a business, or exit a lucrative investment, or come into a substantial inheritance that changes plans.

Coast FIRE 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.

How much do I save to reach my coastFI goal?

It'll depend on several factors. The biggest factors are 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.

Coast FIRE 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

Coast FIRE offers a more balanced, flexible approach to financial freedom. It also gets you hyped about the true power of compounding, and we like a lil' extra motivation mojo.

For ambitious young professionals, coastFI creates the option to downshift into a different career or household income setup. 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 embraces your ambition, but also holds space for different life stages, calculating your coastFI number is a great place to start. ⬥