How thick should a section of hair be when you style it with heat?

Last updated: 19 September 2026

As a rule of thumb, a section of hair is right when it's no thicker than about 1 centimetre (roughly the thickness of a pencil) and no wider than your plates or your curling barrel. If you can faintly see through it, you're good. Sections that are too thick force you into extra passes, and that's exactly what stacks up the heat.

Further down you'll also read why section thickness works differently with an airstyler than with a straightener, and what to look for when choosing a tool if you have thick hair.

How thick should a section of hair be, exactly?

Stick to 0.5 to 1 centimetre of thickness, measured at the scalp, and make the section as wide as the working surface of your tool. That's the safest baseline for nearly every hair type, because the heat then reaches the core of the section without you having to repeat.

Plenty of advice online only mentions width ("take 3 cm sections"). But width mainly determines how quickly you're done. Thickness determines whether the heat reaches every hair. A section 5 cm wide and 1 cm thick is fine. That same section at 2 cm wide and 3 cm thick is a problem.

A blow-dry brush can handle thicker sections because it works with air rather than a hot plate pressed against your hair.

Why does section thickness matter so much?

Because a bundle of hair behaves like insulation. The heat doesn't simply travel to the middle of the section, and the hairs on the outside take the full hit.

That's not a figure of speech. Researchers testing human hair fibre as a building insulation material arrived at a thermal conductivity of around 0.037 W/m·K. That's in the same range as classic insulation wool. On top of that, there's air between the hairs, and air conducts heat even more poorly.

What that means in your bathroom: the outer layer of a thick section sits at the full temperature of your plates, while the hairs in the middle barely warm up. You can see that your section isn't smooth or curled yet. So you go back. And back again. The outer layer takes that heat one more time, while the core still lags behind.

Then there's water. At normal humidity, hair is about 13% water. That moisture needs to evaporate gradually. If it's trapped under a thick layer of hair, the core stays damp while the outside is already dry and hot. That exact combination of moisture and fast, localised heat is known in dermatology as the cause of bubble hair: bubbles inside the hair shaft that make hair brittle and prone to breaking. This has been reproduced experimentally with dry heat around 175 °C, and cases have also been described after using a curling iron.

Is a thick section worse than a high temperature?

Often, yes. Heat damage depends on temperature and on how long your hair feels that heat. A thick section extends both: you hold the tool in the same spot for longer, and you go over it more often.

Do the maths. A thin section done in a single three-second pass gets three seconds of contact. A thick section you go over three times, while also lingering a bit, easily reaches ten to twelve seconds. The outer hairs get all of that time, and always at the same temperature.

How do you know if your section is too thick?

Your hair tells you. These five signs almost always mean your section is too thick, not that your tool is set too low.

1. You have to go over the same section more than once.
2. You can't see light through the section when you hold it up to the lamp.
3. The top is smooth or curled, the layer underneath is frizzy or limp.
4. After styling, the inside of the section still feels cool or damp.
5. The curl drops within an hour, while the rest of your hair holds up fine.

That last point happens more often than you'd think. A curl only holds if the hair first warms all the way through and then cools down in the curled shape. If the core of the section never properly heats up, that shape never really set.

How many hairs are actually in one section?

More than you'd expect. A healthy scalp has roughly 150 to 230 hairs per square centimetre, and in total you've got around 80,000 to 120,000 hairs on your head.

Take a section 5 cm wide and 1 cm thick and you're grabbing roughly 750 to 1,150 hairs at once. Make that section 2 cm thick and suddenly there are 1,500 to 2,300 hairs in it. That doesn't make your tool any hotter or your pass any longer. The only thing that changes is that the heat has to travel that same distance inward through twice as much hair.

How do you divide your hair into the right sections, step by step?

Sectioning properly takes two minutes up front and saves you passes afterwards. Always work from the bottom up.

1. Comb your hair through completely. Tangles force you to tug and to make extra passes in the same spot.
2. Make a parting from ear to ear across your crown. Clip the top up. If you have a lot of hair, work in three layers instead of two.
3. Split the bottom layer into left and right. That way you work side by side and don't miss anything.
4. Take one horizontal section at a time, max 1 cm thick. Use the tail of a tail comb to lift the section cleanly, not your fingers.
5. Do the light test. Hold the section up, stretched out. If you can see the outline of your fingers through it, it's thin enough.
6. Apply heat protection section by section, not as a cloud over your whole head. That's the only way it also reaches the hairs in the middle of the section.
7. Style in one calm, continuous movement. If you have to go back, make the next section thinner instead of turning the temperature up.
8. Let each section cool down before you touch it, brush it out or squeeze it.

Does this work differently with an airstyler than with a straightener?

Yes, and it hasn't become any less important. An airstyler works with warm air instead of hot plates, and air needs somewhere to flow. A section that's too thick simply blocks it.

With a straightener you notice a too-thick section through streaks and frizz. With an airstyler it shows up differently: the section doesn't wrap tightly around the barrel, the outer hairs blow loose, and the inside stays damp. A curl that's still damp when you unwind it will drop within half an hour. That's why 0.5 to 1 cm of thickness is the standard here, and no more.

The barrel itself plays a role too. On the Airstyler Pro Ceramiq™, the curling barrel is fully ceramic coated, so no thin layer of spray over plastic. Ceramic spreads heat evenly across the entire surface. That means your section feels the same temperature across its full width and you're less likely to need a second round. And fewer rounds is exactly what this whole article is about.

In our own customer survey (214 users, 30 days of use, so not independent research), 96% reported curls that held for at least 24 hours. Among the people who didn't get there, a section that was too thick was by far the most common cause.

Should you adjust the thickness to your hair type?

The maximum thickness stays roughly the same, but you can adjust the width and the temperature. Fine hair can take wider sections, thick and coarse hair needs narrower ones.

• Fine or thin hair: sections can be wider (4 to 5 cm) and they stay thin all by themselves. Set your tool lower, because fine hair heats up fast.
• Normal hair: 3 to 4 cm wide, 1 cm thick. The standard from the table above.
• Thick, heavy hair: make sections narrower (2 to 3 cm) and work in more layers. It takes longer, but you never have to go back.
• Curly and coily hair (3A to 4C): the curl itself makes the section thicker than it looks. Stretch the section slightly before judging the thickness.
• Coloured or bleached hair: keep sections extra thin. That way you can get it done in one pass at a lower setting, and for damaged hair that's the biggest win there is.

Frequently asked questions

How thick is "pencil thickness" exactly?

About 0.7 to 1 centimetre. Pick up your hair at the scalp and pinch it flat between thumb and index finger: if it feels like a pencil, you're good. If it feels like a chunky marker, it's too much.

Can I take thicker sections if I'm in a hurry?

Take wider sections instead, not thicker ones. Wider goes faster without the heat having to travel any further. Thicker actually costs you time, because you'll need several passes.

Why does my curl stay limp on the inside?

Almost always because the core of the section didn't get warm enough. Hair only holds a shape if it first heats all the way through and then cools down in that shape. Make the section thinner and let it cool completely before you touch it.

Roughly how many sections do I need?

With shoulder-length hair of normal thickness you'll usually end up with 20 to 35 sections. If you have a lot of hair or long hair, count on 40 or more. That sounds like a lot, but each section takes less time.

Does this apply to a blow-dry brush too?

There you can take thicker sections, up to about 2 cm. A blow-dry brush works with air at a lower temperature and there's no hot plate pressed tight against your hair. What does matter is that the bristles reach the core of the section.

Should I take different sections with wet hair?

Yes, thinner. Wet hair holds far more water that has to come out, and it's precisely moisture plus localised heat that causes the most damage. Work with towel-dried hair and thin sections, or dry to about 80% first with cool air.

Written by Anne Leensen, Ace & Taylor