Porosity measures how tightly your cuticle sits against the hair’s cortex. A tightly closed cuticle (low porosity) beads water like a freshly waxed car hood. A lifted or damaged cuticle (high porosity) drinks water immediately but can’t hold onto it — hair soaks through in seconds but dries frizzy within minutes.
The classic test (imperfect but useful): drop a clean, dry hair strand into a glass of room-temperature water. If it floats on top for minutes, you lean low-porosity. If it sinks immediately, you lean high-porosity. Most people sit somewhere in between and have different porosity in different sections.
Porosity is both genetic and acquired. Coarse, straight, or chemically unprocessed hair tends toward low porosity. Color, bleach, relaxers, and heat damage raise porosity over time. High porosity isn’t a life sentence — repair treatments, protein, and proper lipid balance can bring a damaged cuticle partway closed again.