Porosity is a real property, badly measured by the float test

This is written to be used rather than admired. Each section below is a decision about hair porosity, and each one has a default.

What porosity describes

Porosity refers to how readily water and other substances pass through the cuticle into the cortex and back out again. A tightly closed, intact cuticle resists both directions, which is described as low porosity.

A lifted or eroded cuticle takes up water rapidly and loses it just as fast, which is high porosity. The property is partly genetic and largely a consequence of damage accumulated over the life of the fibre. Because the ends are older than the roots, porosity usually varies along a single strand rather than being one value.

Why the float test fails

The test asks you to drop a shed hair into a glass of water and observe whether it sinks or floats. Surface tension holds a light fibre at the surface regardless of its internal structure, particularly if any air is trapped along it. Product residue, natural oils and how the hair was handled all change the result more than porosity does.

Repeating the test with hairs from the same head frequently produces contradictory answers, which is the clearest evidence it is unreliable. It survives because it is simple and visual, not because it measures anything.

Better everyday signals

Hair that takes a long time to become properly wet in the shower is behaving as low porosity would predict. Hair that soaks instantly and then dries very quickly is behaving as high porosity would predict. Products sitting on the surface without absorbing suggests a closed cuticle, while products vanishing without effect suggests an open one.

How hair responds over a week is far more informative than any single test performed once. These are observations rather than measurements, which is the appropriate level of confidence to have.

What to do with low porosity

Lighter products applied to damp hair spread further and are less likely to sit on the surface as a coating. Warmth helps, so applying conditioner in a warm shower or with a towel wrapped around the head improves uptake. Heavy butters and oils frequently sit on top of low porosity hair and weigh it down without conditioning it.

After four to six weeks, clarifying occasionally matters more here, since anything that does not absorb accumulates.

Diluting products slightly with water can improve distribution noticeably on this hair type.

What to do with high porosity

Hair that loses water quickly benefits from sealing, which means applying a heavier product over a hydrating layer. Leave-in conditioners and light oils on damp hair slow the rate at which water escapes as the hair dries. Protein treatments help temporarily, since filling gaps in the cuticle reduces how open the structure is.

After four to six weeks, the underlying cause is usually accumulated damage, so reducing heat and chemical processing addresses the source. Very high porosity from severe bleaching may not be manageable indefinitely, and cutting is sometimes the honest answer.

Persistent, painful or changing skin conditions belong with a dermatologist rather than a routine.

Where the concept gets overextended

Porosity has become a category that entire product ranges are built around, which outruns what it actually predicts. It tells you something about product uptake and drying speed, and very little about curl pattern, thickness or growth. Hair of the same porosity can behave differently depending on density, diameter and curl, which are separate properties.

At the strengths actually sold, treating porosity as the master variable leads to buying products for a category rather than for an observed problem. Watching how your own hair behaves remains more useful than sorting it into a type.

The takeaway

Watch how fast your hair wets and dries, and put the glass of water away.

A short routine you finish beats a long one you resent.