Fine, thin and thick describe three different measurements

These are listed in the order worth acting on, which with hair diameter and density is not the order they are usually presented in.

The two measurements

Strand diameter is the width of an individual hair, and it varies between people from around fifty to a hundred micrometres. Density is the number of hairs growing per unit area of scalp, which is a separate property entirely. A person can have fine strands at high density, producing hair that looks abundant and behaves softly.

Another can have coarse strands at low density, producing hair that feels substantial and shows scalp between sections. Because both look like a lot of hair or a little from a distance, the two get conflated constantly.

Working out which you have

Roll a single shed hair between your fingers, and if you can barely feel it the strand is fine, while a distinct thread indicates coarse. Comparing a strand against a sewing thread is the crude version of the same test and works well enough. For density, look at how much scalp is visible when hair is parted or pulled into a ponytail.

Ponytail circumference is a rough proxy for density and is affected by length and curl pattern as well. Neither test is precise, and knowing roughly where you sit on both scales is enough to make better product choices.

Why fine hair behaves as it does

A fine strand has less mass, so it is easily weighed down by conditioners, oils and silicones that coarser hair handles easily. It also has proportionally more cuticle relative to cortex, which can make it feel deceptively resilient to touch.

At the strengths actually sold, fine hair breaks more readily under mechanical stress because there is simply less material resisting the force. It holds styles poorly because there is less structure to retain a shape once heat or product wears off. Lightweight products, applied to lengths only and in small quantities, are the consistent answer here.

Why coarse hair behaves as it does

A coarse strand has more cortex, more internal structure and greater resistance to bending, which is why it feels wiry. It tolerates and often requires richer products, since a light conditioner leaves it feeling rough and difficult to manage.

It also resists chemical processing more, which is why colour can take longer to lift on coarse hair. Heat styling holds better because there is more structure to reshape and hold the new form. Coarse hair with low density often needs volumising approaches despite the strand being substantial.

Density and how much scalp shows

Density is largely genetic and changes over a lifetime, generally decreasing gradually with age. Low density is not the same as hair loss, and someone can have naturally sparse hair that has never changed.

At the strengths actually sold, products cannot increase the number of follicles, and volumising products work by increasing the apparent space each strand occupies. Cutting techniques, colour placement and styling do more for the appearance of density than any bottle. A noticeable and ongoing reduction in density, particularly at the parting or temples, is a matter for a doctor.

Choosing products from both numbers

Fine and dense hair usually wants light conditioning on the ends only, since the volume of hair holds product easily. Fine and sparse hair wants the lightest possible everything, because anything heavy removes what little volume exists.

At the strengths actually sold, coarse and dense hair usually needs richer conditioning and benefits from products that reduce bulk rather than add it. Coarse and sparse hair often needs richness for the strand and volume at the root, which pulls in two directions. Naming your combination takes two minutes and makes the shampoo aisle considerably less confusing.

Everything above, in order of what to do first

1. The two measurements. Strand diameter is the width of an individual hair, and it varies between people from around fifty to a hundred micrometres.
2. Working out which you have. Roll a single shed hair between your fingers, and if you can barely feel it the strand is fine, while a distinct thread indicates coarse.
3. Why fine hair behaves as it does. A fine strand has less mass, so it is easily weighed down by conditioners, oils and silicones that coarser hair handles easily.
4. Why coarse hair behaves as it does. A coarse strand has more cortex, more internal structure and greater resistance to bending, which is why it feels wiry.
5. Density and how much scalp shows. Density is largely genetic and changes over a lifetime, generally decreasing gradually with age.
6. Choosing products from both numbers. Fine and dense hair usually wants light conditioning on the ends only, since the volume of hair holds product easily.

The takeaway

Measure one strand and count the rest separately, because the two numbers point at completely different products.

Sunscreen, consistency and time outperform nearly everything sold as a breakthrough.