'Girl math': TikTok's latest financial craze is helping young people justify any kind of spending — and makes anything below $5 'basically free.' But it's getting plenty of backlash

“Now there’s the tone of a woman who’s dropped a lot of money,” quipped one of the hosts as the trio took a call in one clip to help a woman run the math on justifying a $699 hair dryer — eventually making enough mental leaps to tell her she was actually coming out ahead by $14,301. “It’s not basically free, you’re running a business here now.”

The trend has since exploded on TikTok, amassing over 53 million views worldwide, as TikTokers cemented the rules of girl math in real time. Important tenets include: any purchase under $5 is free and drinks purchased through your pre-loaded Starbucks app are also free since that money had already left your account.

Girl math may be a new term, but the concept has been well-documented in cognitive psychology. The field calls it “mental accounting,” a mindset where “two dollars are not treated as equals,” according to Dan Egan, director of behavioral finance and investing at Betterment.

And although the name itself assigns a gender to this mental accounting, the viral videos have simply lifted the lid on a trend people of any age and gender frequently and unknowingly participate in.

“This has nothing to do with being a girl per se,” Egan said via email. “There’s an in-group inclusiveness and non-judgmentalness to these videos, which makes it easier to be vulnerable and talk about mistakes/silly things we do.”

While that might provide some catharsis, leaning into the logic of girl math carries some major financial risks — here’s how.

If I paid in cash, it’s free

One of the central beliefs of girl math is that cash doesn’t count. Girl math adherents posit that when you pay in cash, the purchase is “free” since your bank balance technically stays the same after.

It might take some mental gymnastics, but in these days of using cards for every purchase, it’s easy to understand how cash feels different. In fact, if you’re doing it right, paying for things in cash can be a great way to curb your spending.

An old budgeting method known as “cash stuffing,” or the “envelope system” for Dave Ramsey fans, has recently seen a resurgence with the TikTok generation. This method allocates specific amounts of cash to physical envelopes designated for different spending buckets, like groceries or entertainment. Whatever’s in the envelope is the maximum amount available to the spender within a certain time period (e.g. two weeks). Once the cash runs out, that’s it until the next spending period starts.

Following a budget may be way less fun than telling yourself your $12 Starbucks afternoon pick-me-up doesn’t count, but it’s a small price to pay for financial stability.

If it’s under $5, it’s free

With Gen Z feeling the crunch of everything from high rental prices to student loans these days, small $5 purchases may seem insignificant to them.

“It’s very hard to kill yourself with a thousand paper cuts, and it’s very hard to really get in trouble spending small amounts of money,” Egan says. “Spending too much time deliberating or worrying about little purchases probably costs you more time and stress than money.”

But this girl math doesn’t add up long-term. If you spend $5 every day for a month, you’ve spent $150. Even according to girl math, that’s not free. Over time, these small costs can erode your financial stability and make it hard to set aside enough for long-term savings.