The hair porosity test that works, and the one that does not

Someone on the internet told you to put a hair in a glass of water. If it sinks, high porosity; if it floats, low. The test spread because it is easy to film, and it survives because nobody checks.

Here is the problem. A single hair weighs on the order of a microgram and has a diameter smaller than a fine thread. Surface tension at the water's surface is more than strong enough to hold it there regardless of what its cuticle is doing. Whether it eventually sinks depends on how gently you dropped it, whether the strand carries silicone or oil from your last wash, how long you wait, and whether anyone breathed near the glass. Take ten strands from one head and you will get a mix of sinkers and floaters. The test does not measure porosity. It measures dust, residue and patience.

Which is a shame, because porosity is a genuinely useful thing to know.

What porosity actually is

Every strand is a rod of protein wrapped in overlapping scales, like roof tiles. That outer layer, the cuticle, is the hair's raincoat: intact and flat, it repels water, holds the inner cortex's moisture in, and reflects light in a way we read as shine.

Porosity describes how easily water gets through it. A tightly packed cuticle with its lipid layer intact — the coating cosmetic chemists call 18-MEA — is hydrophobic. Water beads on it. That is low porosity. When the cuticle is lifted, chipped, or has had that lipid layer stripped off by bleach, water goes straight in. That is high porosity.

The important part is what follows from it: the hair that lets water in fastest also lets it out fastest. High porosity hair soaks a conditioner up in seconds and is dry and rough again by mid-afternoon. Low porosity hair takes twenty minutes to wet through properly in the shower and then holds onto everything, including the conditioner it never really absorbed, which sits on the surface and reads as greasy roots and limp lengths.

Three hair porosity tests that tell you something

None of these are laboratory-grade. All three beat the glass.

The wetting test. Step into the shower with dry hair and count how long it takes to be properly wet — not damp on the outside, but soaked through so it clings to your neck. Under 15 seconds and your cuticle is letting water straight in. Approaching a minute, with water still beading on the surface while you scrub, and it is not.

The drying test. How long does your hair take to air-dry, compared with other people's hair of the same length and thickness? High porosity hair dries fast because the same open cuticle that let water in lets it evaporate. Hair that is still damp three hours later is low porosity, and this is the single most reliable signal on the list.

The conditioner test. Apply conditioner to soaking-wet mid-lengths. High porosity hair drinks it: the strands feel slippery almost immediately and the product seems to vanish. Low porosity hair sits there with a film on top that never quite goes anywhere.

The porosity nobody mentions: it is not uniform

Your hair is not one porosity. It is a gradient.

The hair emerging from your scalp today has never been bleached, brushed, blow-dried, slept on, or exposed to a summer. The hair at your ends, if it is long, is four or five years old and has been through all of it several thousand times. On a head of hair past the shoulders, the difference in porosity between root and tip is usually larger than the difference between two random people.

This is why "I have low porosity hair" as an identity leads people wrong. It leads to protein-free routines on ends that are crying out for structure, or heavy oils on roots that cannot take them. Treat the ends as high porosity and the roots as low porosity, because on most heads that is simply true.

What to actually change

If your hair reads as high porosity, the goal is to slow the exit. That means conditioner while the hair is soaking wet rather than towel-dried, so you are sealing water in rather than trying to add it later. It means finishing with something occlusive — a light oil or a cream on the ends — because a lifted cuticle needs something over the top of it. And it means treating heat as expensive, since every pass with an iron lifts the cuticle further. The protein side matters too: hair that has lost cuticle has often lost cortex protein along with it, and the protein and moisture balance is the thing to get right before anything else.

If your hair reads as low porosity, your problem is getting anything in at all. Warmth helps: conditioner on hair in a warm shower, or under a towel for a few minutes, does more than the same product on cold, damp hair. Lighter formulas beat heavy butters, which will only sit on the surface. And you can usually cut the protein back — that stiff, straw-like feeling low porosity hair gets after a protein mask is protein sitting on the outside of a cuticle it never entered.

Why this is worth ten minutes

Almost every routine failure we see is a mismatch between what hair needs and what it is being given: moisture piled onto hair that has lost its protein structure, protein piled onto hair that is simply dry, heavy creams on a cuticle that cannot take them. Porosity does not tell you everything, but it tells you which direction to move in.

Then check whether the hair is shedding or breaking, because that answer changes the plan completely — and it is the one people most often get wrong.