How often to wash your hair

Two variables, and neither is hair type

Every article on this subject sorts people by hair type - straight, wavy, curly, fine, thick. That is largely the wrong axis. What actually determines how often hair needs washing is how fast it gets dirty, and hair gets dirty two ways.

• Sebum, from your scalp. Output varies enormously between people and is set by hormones and genetics. This is why two men with identical hair need different schedules.
• Product, from you. A wax-based pomade does not come out with water. If you use one daily, you wash daily, and no amount of scalp virtue changes that.

Hair type does matter for one thing: how visible the sebum is. Straight hair shows oil sooner because sebum travels down a straight shaft easily; tightly curled hair shows it much later because it does not. Same amount of oil, different appearance, different perceived need.

Where most people land

The myth, and the true thing inside it

The claim is that hair “trains” itself to be less oily if you wash it less. As usually stated, that is not right - sebaceous glands respond to hormones, not to your shower schedule, and they do not learn.

There is a real effect underneath it, though, and it is worth separating out. A harsh sulfate shampoo can strip the scalp badly enough to provoke a compensatory increase in oil production. Switch to a milder shampoo and that compensation stops. People experience this as “my hair got less oily when I washed it less”, but the change that mattered was the product, not the interval.

Medicated shampoo, where the label overrides you

This is the one part of the page where a specific number is available, and the reason is that dandruff actives are regulated drug ingredients with published directions filed with DailyMed.

• Pyrithione zinc 1 per cent (Head & Shoulders Classic Clean) - published for regular use, which makes it the one that fits into an everyday routine.
• Ketoconazole 1 per cent (Nizoral A-D) - published for twice-weekly use, with a mild shampoo on other days.

Follow the label rather than your preferred interval, and give one mechanism a full month before switching. Moving from one pyrithione zinc product to another is changing the bottle, not the treatment - the shampoo roundup sets out both actives properly.

Finding your own number in two weeks

Rather than adopting somebody else's schedule, run this. It takes a fortnight and it produces an answer that is actually about you.

• Week one: wash every other day, same shampoo, no product in your hair. Note at the end of each day whether the hair looked and felt fine.
• Week two: stretch to every third day. Water rinse on the off-days.
• Your interval is the longest gap where day two still looked fine. If day two of a three-day gap was already greasy, you are a two-day person. If day three was fine, try four.
• Then add your product back and adjust. Water-based products usually cost you nothing; wax and clay usually cost you a day.

Washing it properly, which changes the answer

Technique makes a genuine difference to how long a wash lasts.

• Shampoo the scalp, not the lengths. The scalp is where the oil is. The lengths get cleaned by the suds running past them, which is enough.
• Fingertips, not nails. Scratching a scalp irritates it, and an irritated scalp flakes and produces more oil.
• Rinse for longer than feels necessary. Residual shampoo is a common cause of both flaking and hair going limp within a day.
• Conditioner on the ends only, unless it is a scalp product - the conditioner roundup covers which is which, and it is the most misapplied product in the category.

One last thing that surprises people: water temperature. Hot water strips sebum efficiently and provokes the rebound described above. Warm is enough, and a cool final rinse closes the cuticle, which is worth about a day of looking clean.