I use a rainy day cash fund to splurge on items like SZA tickets. My method is a little 'girl math' and a little 'cash stuffing,' but it works for me.

• I bought SZA concert tickets with a rainy day cash fund.
• I keep cash from birthdays and holidays in an envelope.

It was a gloomy Friday afternoon and I'd refreshed Ticketmaster and SeatGeek too many times to count.

Should I buy a ticket or not? I couldn't make up my mind.

SZA, one of my favorite musicians, would be on stage at the Barclays Center in a matter of hours. Tickets were going for about $150, much less than they were months earlier. It certainly wasn't Taylor Swift concert pricing, but I still hated the $60 fee the ticketing sites charge.

Then, as I rifled through some papers in my desk drawer, a little white envelope caught my attention. On the back in gold gel-pen letters, I'd written "Rainy Day Cash." Inside was a small amount of cash I'd received for my birthday a couple of months ago.

I counted the money and looked out the window. It was indeed a rainy day.

I knew I would regret not going to the SZA concert more than I'd regret spending the money, so I did a bit of "girl math" and decided to go for it.

Girl math became the topic of viral videos on TikTok in which women justify spending money on coffee, clothes, or concert tickets because the cost is relative to the value they place on it. As described by writer Imani Barbarin on X, girl math is a concept that "time, convenience, and money are interchangeable currencies."

Obviously, I couldn't buy the ticket with the cash in my rainy day envelope, but if I put it on my credit card, I could essentially use the cash to pay myself back. The cash didn't cover the full cost, but it was just enough to make the ticket feel less expensive. Now, instead of spending what I might on a new handbag, the cost was closer to a nice dinner out that I might have once a month anyway.

My cash envelope was similar to another personal finance strategy called "cash stuffing," by which people budget their monthly expenses in cash envelopes and only spend what they have set aside for each category. Also known as envelope budgeting, this method has been around for years but gained recent attention on TikTok from people who use it to pay their monthly expenses or even pay off debt.

While I don't personally budget with the cash-stuffing method, I do put my own spin on the concept. Whenever I receive cash gifts from friends or relatives, I store it in an envelope instead of depositing it into my bank account or spending it right away. Then, I let it accumulate over multiple holidays and birthdays to save up for one big thing like a nice dinner, vacation, or shopping spree.

Mentally, when the cash is out of sight, it's easy to forget that it's even there. Then, when I stumble upon it, like on a rainy Friday afternoon, it's a pleasant surprise. I feel better about splurging on something that wasn't in my regular monthly budget.