Plotting your shampoo schedule around workouts and dinner plans? Welcome to 'hair wash math.'

Wash your hair today and tomorrow's hot yoga class will ruin it. Wait until after your class and you'll have greasy hair at dinner tonight. Wash it twice? Sure, but then you'll have to double up on your whole washing-and-styling routine, which no one wants to do.

Now, this surprisingly complicated calculation has a name: "hair wash math."

Content creator Julie Kahut perfectly captured the thought process in her YouTube short : "Me debating whether I really need to wash my hair... If I wash it now, I have to wash it again after I go to the gym in two days. But I want clean hair when I go out tomorrow night. So, do I wash it now and wash it again later, or leave it greasy for a day?" As another creator put it, it can feel like an " eternal struggle ."

Blogger Kerry Schicker of Jacksonville Mom handles it by mapping out her hair-washing schedule every week. "I will plan out on Sunday what days I'm going to wash my hair based on days I'm in the office, have a work dinner or happy hour with friends, am volunteering in my son's class, or something to do where I can't show up with my hair looking a mess," she wrote.

For some, the mental gymnastics can feel like yet another annoying item on their to-do list. As one person joked on Instagram : "Studies show that the mental load a woman carries about hair wash timing would break a man."

But here's one less thing to stress about: Dermatologist Dr. Joshua Zeichner , director of cosmetic and clinical research at Mount Sinai Hospital, tells Yahoo that there aren't many hard-and-fast rules for when — or how often — you need to wash your hair. Instead, how frequently you suds up depends on your hair type and your scalp. People with thick, curly or coily hair, who tend to have a drier scalp, for example, can get away with washing their hair less frequently than people with oilier scalps who have thinner, less dense hair.

Dr. Samila Asad, a dermatology and trichology specialist at The Female Health Doctor clinic, tells Yahoo that attending a sweaty workout class doesn't necessarily mean you need to wash your hair immediately after. You can, she says, let your sweaty hair dry rather than hopping in the shower. "You don't need to sacrifice every blow-dry just because you've had a workout," she says. "If you're not sweating that much and your scalp feels comfortable, there's no odor, there's no itching, it doesn't feel oily — you could just put a shower cap on and have your regular shower, and off you go."

After exercise or other sweat-inducing activities, let your scalp dry rather than immediately tying up sweaty hair. "When you do that, you can get more sweat buildup, and that plugs the follicles, so it prevents [hair] from growing properly," Asad says.

And while it's OK to skip a wash day after a workout, if you have flaking, itching, redness or other signs of inflammation, it may be time to properly clean your hair. This is especially true if you extend your hairstyle by using styling products like dry shampoo, which absorb some of the oils but don't actually cleanse the hair.

Going too long between washes can also contribute to dandruff. Zeichner explains that oil buildup can allow yeast to flourish on the scalp, triggering inflammation and those annoying flecks of dead skin.

The one time you really should not skip washing your hair is after swimming in a pool or the ocean. "If you go into a pool, it's either going to be salt or chlorine," says Zeichner. "And you are going to want to remove that, especially if you have color-treated hair" or blond hair, since chemicals in some pools can tinge hair green .

Bottom line: "The reality is that we need to maintain a basic level of hygiene," Zeichner says, "but the frequency of hair washing really depends on your hair type and personal needs and preferences."