If you've spent any time in natural hair spaces, you've heard the word porosity. People throw it around like everyone already knows what it means. Most don't. And that's a problem, because porosity is the one factor that determines whether a product works for your hair or just sits on top of it doing nothing.
Let me give you the real explanation.
Your Hair Is Covered in Scales
Every strand of hair has an outer layer called the cuticle. The cuticle is made up of tiny overlapping scales, like shingles on a roof. Those scales open and close based on heat, pH, and the health of your hair. Porosity describes how open or closed those scales are in a resting state.
Low porosity means the scales lay very flat and tight. High porosity means the scales are lifted, with gaps between them. Medium porosity is somewhere in the middle.
Why does this matter? Because moisture has to get past those scales to actually hydrate your hair. And once it's in, those scales are what keep it there.
Low Porosity Hair
Water beads on low porosity hair. Products sit on top instead of absorbing. This hair takes a long time to get fully wet, and it takes a long time to dry. If you've ever felt like your hair rejects every product you put in it, that's likely low porosity.
Low porosity hair needs heat to open those scales. Steaming, warm water rinses, and heat caps during deep conditioning all make a real difference. Water-based products work better than heavy butters and oils, which just create more buildup on top of scales that are already reluctant to let anything in.
High Porosity Hair
High porosity hair absorbs everything fast. It gets wet quickly and dries quickly. It looks thirsty again within hours of styling. This hair often feels rough, tangles easily, and breaks more than it should.
The gaps in the cuticle let moisture in fast, but they let it out just as fast. The goal with high porosity hair is sealing. After you moisturize, you need something to close those scales back down temporarily. That's where heavier oils, butters, and protein treatments come in. Protein helps fill those gaps and temporarily reduces porosity.
How to Test Your Porosity
The float test you've seen online (drop a strand in water and see if it sinks) isn't very accurate. Shed hairs carry product buildup and oil that affects the result.
A better method: wash your hair with a clarifying shampoo, then observe. How long does it take to fully saturate under water? How quickly does it dry? Does product absorb in or sit on top? The answers tell you more than a float test ever will.
What Changes When You Know Your Porosity
Everything. Product selection, how long you deep condition, whether you use heat during conditioning, what you seal with, how you layer products. You stop buying products based on what worked for someone else's hair and start buying based on what your hair actually needs to receive and hold moisture.
Low porosity: lighter products, heat, more water.
High porosity: protein, heavier sealants, cold water rinses to close the cuticle.
This is the foundation. Every other decision in your routine sits on top of this one.