Perm rod sizes and colour codes — a convention, not a standard

Why the rods are coloured at all

The reason is entirely practical and it is worth knowing, because it explains both the usefulness of the system and its limits.

Early cold wave rods were wood or early plastics, made in a handful of diameters, with the measurement engraved on the barrel in millimetres. That is workable when you are choosing one rod. It is not workable when you are winding eighty of them onto one head, wet-handed, against a clock, reaching into a tray without looking. Nobody stops mid-wind to read an engraving.

So the trade converged on colour. Diameter became something you could recognise from across the room and grab by feel, and the whole tray became legible at a glance. By the time colour coding had settled into cosmetology training in the 1960s and 1970s it was universal in practice, and it has stayed that way — which is a genuinely useful piece of standardisation that happened without anybody standardising anything.

That is the crucial distinction, and the rest of this page rests on it. The colour convention became universal by habit. It was never made official, and nothing keeps it honest.

Size, incidentally, is only half the decision. The other half is the profile of the barrel — whether it is a plain cylinder or an hourglass — and that changes the character of the curl rather than its tightness. Salons discuss colour constantly and profile almost never, which is why so many clients get the diameter they asked for and a curl they did not. That argument is at concave and straight rods.

What each colour usually means

Here is the convention as most manufacturers follow it, with the diameters that generally sit behind each colour. Read the millimetres and treat the colours as labels on them.

Colour, typically	Roughly	What it gives	Where it is used
Yellow, red	4–6 mm	Tight coil	Very short hair, nape sections, deliberate ringlets
Blue, pink	8–10 mm	Defined corkscrew	Short to mid hair where the curl should be obvious
Grey, white	11–14 mm	The classic perm curl	The salon standard, and the most-used size on most heads
Purple	~16 mm	Loose, voluminous	Mid-length hair; body rather than curl
Peach, orange	19–22 mm	Beach wave, body wave	Long hair, soft movement
Teal, brown, black	25 mm and up	A bend rather than a curl	Root lift and heated rods; usually too large for a cold wave

Indicative, and it varies by maker. These bands are what you will find on most professional trays; they are not a specification, and a rod that measures 15 mm may be sold in any of three colours depending on who made it.

The bottom row is worth pausing on, because it is where most disappointments start. Above about 25 mm a cold wave generally cannot produce a curl at all on ordinary length — there is not enough travel round the barrel — and rods that size are doing a different job: lifting at the root, or working on a heated system where the shape is set thermally as well as chemically. See the digital hot perm.

How much the colours will bear

The colour system gets described as universal, and you will find claims that some very high percentage of professional rods follow the hierarchy exactly. Both are overstatements, and the second is invented precision. It is worth being blunt about what the system actually is.

Nobody owns this system

The rod colour convention is a convention, not a standard. No trade body defines it, no cosmetology board enforces it, and no manufacturer is obliged to follow it. Two rods labelled grey by two different makers need not be the same diameter, and in practice sometimes are not. A professional trusts the measurement. The colour is shorthand for it, not a substitute.

None of which makes the convention useless. It makes it useful for two specific things, and it is worth naming them because they are what the system is genuinely good at.

Inside one salon

A shared vocabulary

Within a single salon, working from a single set of trays, colour is precise. “Greys through the canopy, pinks at the nape” is unambiguous to everyone standing there, and it is faster than any other way of saying it. That is a real benefit and it is the reason the system exists.

Between appointments

A record you can repeat

The far more useful application, and the one clients almost never use. If a perm came out well, ask what was used and write it down. Then it can be reproduced deliberately, or changed deliberately — a size up if it was tighter than you wanted, a size down if it dropped. That is a much better conversation than “a bit looser this time”.

What the colour does not travel well between is salons. Taking a colour name to a different salon, or ordering rods from a different supplier on the strength of one, is where the convention quietly fails. Take the millimetres, or better, take the photograph.

The rule that overrules both

Hair must travel round the rod at least two and a half times to form a complete S. Below that, no chemistry in the world produces a curl.

This is arithmetic, not a school of thought, and it settles most arguments about rod size before they begin. One and a half turns around a barrel gives a single wide bend, which dries as a slightly odd blow-dry — not a wave, and not something that survives a wash. Two and a half turns gives a complete curl with the beginning of a repeat in it, which is what the eye reads as texture.

The consequence is direct: your length sets your maximum rod diameter, not your taste. A hand-span of hair on a 22 mm rod does not make a beach wave; it makes a bend. If the brief is soft loose movement and the length is short, there are two honest answers — a smaller rod and therefore a tighter curl than the photograph, or wait. Hair grows about half an inch, a little over a centimetre, a month, so waiting is a real and quantifiable option rather than a fob-off. Measuring hair length is the sober way to work out where you actually are.

Doing the sum

The circumference of a rod is a little over three times its diameter, so two and a half turns round a 20 mm rod needs somewhere near 160 mm of hair — about six and a half inches — travelling round the barrel, before you account for the section standing away from the head at the root. Round a 10 mm rod the same two and a half turns need half of that. This is why the answer to “can my hair take big loose waves” is a tape measure rather than an opinion, and why a stylist who says no to a large rod on short hair is not being unadventurous.

A stylist who takes the booking without doing that sum out loud is selling you a disappointment, and you will find out in about three weeks. The failure has a name in salons — the drop — and it is nearly always large rods on hair too short, too heavy or too resistant to hold them.

Rod size is one variable of four

The commonest version of this question is a client who used the same rod colour as a friend, or the same colour as last year, and got a different result. Two things are going on and the second is much the larger.

The first is the one already covered: the colour is not enforced, so the two rods may not have been the same size. The second is that even if they were identical, diameter is only one of four things deciding the outcome.

Rod diameter. The size of the curl. The one everybody discusses.
The wrap. Rolled from the tips towards the scalp, or wound down a length as a spiral, or stacked — the same rod produces a completely different architecture in each.
Base elevation. Whether the section is held above, on, or below its base as it is wound. That decides root lift and how the curl sits, and it is invisible in any photograph of a rod. See base control.
Your hair. Length, density, texture and condition, which between them decide how much of the rod’s geometry the fibre will actually keep.

Which produces the single most useful instruction on this page. If you want to reproduce a result you have seen, bring a photograph of the finished hair, not a rod colour. The photograph carries all four variables at once and lets the stylist work backwards to the tools. The colour carries one of them, unreliably.

And do not request a colour by name. The arithmetic is the stylist’s to do, and asking for a rod is asking them to solve for the wrong thing.

Why a considered head uses three or four sizes

One diameter used uniformly over a whole head is the single thing that dates a perm. It produced the eighties look and it still produces it, and it is what a drugstore kit containing one size of rod can only ever do.

A professional head mixes sizes for structural reasons rather than decorative ones. Hair at the nape is shortest, so it needs a smaller rod to reach two and a half wraps at all; hair over the canopy is longest and most seen, so it takes a larger one and reads as movement. Natural curly hair is tighter underneath and looser on top, and mixing sizes is simply an imitation of that.

Fine hair

Smaller than you would guess

Fine hair struggles to hold the weight of a large loose wave — there is not enough fibre to keep the shape standing — so it generally takes a grey or white rather than a peach, even when the brief is soft.

Coarse, resistant hair

Also smaller than you would guess

Thick resistant hair pushes back against the shape and loosens the curl on its own within a fortnight. The counter-intuitive answer is a smaller rod than the reference photograph, for the opposite reason to fine hair.

Weakened hair

Smaller still, if at all

Hair that has been tinted repeatedly or is porous holds curl memory poorly and drops faster. Where it can be permed, a specialist aims ahead of the photograph deliberately.

That last point is worth spelling out because it looks like a stylist ignoring your picture. Choosing the rod that matches the photograph exactly, on hair that will relax, produces a client who is delighted on the day and disappointed by the end of the month. Choosing a size smaller produces a client who thinks it is slightly too tight on day one and exactly right by week three. The second is the better service and it takes some explaining at the basin.

The limit no diameter gets round

On bleached or highlighted hair the answer is usually no, and usually is doing real work in that sentence. Bleaching oxidises cystine to cysteic acid, and a neutraliser needs two sulphur atoms it can no longer find, so most bleached or highlighted hair takes a curl that drops out within weeks while the hair pays for it. That is the default and the default is no. Foils count, and so does grown-out bleach still sitting on the lengths. There is no rod size that compensates for a bond that is not there — going smaller does not help and going larger does not either. It is a judgement made on the hair in front of me: how much lightener, how long ago, how much has grown out, what condition the rest is in. Occasionally, in the right hands and at the right strength, it is possible. Assume no, and come and let me look rather than deciding from a page. See bleach damage.

What diameter costs you in months

The last thing diameter decides is how long the result lasts, and the relationship is steep enough to be worth knowing before you choose.

Rod	Result	Reads as gone after
Small, 4–10 mm	Coil or firm corkscrew	Six to eight months
Standard, 11–16 mm	Classic curl	Four to six months
Large, 19 mm and up	Body wave, movement	Two to three months

The chemistry is identical in all three rows. What differs is how much shape there is to lose. A tight coil can relax a long way and still read as a curl; a loose wave has very little in hand, so a modest amount of relaxing takes it to the point where the client says it has dropped. Nothing has gone wrong in the bottom row — a large rod is buying movement, and movement is inherently less durable than a coil. Every bit of curl you soften is a bit of longevity you spend.

Two things that shorten the bottom row further

Weight, and water. Heavy conditioners and rich creams drag a large lazy wave straight within minutes of drying, so a large-rod perm wants light mousses and a diffuser rather than the same products a coil would take. And on London’s hard water, calcium settling into hair that is now more porous than it was stiffens and flattens a loose wave faster than it does a coil — a chelating shampoo every three or four weeks is not optional on a large-rod perm. More at curl memory and water.

Whatever the diameter, the first three days belong to the chemistry rather than to you. Seventy-two hours: neutralising reforms the bonds but does not finish the job on the spot, oxidation carries on in air, and the fibre keeps hardening for about three days. In that window what you press into the hair tends to stay — no tight ties, no clips, no hard brushing, and go gently with heat. Washing follows the instruction for the product actually used on you.

Questions

Is the rod colour system a standard?

No. It is a widely followed convention that grew up because a stylist winding eighty rods cannot stop to read engraved millimetres. No trade body defines it and none enforces it, so two rods labelled grey by different makers need not match. It is precise inside one salon and unreliable between two. Trust the measurement.

Should I just ask for the biggest rods?

Only if you have the length for them. Hair has to travel round the rod at least two and a half times to form a complete S, and on a 22 mm rod that needs something close to six and a half inches of hair going round the barrel. Below that you get one wide bend that reads as an odd blow-dry, not a wave.

Why did the same rod colour give my friend a different result?

Partly because the colours are not enforced, so the rods may not have been the same size. Mostly because diameter is one variable of four: the wrap, the base elevation and your own hair’s length and condition all change the outcome as much as size does. Bring a photograph of finished hair rather than a rod colour — it carries all four.

Will my curls look exactly like the rod size?

On the day, close to it. Over the following fortnight gravity and washing relax and elongate the curl, which is why a good stylist deliberately picks a rod slightly smaller than the look you asked for. If it seems a little tight when you leave, that is often the plan rather than a mistake — though you should be told that at the time.

Why would a stylist use three different sizes on one head?

To imitate how curly hair actually grows: tighter underneath, looser on top. There is a mechanical reason too — the nape is the shortest hair on the head and often cannot reach two and a half wraps on a large rod. One size used everywhere is the single thing that makes a perm look dated.

Do loose waves last as long as tight curls?

No, and the gap is large: two to three months against six to eight, on identical chemistry. The difference is how much shape there is to lose before it reads as gone. A large-rod perm is buying movement, and movement is inherently less durable than a coil.

Should I ask for a particular rod colour when I book?

Bring a photograph instead. The arithmetic — your length against the diameter that look needs — is the stylist’s to do, and asking for a colour asks them to solve for the wrong thing. What is worth recording is what was used on a perm you liked, so it can be repeated or changed deliberately next time.

Rod Sizes and Colour Codes