How often to wash your hair

Key takeaways

• There is no single right number. Wash often enough that sebum, flakes and product do not build up on the scalp, and rarely enough (or gently enough) that the lengths do not get dry, tangled or faded.
• Frequent washing does not make the scalp produce more oil. The one direct test found that washing once versus three times a week did not change how fast hair re-greased (Shaw 1979). Sebum output is driven by hormones; washing only changes how much has built up on the surface.
• Infrequent washing is reliably bad for the scalp. Studies that looked found more flaking, itch and scalp yeast as the gap between washes grew. Expedition members who could not bathe for 3 months had roughly 100- to 900-fold more scalp Malassezia, with itch and scaling (Sugita 2015).
• The headline "daily washing is best" study is weaker than it sounds. It was fully funded by Procter & Gamble, the daily washers were mostly aged 18-30 while weekly washers were mostly over 45, the treatment part had no control group, and the daily phase used a zinc pyrithione anti-dandruff shampoo (Punyani 2021).
• For straight, fine or oily hair, daily to every-other-day washing with a mild shampoo is supported, with no measurable loss of internal hair lipids over 4 weeks of daily washing (Punyani 2021).
• For curly, coily and very dry hair, less frequent washing is normal, but it has a floor. In 60 South African women with afro-textured hair, dandruff and itch were back at their peak within about a week of a wash, and weekly washing was recommended for dandruff-prone scalps (Fajuyigbe 2024).
• Hairs in the drain were already shed. Hairs that come out during washing are mostly resting hairs that would have fallen anyway. In healthy Chinese adults washing daily, about 28 hairs per wash was normal (Li 2022).
• Washing does fade dye. Oxidative hair colour leaches out a little with every wash (Zhang 2016), so coloured hair is the main legitimate reason to space washes out or use dry shampoo between them.

The science

Sebum builds up on a predictable schedule

After shampooing, the scalp re-greases along a curve: fast at first, then levelling off after about 2-3 days. L'Oreal measured this in 1,325 people from seven ethnic groups. The curve had the same shape everywhere but the amounts differed a lot: highest in African American subjects and lowest in Indian subjects. Men produced more than women and younger people more than older people (Pouradier 2017). In 123 Chinese men, sebum crept down the hair shaft at a steady rate, and self-described "greasy" men had about twice as much along the hair at 48 hours (Gao 2019). That is why fine, straight, oily hair looks dirty fastest.

Washing does not trigger "rebound" oil

The idea that the scalp makes more oil when you wash often, or can be "trained" to make less, has one direct test, and it found no effect on re-greasing and no sign that surfactants stimulate the oil glands (Shaw 1979). It is an old, small industry study, but nothing since has contradicted it. On facial skin, sebum returns to its usual level within about 2 hours of degreasing, which is the follicle refilling the surface rather than the gland speeding up (Rode 2000).

What builds up when you wash less

Malassezia yeasts break sebum down into free fatty acids, and sebum oxidizes over time. These by-products irritate the scalp and are central to dandruff (see scalp health and oxidized sebum).

• Antarctic expedition: scalp Malassezia rose by roughly 97- to 917-fold in 16 team members who could not bathe for 3 months, with itch and heavy scaling (Sugita 2015).
• Astronauts: 10 astronauts washing with wet tissues had rising Malassezia on cheek and chest over 6 months (Sugita 2016).
• Afro-textured hair: in 60 women followed for 3 weeks after a wash, dandruff severity peaked by the end of week 1 and then plateaued. One wash improved dandruff and itch, but the benefit did not last beyond a week (Fajuyigbe 2024, L'Oreal).
• Seborrheic dermatitis or psoriasis: 18 Japanese patients taught to wash daily for 12 weeks had less scaling and itch by weeks 8 and 12 and lower Malassezia counts. It was small and uncontrolled, and a pharmaceutical company co-authored it (Kobayashi 2016).
• "Bad hair days": in 13 women tracked through three menstrual cycles, bad hair days were more common in infrequent washers (p = 0.06, just short of significance). Scalp sebum level did not predict them (Birch 2003).

The main wash-frequency study

Punyani 2021 has two parts, both in Xi'an, China, both paid for by Procter & Gamble.

The authors state that both studies involved people with straight or low-texture hair and that it is not known whether the results apply to curlier hair.

Textured hair

Low wash frequency is common and often deliberate in tightly curled hair, because washing and detangling take time and can cause breakage. Women of African descent in London washed on average every 18 days, and every 33 days while wearing braids or weaves (Dadzie 2016, n = 242). In 201 African American girls, 61% had their hair washed every 2 weeks, and wash frequency was not linked to seborrheic dermatitis or scalp ringworm (Rucker Wright 2011). A comparison of African American and Caucasian women found large differences in washing frequency alongside more broken hairs and lower density in the African American group, but could not separate cause from correlation (Lewallen 2015). In a US survey, Black and mixed-race children with seborrheic dermatitis were much more likely to use medicated washes weekly or less (62.5% versus 30.8% of White children), which matters because medicated shampoos need regular contact to work (Hollins 2022).

What it means by hair profile

Myths and facts

Bottom line

Start from the table, then adjust. If your scalp gets itchy, flaky, smelly or visibly oily before your next wash, wash more often. If your lengths get dry, frizzy or tangled, keep the scalp schedule but switch to a milder shampoo, shampoo only the scalp and condition the lengths (see shampoo types and technique). If you wash rarely because of time or styling, cleansing just the scalp is better than nothing. With dandruff, wash more often and use a medicated shampoo two to three times a week. Do not judge hair loss by what is in the drain.