Hair porosity: what it is and how to really check it
Updated
Hair porosity is how easily your hair takes in water and how quickly it lets it go. It mostly depends on the condition of the outer layer of the hair, the cuticle. Low porosity hair is hard to wet and slow to dry, high porosity hair soaks up water at once and loses it fast, and medium porosity sits in between. It is a useful clue when choosing products, but it is not a fixed trait or a rigid label, and a glass of water cannot measure it reliably.
What is hair porosity?
Each hair is covered by overlapping cuticle scales, a bit like roof tiles. On undamaged hair the scales lie flat under a thin layer of lipids, which makes the surface slightly water-repellent, so water tends to run off.
When that layer is worn down, water and product ingredients get in more easily, but also leave more easily. That is higher porosity. "Porous hair" does not mean the hair has holes; it describes how tight the outer layer is.
What changes hair porosity?
- Chemical treatments. Bleaching, colouring, perms and chemical straightening raise porosity the most.
- Heat. Flat irons, curling tongs and very hot blow-drying.
- Wear and tear. Friction from pillows, brushing wet hair, tight styles.
- Sun, chlorine and hard water. Slower, but constant.
- Length. Ends are years older than the hair near the scalp, so usually more porous.
Genetics sets your starting point, but today's porosity is largely the history of what your hair has been through.
Can hair porosity change?
Yes, it goes up with bleaching, heat styling and every extra centimetre of length, and it does not go back down. Hair is dead tissue, so a damaged cuticle cannot be rebuilt for good. Products can smooth and seal it for a while, but the only real way to "lower" porosity is to grow out new hair and cut off damaged lengths.
Low, medium and high porosity hair
Three types is a simplification: one head of hair can be low porosity at the root and high porosity at the ends. Read these as sets of signs, not boxes to fit into.
| Low porosity | Medium porosity | High porosity | |
|---|---|---|---|
| Wetting | Water beads up, hard to soak | Wets normally | Soaks through at once |
| Drying | Very slow | Typical | Fast |
| Products | Easily weighed down, sit on top | Respond to many formulas | Absorbed quickly, hair still feels dry |
| Typical look | Smooth, shiny | Healthy, some movement | Dull, frizzy, fragile ends |
Low porosity hair
A tightly closed cuticle means hair is slow to wet and slow to dry. It tends to be smooth and shiny but easy to weigh down: a rich mask or a lot of oil can leave it limp and quicker to look greasy. Light formulas, small amounts and thorough rinsing usually work best. Low porosity hair can still frizz in humid weather, and it can be curly. Porosity and curl pattern are separate things.
Medium porosity hair
The most common result for people who have never bleached their hair. It copes with many formulas, but its needs shift after colouring, with heavy heat styling and as it gets longer. It is not "the best", just the least demanding. How your hair reacts to a particular product matters more than the category.
High porosity hair
A damaged cuticle lets hair absorb water quickly and lose it just as quickly. It is prone to frizz, tangles and breakage. It is most often bleached or heavily styled hair, though some people with very tight curls describe it too, because the bends in a strand are more exposed to wear.
Conditioners that reduce friction, film-forming ingredients (silicones included), oils and some hydrolysed proteins can help. None of them repairs the strand for good; they improve how it behaves while they sit on the surface.
The float test: how to do it and why it fails
The most popular home test goes like this: drop a single clean hair into a glass of water and watch. If it floats, it is meant to be low porosity. If it hangs in the middle, medium. If it sinks quickly, high.
The problem is that the result depends on things other than porosity:
- Surface tension of the water can hold up any fine hair, whatever the state of the cuticle.
- Sebum and product residue. Silicones, oils and conditioner change how water wets the hair, so the same hair behaves differently before and after a wash.
- Thickness and length. A thin, short hair acts differently from a thick, long one.
- Time and temperature. Hair is denser than water, so it eventually sinks. The verdict depends on when you stop watching, and warm or cold water changes it too.
- One hair is one spot. Porosity differs between root and ends.
So the float test can give one person different answers on the same day. It does no harm, but it is a poor basis for choosing products.
Other home tests
The slide test (run your fingers up a strand towards the root and feel for roughness) says something about the surface, but a leave-in can hide it. The spray bottle test (mist a dry section and see whether droplets soak in or sit on top) is closer to what porosity is, but needs clean hair with no product on it.
How to check hair porosity in practice
Instead of one test, answer a few questions about what you already see:
- How fast does your hair get wet in the shower? Water runs off and it is slow to soak: low porosity. It soaks through at once: high.
- How long does it take to air-dry? Damp for hours: low. Dry faster than other people's: high.
- What happens when you apply conditioner or oil? It stays on top and weighs hair down: low. It vanishes and the hair feels dry again soon after: high.
- What is your hair's history? Bleaching, perms or regular straightening almost always mean higher porosity, at least on the lengths.
- Do the ends behave differently from the roots? If so, you have two porosities and can treat them separately.
If the answers disagree, that is normal. Go with the pattern that repeats and watch how your hair reacts to products over several washes.
Porosity, density and texture are different things
People often mix these up.
| Term | What it describes |
|---|---|
| Porosity | How easily the cuticle lets water in and out |
| Density | How many hairs you have on your head |
| Strand thickness (texture) | How wide a single hair is: fine, medium, coarse |
| Curl pattern | The shape of the strand: straight, wavy, curly, coily |
Any combination is possible. Porosity is not determined by ethnicity or curl type; it is mostly a record of treatment and wear. It also says nothing about how fast hair grows, because growth happens in the follicle and porosity describes the surface.
Porosity and hair care
Porosity narrows down choices but does not replace observation. Change one thing at a time, judge it over several washes, start with a small amount of product and care for the lengths separately from the scalp, which has no porosity, only its own needs. Protecting hair from heat and friction slows the rise in porosity better than any mask. If your scalp itches, flakes or looks red, that is a question for a dermatologist, not for a porosity test.
For how proteins, emollients and humectants fit together, see the guide to protein moisture balance (PEH). To choose an oil, read about hair oiling. And if hair turned stiff after a protein mask, check the signs of protein overload.
Hair porosity FAQ
Does low porosity hair need protein?
There is no rule against it. Some low porosity hair turns stiff quickly with a large dose of protein, while other hair copes well with light hydrolysed proteins in a conditioner. Try a small amount and see.
Can you oil low porosity hair?
Yes, but lighter oils, smaller amounts and applying to damp hair or before washing with a thorough rinse usually work better. Details are in the oiling guide.
Does high porosity hair mean damaged hair?
Most often, yes, because high porosity is largely the result of cuticle damage. That does not mean it cannot look good. It needs more protection.
Which porosity is best?
None is "better". Low porosity hair is more resistant but easy to weigh down; high porosity hair takes in care easily but loses it fast.
How Flomsy helps
Your Flomsy profile includes a short porosity check: three questions about how your hair wets, dries and reacts to products, the same ones described above. The result is a hint, not a measurement, and you can change it at any time. You can also plan care on specific weekdays and tick it off on the day screen, and the ingredient scanner shows an estimated split of proteins, emollients and humectants in the products on your shelf.
