Grant Sabatier, creator of finance website Millennial Money and author of "Financial Freedom," technically isn't retired. But he could be. He has enough money in his portfolio to live on without ever needing to work again. And that's sort of the point.

Sabatier is one of the leading voices in the so-called FIRE movement short for financial independence, retire early. Adherents to this philosophy aim to save and invest large portions of their income in their early earning years in order to have enough money to retire decades before they reach their mid-60s.

By 2015, at age 30, Sabatier had saved $1.25 million, enough to ensure that he'd never have to work again. But instead of kicking back on a beach, he has embarked on a new career teaching others how to achieve financial independence.

Over the last seven years, Sabatier has seen his share of FIRE success stories, as well as common pitfalls early retirees run into. If you're considering embarking on a FIRE journey, here are two potential problem areas to understand now so you don't run into them down the line, Sabatier says.

Planning for an early retirement requires you to have an idea of what life after work will look like, which can be difficult in a society where people are often defined by their work.

"So much of our identity is tied up to our work and the things that we do in our professional life," says Sabatier. "A lot of people spend all this time working and saving and investing in order to retire early, then they don't have an idea of what they want to do after."

That can make knowing how much money to save tricky given that retiring to a beach in Thailand, writing your novel at a caf or traveling the country in a van require different financial pictures to pull off.

One way to narrow things down is to focus on your core values. Interrogating which parts of your life bring you the most happiness can help you form a clearer idea of what you want, Jim Crider, a certified financial planner who specializes in clients seeking financial independence, recently told CNBC Make It.

"If you can be articulate about what's important to you, your vision is clear," he said. "You can spend money in the most efficient manner. You can make the things that are most important to you happen in a bigger, grander way."

Still, no matter how clear your retirement vision is, it may require some field testing, Sabatier says. If you've accumulated enough cash savings to cover a year or more of expenses, try a "mini retirement" to get a sense of how life away from the office actually feels, he suggests.

Or begin pursuing your passions on the side while you're still working. "This is one of the biggest reasons I recommend trying a side hustle, so you can start making money doing something that you enjoy. And actually then use that as a bridge to when you want to retire early."

None of your early retirement dreams are likely to come to fruition if you don't stash away enough money.

"I see a lot of people retiring with enough money to cover their annual expenses today, but they're not estimating what adding two kids or moving to a higher cost of living area could add to their expenses," Sabatier says.

The number that would-be early retirees are aiming for is known as their "FIRE number" the amount of money they need in their portfolio to live off of in perpetuity.

The calculation used to find it is based on the "4% rule," an investing concept borne of an influential 1998 financial study which posited that investors holding a mix of stocks and bonds could withdraw 4% of their portfolio's value per year.

To find your FIRE number, if you assume a 4% withdrawal rate, you'd multiply the annual income you expect to need in retirement by 25. Someone hoping to live on $50,000 annual withdrawals from their portfolio would need $1.25 million to retire.

People get themselves into trouble, Sabatier says, when they fail to account for how that equation can change for them over time.

You may have thought $50,000 was plenty to live on when you embarked on your FIRE journey in your 20s, but by the time you're 45, your needs may have changed drastically. You may need to adjust your number upward before you can have the retirement you were envisioning.

You may have to adjust your assumptions for how soon you could hit your number too, Sabatier adds. That's because safely drawing down your investments relies on the assumption that markets will consistently move upward. And while that has been the trend over long periods, the direction of your investments is far less predictable between now and when you're hoping to call it quits.

"We know we're living in increasingly uncertain times. I see a lot of people under-saving and overestimating the potential future performance of the stock market."

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37-year-old self-made millionaire: Don't retire early before you consider these 2 things - CNBC