Does Hair Porosity Really Change How You Should Use Hair Oil?
Does Hair Porosity Really Change How You Should Use Hair Oil?
If you’ve spent any time on hair-care TikTok or scrolling through beauty blogs lately, you’ve probably seen the porosity test: someone plucks a strand of hair, drops it into a glass of water, and waits. If it floats, congratulations, you supposedly have low-porosity hair and should avoid heavy oils at all costs. If it sinks, you’re told you have high-porosity hair and need to reach for the richest oils and butters you can find.
It makes for a great viral video. But from a cosmetic-chemistry perspective, the reality is far less definitive. The internet has turned hair porosity into a rigid identity, almost like a zodiac sign for your strands, with a prescribed list of oils you should use and those you should avoid. In reality, porosity exists on a spectrum, can vary across the same head of hair, and doesn't dictate your routine quite as neatly as social media would have you believe.
So, what does hair porosity actually tell you about your hair; and does it really change the way you should use hair oil?
Let's look beyond the float test and into the biology of the hair fiber to find out.
What Is Hair Porosity, Actually?
To understand porosity, you have to visualize the outer layer of your hair: the cuticle.
Think of the cuticle like microscopic shingles on a roof. When hair is incredibly healthy and hasn’t been chemically processed or subjected to intense heat, those shingles lay flat and tight. This is what the internet calls "low porosity." Water and oils take a little longer to get past that tight barrier.
"High porosity" hair simply means those shingles are lifted, chipped, or missing altogether. There are gaps and voids in the fiber. While some people naturally have slightly more raised cuticles (often those with tighter curl patterns), high porosity is almost always a byproduct of damage. Bleaching, permanent color, relaxing, daily flat-ironing, and even just the natural weathering of long hair over several years will increase a strand's porosity.
So, porosity isn’t really a fixed genetic category. It’s a sliding scale of wear and tear. The ends of your hair are almost always more porous than the roots simply because they are older and have survived more brushing, washing, and styling.
The Problem With the Float Test
Before we talk about oil, we need to retire the glass of water test.
If you drop a strand of hair into a glass of water, whether it sinks or floats is determined by a few things: the surface tension of the water, the natural weight and density of your specific hair strand, and, most importantly, the invisible layer of natural sebum (scalp oil) or leftover product sitting on that strand.
Oil naturally repels water. If your strand has even a microscopic coating of natural scalp sebum, it’s going to float. The float test doesn't tell you about the microscopic voids in your hair's cuticle; it mostly tells you about physics and surface tension.
You don't need a glass of water to know your porosity. If your hair takes hours to get fully wet in the shower and takes all day to air dry, your cuticles are likely tight (lower porosity). If your hair gets instantly soaked the second water hits it, dries incredibly fast, and is color-treated or heat-styled daily, you are dealing with higher porosity.
How Oil Actually Interacts With Your Hair
Here is the biggest myth about hair oil: Oil seals moisture into the hair.
Oils do not physically act like glue, sealing the cuticle shut or trapping water inside forever. Instead, what oil actually does depends entirely on its molecular size.
Penetrating Oils Certain oils, like coconut oil, have a very small molecular structure (specifically, straight-chain fatty acids like lauric acid). Because they are so small, they can actually slip right past the cuticle and absorb directly into the inner cortex of the hair fiber. This happens regardless of whether your hair is low or high porosity.
Coating Oils Other oils, like castor oil or jojoba, have much larger, bulkier molecular structures. They are simply too big to squeeze past the cuticle. Instead, they sit on the outside of the hair strand. They create a lubricating film that reduces friction, makes the hair feel incredibly soft, and creates serious shine.
Does Porosity Change Your Routine?
Yes and no. The fundamental biology of how oil penetrates doesn't change, but the condition of your cuticle dictates what kind of help your hair needs most.
If you have highly porous, damaged hair: Your cuticles are chipped. Your hair feels rough and tangles easily because those raised cuticles catch on each other like velcro. You benefit massively from heavier, coating oils. Why? Because those thick oils fill in the gaps and voids on the surface of the hair. They act like a temporary spackle, smoothing over the damage, masking the rough texture, and providing the lubrication your hair desperately needs to prevent snapping when you brush it.
If you have low-porosity, healthy hair: Your cuticles are flat. If you apply a heavy, bulky oil to low-porosity hair, it has nowhere to go. It just sits on the surface, weighing the hair down, making it look greasy, and causing buildup that requires harsh shampoos to remove. This is where the myth that "low porosity hair can't absorb oil" comes from. It absolutely can : you just have to use penetrating oils with smaller molecules, and you need to apply them strategically.
The Great Equalizer: Pre-Wash Hair Oiling
Whether your cuticles are tightly sealed or highly porous, the single best way to use hair oil is as a pre-wash treatment.
When you wash your hair, it absorbs water and swells. This swelling forces the cuticles to lift up. As the hair dries, it shrinks back down. Doing this repeatedly causes something called "hygral fatigue" ; essentially, the structural weakening of the hair fiber from constant expanding and contracting.
Applying a proper oil treatment before you wash your hair changes everything. The oil acts as a hydrophobic (water-repelling) barrier. It prevents the hair fiber from swelling up like a sponge in the shower. It also protects your hair’s natural proteins from being aggressively stripped away by the surfactants in your shampoo.
For high-porosity hair, pre-wash oiling protects an already fragile strand from taking on too much water and snapping. For low-porosity hair, it offers deep nourishment without the risk of leaving your hair flat and greasy after you style it, because the excess surface oil is gently washed away.
The Final Verdict
Don't let a glass of water dictate your hair care routine. Hair porosity is real, but it’s a measure of your hair's current condition, not a permanent label.
Highly porous hair thrives on the surface lubrication that heavier oils provide, while low-porosity hair prefers lightweight, penetrating oils that won't cause buildup. But regardless of where your hair falls on the spectrum, the biology of the hair fiber shows that protecting the strand before you expose it to water and shampoo is the ultimate game-changer.
Focus less on whether your hair floats, and focus more on consistent, high-quality pre-wash protection. Your hair will thank you for it.