Tanner Firl doesn't understand why anyone would need a budget.

"It never made sense to me," the 29-year-old tells CNBC Make It. "Most people have a problem not spending money. We have almost the opposite problem."

Firl and his wife, Isabel, who live in Minneapolis, are practically allergic to spending money on anything they don't see as a necessity, a shared attitude that plays into the couple's financial strategy.

Firl is part of the FIRE — short for financial independence, retire early — movement, and specifically adheres to a strategy known as "lean" FIRE. Practitioners of this version of FIRE look to supercharge their savings rate by cutting out as many extraneous expenses as possible.

So far, Firl stashed away roughly $380,000 and hopes to save at least $625,000 to fund an early retirement at age 35. A portfolio of that size would provide his family $25,000 per year in annual income.

Currently, Firl is the primary breadwinner for his family, which also includes his 1-year-old son, Teddy, and three cats. He earns $135,000 a year as a software engineer, and about half of each paycheck goes toward food and living expenses every month. He invests the rest.

Here's how Firl says his super-saver lifestyle is setting him up for early retirement.

Pursuing FIRE from an early age: 'It all seemed to make a lot of sense to me'

Growing up as one of six kids in Rochester, Minnesota, Firl learned about the importance of stretching a dollar. Family vacations meant packing into the van to visit family members or spend time at free-to-enter national parks or monuments. Lunch breaks typically meant pulling over for roadside PB&Js.

The Firl family wasn't hurting for money. They just wanted their kids to learn to work for the things they valued. "Whenever we wanted something as a kid growing up, we would have to spend our own money to buy it or wait until a birthday or Christmas," Firl says.

As a result, Firl followed his siblings' lead and got a paper route as a school-age child and worked all the way through high school. By the time he got to college, he'd discovered a blog written by Peter Adeney, also known as Mr. Money Mustache, one of the foremost figures in the FIRE movement. Something instantly resonated.

"It all seemed to make a lot of sense to me: basically just spending as little as you can so you can live your life as fully as you want," says Firl.

Firl graduated from the University of Minnesota in 2015 with a degree in mathematics and scored a job at the National Security Agency that paid an annual salary of about $66,000 a year. That same year, he and Isabel, who had been high school sweethearts, got married.

Within two and a half years, the couple had saved enough to put a down payment on a house in Minneapolis, where they lived upstairs and covered mortgage payments by renting out the basement on Airbnb.

The frugal path to FIRE

The Firls purchased their second home for $185,000 in 2018 and sold their first home not long after when managing it as a rental became cumbersome. They put up the basement in their new home for short-term rentals, but had to abandon that plan when Teddy arrived in 2021. The house's floor plan is just 675 square feet — close quarters for a couple of new parents, one of whom intermittently works from home.

As Firl's salary has risen to the $135,000 he currently makes, the couple have resisted "lifestyle creep" and maintained their commitment to frugality. If the family needs something, they hunt for it for free on online marketplaces such as Craigslist.

"We also have garnered a reputation with our friends and family as being very frugal and thrifty," Firl says. "We do end up getting a lot of free things just because a family member will see something free on the side of the road, and they'll think that we might we might like it."

Additionally, the couple doesn't have to shell out much for any of their hobbies. Tanner is an avid runner, podcast listener and board game player, while Isabel writes and runs a Twitch stream. They both enjoy playing video games in the evenings.

Plus, the couple have whittled their food and pet supply budget down to $200 a month thanks to frequenting a food waste non-profit to cover groceries.

"For 25 bucks a bundle — a bundle is about half a carload — they just give you a ton of food and then you drive off and you have probably half your groceries for the month, if not more, depending on how much you want," Firl says.

Looking forward to a financially independent future

Looking to the future, Firl acknowledges that aiming to live off $25,000 a year may not seem like enough. But he's confident they can make it work.

The figure assumes the house will be completely paid off by then (he currently pays about $1,100 a month in mortgage payments, homeowners insurance and property tax) and that he'd theoretically qualify for deeply discounted health insurance through the state of Minnesota.

Still, given how his life has changed in the past few years, Firl understands that he'll have to be flexible when it comes to the exact dollar amount and timing of his early retirement. If he and Isabel decide to have more children and move to a bigger home, for instance, that might change his calculus.

When Firl does hit $625,000 in savings, he doesn't necessarily plan to switch to a life of leisure. "Retiring … is not about sitting on your couch watching Netflix all day or going to the beach and getting a really nice suntan," he says. "It's about getting to do whatever you want in life."

For Firl, that means having the power to walk away from a job that isn't bringing him fulfillment or taking a gig that he's more passionate about, even if it pays less money.

Ultimately, no matter what his retirement looks like, you can be pretty certain of one thing: The frugality isn't going away.

"In life there's no short supply of experiences, and most experiences that will make you happy are probably free or extremely cheap," he says.

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