• TikTok's latest trend aims to reframe the narrative around luxury indulgences.
• "Girl math" breaks down the price of an item by the cost per wear.
• However, with any big-ticket purchase, consumers should first consider the trade-offs, experts say, especially if it comes at the expense of your financial well-being.

By nearly every measure, Americans are financially strained. Yet, we're coming up with new ways to justify expensive purchases.

The latest way, coined "girl math," breaks down the price of an item by the cost per wear. If you use an expensive handbag every day for a year, for example, then it might only set you back a few bucks each time you wear it.

Alternatively, buying something on sale not only means you'll spend less, but the difference can be considered "found money," which can be put toward something else.

While the term originated from an unflattering view of women and their finances, TikTok's latest trend aims to reframe the narrative around luxury indulgences.

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In addition to how these purchases are rationalized, there's also a psychology to which payment method is used.

Buying big-ticket purchases in cash, for instance, makes them easier to forget because there's no paper trail, according to a recent paper by Christopher Bechler and Szu-chi Huang at Stanford's Graduate School of Business.

This works well for "an indulgent purchase that doesn't feel super justifiable," Bechler said.

Otherwise, consumers increasingly turn to buy now, pay later to spread out the cost of their "retail therapy" with small installments.

All this coincides with TikTok's other recent trend, "treat" culture, which promotes spending money as a form of self-care, providing a temporary boost during stressful economic times.

By whatever means, "we are all vulnerable to destroying ourselves financially," said Brad Klontz, a Boulder, Colorado-based psychologist and certified financial planner.

However, there are some benefits to this approach.

When 'girl math' works

Most experts recommend thinking about major purchases more carefully, and that's where "girl math" can come in handy.

"Sometimes, 'girl math' is perfect math," Klontz said. Factoring in how often you will use or wear an item helps "calm down the emotional part of the brain and turn on the rational part of your brain."

When the math doesn't add up

"The math often won't end up in favor of splurging on luxury goods. Jewelry and watches may hold their value but most of the rest are depreciating assets," said Christine Benz, Morningstar's director of personal finance and retirement planning.

That underscores the importance of weighing those types of purchases carefully, she added, and considering the trade-offs, especially if it comes at the expense of your economic standing.

"'Girl math' is just the latest iteration of us trying to rationalize financial behaviors that we know we shouldn't be doing," said Klontz, who is also managing principal of YMW Advisors and a member of CNBC's Financial Advisor Council.

"Why the need to justify it?" he added. "The answer to that is because you can't afford it."

In the wake of the Covid-19 pandemic, a spike in interest in luxury goods has also driven prices sky-high.

The "quiet luxury" trend is partly to blame, which glorifies the most high-end lifestyles even though most Americans are more likely to live paycheck to paycheck.

Just take the HBO series "Succession," where the central characters travel by helicopter in $600 Loro Piana cashmere baseball hats and scoff at a Burberry tote bag that retails for $2,890.

Benz advises others to think broadly about luxury. Rather than an expensive bag, being financially healthy is a more valuable possession, she said.

"For me, financial well-being and security have evolved to be more important than luxury goods."

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