'Girl math' is all over TikTok, but it 'straight-up does not work,' says self-made millionaire

All over TikTok, "girl math" is being used to justify almost any purchase, from everyday things like $5 coffees to big-ticket items like concert tickets. While everyone's version of girl math is different, it typically involves rationalizing your spending in often illogical ways.

Say you buy your daily coffee with cash, rather than your credit card — it's basically free, according to girl math. Or perhaps you make an expensive purchase, like a designer handbag — just use girl math to break the price down on a "cost per wear" basis so it doesn't seem as pricey as it really is.

However, it's not necessarily a healthy outlook.

"It's like burying your head in the sand," says Tori Dunlap, a self-made millionaire and founder of financial education platform "Her First $100k."

On TikTok, there are hundreds of videos tagged as "girl math," with some amassing millions of views. And although these videos may be intended as a joke, the trend may inadvertently normalize poor spending decisions, Dunlap tells CNBC Make It.

While most of us are familiar with the phrase "ignorance is bliss," that mindset shouldn't be applied to your finances, says Vivian Tu, a fellow self-made millionaire and author of "RichAF: The winning money mindset that will change your life," which will be released Dec. 26.

"That strategy straight-up does not work because it will catch up with you," Tu tells CNBC Make It.

If you dismiss buying a $5 cup of coffee every day, you'll have spent around $1,800 by the end of the year without even realizing it, Tu says.

"That daily $5 expense that you're not accounting for adds up," she says. "I'm not saying don't buy coffee. I'm saying recognize that your budget for coffee annually is $1,800."

While the girl math trend may seem harmless on the surface, it can actually perpetuate the baseless stereotype that portrays women as being bad with money, Dunlap says.

"Where it starts to get damaging is when it plays into the stereotype that women are frivolous spenders," she says. "Frivolous is never NFL season tickets or golf clubs or video games. It is innately feminine purchases. It's the latte. It's the manicure."

And calling mental accounting "girl math" infantilizes people who primarily identify as female, says Tu.

"It paints a very specific picture of somebody who needs their hand held and somebody who isn't smart enough to manage their own finances," she says.

The girl math trend also conceals the undue guilt women often feel about spending money on things they like, which leads to trying to justify every purchase, Dunlap says.

"If you can afford something and you want to spend money on it, great. Spend money on it," she says. "You don't need to apologize for it or use 'girl math.'"

You don't have to use girl math to justify your everyday purchases or treating yourself, says Tu. Instead, you can plan for splurges or "little treats" ahead of time using the 50-30-20 method.

To follow this strategy, set aside 50% of your take-home pay for essentials like your rent, mortgage payment, car payment and other recurring bills. Use 30% for your wants, such as brunch with friends or going to a concert, and put the last 20% into savings and investments to "take care of future you," Tu says, whether that's contributing to a retirement savings account or building up your emergency fund.

"For a long time, budgeting was associated with depriving yourself of all the things you want," she says. "Now, I think budgeting affords you all of those moments that are priceless."

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