Heat Damage & Protection
The Best Flat Iron Temperature by Hair Type
Two manufacturers set their irons at 365°F. Peer-reviewed work puts the melting point of dry hair keratin at 401°F. A lot of irons dial to 450.
Updated September 2026 · Sawyer V.
Straighten in the mid-300s Fahrenheit. ghd fixes its Platinum+ at 365°F with no dial, and Dyson tells Corrale owners to start at 365°F. Published research puts the melting point of the ordered keratin in dry hair at about 401°F — below the 450°F many irons will happily reach.
The number the manufacturers picked for you
Two companies with every commercial reason to make their tools look powerful have both landed on the same temperature for typical hair.
- ghd sells the Platinum+ with a single fixed temperature of 365°F and no adjustment at all. There is no dial to turn up.
- Dyson tells Corrale owners to begin at a medium heat, 365°F, and adjust from there.
Meanwhile plenty of irons dial past 450°F, and people use those settings routinely because the number is available and the first pass did not take. The gap between what the makers recommend and what their hardware permits is the single most actionable thing on this site, and closing it costs nothing.
What the published science adds
Two peer-reviewed findings make this concrete rather than a matter of taste.
Cao and Leroy, measuring by differential scanning calorimetry, reported that the melting temperature of the alpha-form crystallites in human hair varies with moisture content, from 205°C for dry hair to 155°C for hair at 23% moisture content. In Fahrenheit that is 401°F dry and 311°F damp.
Separately, Lima and colleagues, studying the effect of flat irons on virgin Caucasian and Asian hair, reported a denaturation temperature of 237°C — 459°F — alongside electron microscopy showing heat damage to the cuticle, and concluded that the results evidenced the importance of the control of temperature in the procedures involving heat.
Put those together and a 450°F setting is not a safety margin. It is at or above the temperatures at which published work observes the ordered structure of hair keratin changing.
The table
Styling temperature reference
| Reference point | °F | °C | What it is |
|---|---|---|---|
| ghd Platinum+ fixed setting | 365°F | 185°C | The only temperature this iron offers — set by the manufacturer |
| Dyson Corrale starting point | 365°F | 185°C | Dyson's own stated starting temperature for the Corrale |
| Melting point, hair at 23% moisture | 311°F | 155°C | Published DSC figure — why you never iron damp hair |
| Melting point, dry hair | 401°F | 205°C | Published DSC figure for alpha-form crystallites in dry hair |
| Keratin denaturation in a flat-iron study | 459°F | 237°C | Published figure alongside observed cuticle damage |
| Where many irons top out | 450°F | 232°C | Available on plenty of consumer irons — above the dry melting point |
Manufacturer settings from the product pages linked below. Melting and denaturation figures from the peer-reviewed papers in the sources. Fahrenheit values are direct conversions of the published Celsius figures.
By hair type
No manufacturer in our registry publishes a per-hair-type temperature chart, so we are not going to invent one and dress it up as data. What follows is reasoning from the two published anchors above, stated as reasoning.
Where to start, and why
| Hair | Where to start | The reasoning |
|---|---|---|
| Fine or fragile | Below 365°F | Less mass per strand means it reaches plate temperature faster, so the general-purpose number is the ceiling rather than the target |
| Color-treated or damaged | Below 365°F | Already-compromised cuticles lose moisture faster, and the melting point falls sharply with moisture content |
| Normal / medium | 365°F | The setting two manufacturers chose when they had to choose one |
| Thick or coarse | 365°F, then up only if one pass genuinely fails | Coarse strands hold more mass, but the published melting figures do not change with strand thickness |
| Coily / 4C, pressing | Higher, but stay below 400°F | Resistant textures may need more, and 401°F is the published dry melting point — there is no headroom above it |
| Any hair, still damp | Do not iron it at all | At 23% moisture the published melting point falls to 311°F, below every common iron setting |
These are our conclusions from the published figures, not manufacturer recommendations. Where a maker publishes a number we say so and name them; where nobody publishes one, this is reasoning and it is labeled as such.
Why damp hair is the real hazard
This is the most under-appreciated finding on the page. The published melting point of the ordered keratin drops from 401°F in dry hair to 311°F at 23% moisture content — a fall of ninety degrees Fahrenheit from water alone.
Which means damp hair at a setting you would consider conservative can be well past the point where published work observes structural change. If you take one thing from this page, take that: it is not the dial that makes ironing damp hair dangerous, it is the water.
Passes matter as much as the setting
Total exposure is roughly temperature multiplied by contact time, and contact time is dominated by how many times you go over a section. Three passes at 380°F is more exposure than one at 420°F, even though the second number looks worse.
So the goal is not the coolest possible setting — it is the coolest setting that finishes a section in one pass. If nothing under 400°F works, the problem is usually section thickness or hair that is not fully dry, not the temperature.
The tools that make one clean pass realistic are covered at flat irons for fine hair and flat irons for coily and 4C hair.
Questions people actually ask
What is the best flat iron temperature to avoid damage?
The mid-300s Fahrenheit for most hair. ghd fixes its Platinum+ at 365 degrees with no dial, and Dyson names 365 as the Corrale's starting point. Published DSC work puts the melting point of ordered keratin in dry hair at about 401 degrees Fahrenheit.
Is 450 degrees too hot for a flat iron?
It is above the published melting temperature of the alpha-form crystallites in dry hair, which peer-reviewed work puts at about 401 degrees Fahrenheit, and close to the 459 degree denaturation temperature reported in a flat-iron study that also observed cuticle damage.
What temperature should I use on damp hair?
None — do not iron damp hair. Published research shows the melting point of ordered keratin falls from about 401 degrees Fahrenheit in dry hair to about 311 degrees at 23% moisture content, which is below almost every iron setting.
What temperature for a silk press on 4C hair?
Higher than for fine hair, because resistant textures need more to finish in one pass, but there is no headroom above the published 401 degree Fahrenheit dry melting point. Blow-drying with tension first is what reduces the temperature you need.
Sources
- Cao J, Leroy F., “Depression of the melting temperature by moisture for alpha-form crystallites in human hair keratin”, Biopolymers 2005;77(1):38–43
- Lima CRRC et al., “Heat-damaged evaluation of virgin hair”, Journal of Cosmetic Dermatology 2019;18(6):1885–1892
- ghd — Platinum+ published fixed 365°F styling temperature
- Dyson — Corrale straightener published starting-temperature guidance