Hair trends: The perfect new term that explains what every woman knows about her hair.

Ever since I started wearing my hair curly a few years ago, things have gotten knotty. Not my actual hair: I always detangle while conditioning. What's knotty is the question of when to wash my hair. It's gone from something I don't think about much to something that consumes, oh, roughly all my waking hours. The thought is in the background usually, but it's definitely always there, hovering over me: Is it time for another wash?

The issue is that doing my entire hair routine is time-consuming, so the goal is to make the wash last. But the longer I go without washing it, and the more nights I sleep on it, the worse it looks. A few months ago, I remember being so confounded by a "to wash or not to wash" question that I sought out a friend's advice. Scrolling back to the text exchange in my phone to reacquaint myself with the details, I almost couldn't believe how convoluted they were: As I explained to my friend, I had washed my hair the day before, but I slept on it in such a way that it looked pretty lackluster on Day 2. I wanted to attend an early-morning exercise class on Day 3, and I thought maybe I should wet my hair and restyle it, but that would mean waking up before dawn if I wanted it to be dry by the time the class started … and so on. Even as I myself can barely follow past-me's logic, I also know in my follicles that all of this made perfect sense to me then. At the time, I compared the conundrum to one of those classic algebra problems involving two trains leaving different stations at different times traveling at different speeds. It was a more apt comparison than I knew, because what I didn't realize then was that I was engaging in the age-old art and science of hair math.

I first encountered the phrase on a podcast called Normal Gossip a few months ago. Hair math, host Kelsey McKinney explained, was her and her producer's way of talking about perplexing scheduling issues involving hair. "It's when you’re like, ‘I need to wash my hair because I’m sweaty from the farmer's market, but also I need to exercise, and I know that I’m gonna need to wash my hair again, like, in two days,’ " she said. "So what's the order of operations?"

Hair math! It explained so much. It was exactly what had been secretly eating up all my extra brain capacity. No wonder I was having trouble functioning—a not-insignificant portion of my RAM usage was, unbeknownst to me, being dedicated to doing complicated math procedures while I was going about my day. I searched for "hair math" online and found that it was kind of a thing already, and up popped several popular TikToks elaborating on the term. It seemingly hadn't been promulgated by any one person, though; it was what we might call a hair-roots phenomenon. I even started recognizing the concept of hair math in contexts that didn't use the actual phrase. For example: When a podcaster I follow on Twitter named Brittany Luse polled her followers about which day of Memorial Day weekend she should take down her twists, she was doing hair math. (Her followers voted overwhelmingly for that Friday, for the record.)

"I don't want to work out," TikTok creator Sheena Melwani complains at the start of one video about hair math that's been viewed millions of times. Her long, straight hair is parted on the side and looks like she performed the kind of dark magic it takes to get juuust the right amount of volume. "Don't work out," her off-screen hubby responds calmly. "Because then I’ll have to wash my hair," she goes on. He suggests that she wear a hat, but she's not listening. "I need my hair to look nice tomorrow, so I need to wash my hair tomorrow," she continues, now in conversation with herself. "But then if I work out today, I’ll have to wash my hair today."

Much to his frustration, Melwani's husband had no idea what she was talking about in the video, but I did: I’ve had this conversation with myself more times than I can count.

"Men really don't get it because they go into the shower most days and they wet their hair, they wash it once or twice a week, and that's the end of it," Melwani told me when I reached her on the phone for a hair math lesson. "They don't have to spend hours and hours on styling or on curling or straightening or both." I’m sure there are some men who do hair math, some women who don't, and nonbinary people on both sides of the issue, to be fair—but for the most part, discovering the phrase felt like giving a name to a tiny, as-yet-uncharted psychic burden that hangs over much of the female population. Hair math!

Curly hair influencer Barbara DelleMonache agreed: "I think actually the term hair math is so incredible," she told me. "I’ve always known about quote-unquote ‘hair math’; I just never put a name to it. Putting a name to things sometimes normalizes it, sometimes makes it a little bit less crazy to deal with."

Because hair math is an emerging term, its definition isn't set in stone—some people also use it to describe the process of estimating when a photo was taken based on a person's hairstyle. And as with other number-related disciplines, we might also divide hair math into subfields: If Melwani was talking about micro hair math (hair math on the day-to-day level of cleaning and styling), other people, like Ceara Jane O’Sullivan, are more concerned with macro hair math (the more long-haul arithmetic of cuts, dye jobs, and growing out old hairstyles). O’Sullivan has made widely viewed comedic videos that deal with the latter topic, focusing on the all-important question of when to color her hair. In one, she holds a marker in her hand and writes like she's planning out a military operation as she explains her plans for when to make a salon appointment.

"I am a blond-as-a-child brunette who insists on getting my hair balayaged," O’Sullivan, who writes for Saturday Night Live, told me when we spoke about the grip hair math holds on her. (Balayage is a highlighting technique, and as everyone knows, you’re not supposed to dye your hair too often.) "So I’m constantly dealing with ‘When do I get it done? When do I wash it?’ Because if I don't wash it too much, it’ll start to look bad. But then if I do wash it, then I can't go running.’ It's Tetris on the daily," she said.

This summer is going to require a particularly complex string of hair math for O’Sullivan: "Right now we’re in looking-really-bad territory, and I want it to look great at my bachelorette party in mid-June. But then I want it to look perfect for my wedding at the end of August. So I’m probably going to break the rules and get it done for both."

As vain and silly as you can start to sound when you ramble on about hair math, it would never occur to most of us not to do it. "I’ve thought before, ‘Well, why do I insist on keeping my hair long? Should I just cut it short?’ " O’Sullivan said. But as is the case for a lot of women, her self-esteem and her hair are pretty much braided together. "It's definitely my physical feature that I spend the most time and energy on," she said. "I think, for a lot of people, the thing that they have going on with their hair is the quick and easy way they can feel great about how they look."

Part of why we spend so much time doing hair math might have to do with how much hair can start to feel inextricable from identity. "I was sort of known as the girl with curly hair," DelleMonache remembered of growing up. "Like, ‘Who's Barbara? Oh, she's the one with the curly hair.’ So it sort of, in that regard, almost becomes a personality trait, and then you focus on it extra hard because you’re like, ‘Well, everyone else is clearly focusing on this.’ "

In math, a theorem is a statement that has been or can be proved, and I believe that we have now conclusively proved the existence of hair math. But it did occur to me to wonder what an actual mathematician might have to say about the concept. That's how I ended up speaking to Sarah Hart, a mathematics professor at Birkbark College at the University of London and the author of a book called Once Upon a Prime. I asked her if she’d heard of hair math before I reached out to her. "I hadn't, but I instantly identified with the issues," she said. She has long hair that was braided when we spoke, while mine, as I explained to her, was an absolute hair math worst-case scenario: It was Day 6 since a wash, and I’d pulled it into an unsightly low pony that brought me great shame. She got it.

"We women have all of these layers of stuff constantly going on in our heads," she said. "A lot of women are responsible for child care or food shopping and all of these other things where we’ve got to have a constant tally going in our head: ‘We’re running low on this, I need to get that, I need to sort this out.’ " And then hair math is "just this extra layer of thinking that goes into just going through our day. It's not advanced calculus, but it is just a complicated series of calculations that we always need to be on top of."

It may indeed not be advanced calculus, but hair math actually does remind Hart of a real mathematical research area called combinatorial optimization, which involves finding optimal solutions to problems with many variables. It has applications in efficient scheduling, production, and logistics, and Hart said it contains some of the most advanced problems in mathematics. I looked up one of the ones she told me about, the traveling salesman problem, on Wikipedia. Trying to understand the page, which expresses the problem in equations using that sigma symbol I haven't thought about since 12th grade, I felt pleased to know that you could theoretically make up an equation just as complex as one of these about almost anyone's hair.

In reality, no mathematicians have tackled hair math yet, but if it weren't such a male-dominated field, maybe that would be different, Hart said: "If it was men who had long hair, we might find there would be mathematical papers maybe about these kind of questions, but there aren't." Naturally, this has really reframed the way I think about my hair routine. Now, when I finally get around to washing and styling it, no longer am I just a girl standing in front of a mirror with a Wet Brush and some mousse: I’m an amateur mathematician. I’m a woman in STEM.