'Girl Math' is the latest financial trend going viral on TikTok where purchases under $5 don't count and big ticket items should be measured by their cost per wear.Henglein and Steets/Getty Images Move over "girl dinner" and "lazy girl jobs," a new personal finance trend has gone viral on TikTok. Called "girl math," this humorous trend has users breaking down purchases into their smallest possible units to justify them. One critic says the trend justifies overspending. A new personal finance trend has TikTok users recommending humorous new ways to rethink their spending habits — like imagining that anything under $5 is free, or measuring the cost of big-ticket items by their cost per wear."Girl Math" was first popularized by the New Zealand podcast Fletch, Vaughan & Hayley.