How to tell your hair porosity (and why the float test gets it wrong)

If you've ever loved a product your friend swore by, only to have it sit on your hair like a film, porosity is probably why.

What porosity actually means

Porosity is how easily water gets into your hair, and how well it stays there. It comes down to the outer layer of each strand, the cuticle. When the cuticle lies flat and tight, water has a hard time getting in but doesn't leave quickly once it's there. When the cuticle is lifted or worn down, water rushes in and escapes just as fast.

That one difference changes almost everything about which products work for you. It's a bigger deal than curl type: two people with 4C hair can need completely opposite routines if one has low porosity and the other has high.

Skip the glass of water

You've probably seen the float test: drop a clean strand in water and watch whether it sinks. It's popular, and it isn't reliable. A single hair is light enough that surface tension can hold it up no matter its porosity. Leftover product, natural oils, or even how thick the strand is all change the result.

Signs you have low porosity hair

Water beads on your hair before it soaks in, and wash day takes a while to get you fully wet.
Your hair takes hours, or overnight, to air dry.
Heavy creams and butters leave it greasy, limp, or coated.
Deep conditioners don't seem to make much difference.
Color and relaxers take longer to process.
Your hair has rarely been bleached, colored, or heat styled.

Signs you have high porosity hair

Your hair gets soaked almost instantly and dries quickly.
It drinks up rich products and still feels dry a day later.
It tangles easily and can feel rough or dull.
Color fades fast.
You have a history of bleach, color, heat, or lots of sun and swimming.

Medium porosity sits in between: your hair takes and holds moisture without much drama, and most products behave.

It can change along the strand

Your ends have been around for years longer than your roots, so they've seen more heat, color, and weather. It's common for roots to be low porosity and ends to be high. If your results feel inconsistent, that's often why.

What to look for once you know

Low porosity: lighter, water-based products applied to soaking wet hair, lightweight oils like argan or jojoba, film-formers like aloe and flaxseed, and a regular clarifying wash to clear build-up.

High porosity: richer products that seal moisture in, like butters and heavier oils such as castor or olive, regular deep conditioning, and protein to help patch damage.