Hat Sizes in cm vs Inches: Which to Work In
Both units describe the same head and one of them causes fewer mistakes. We work in centimeters when precision matters.
Centimeters are the better unit for hat sizing because the gradations are finer — one fitted size step is about one centimeter but only three eighths of an inch, which people round badly. For adjustable caps either unit is fine. For fitted, stretch-fit and any international order, centimeters remove a whole class of rounding error.
- 1 fitted size step &asymp. 1 cm, or 3/8 inch.
- Centimeters round more cleanly. Fewer conversion errors.
- International sizing is centimeters. EU and JP use it directly.
- Made in the USA, no minimums. One hat is a real order.
The Two Side by Side
| Inches | Centimeters | Fitted size |
|---|---|---|
| 21 1/4 | 54 | 6 3/4 |
| 21 5/8 | 55 | 6 7/8 |
| 22 | 56 | 7 |
| 22 3/8 | 57 | 7 1/8 |
| 22 3/4 | 58 | 7 1/4 |
| 23 1/8 | 59 | 7 3/8 |
| 23 1/2 | 60 | 7 1/2 |
| 23 7/8 | 61 | 7 5/8 |
| 24 1/4 | 62 | 7 3/4 |
The centimeter column advances by whole numbers. The inch column needs eighths to say the same thing.
That is the practical case in one table. Each size step is a clean single centimeter, where in inches it is three eighths — a fraction people round to a half or a quarter, which is enough to shift a size.
Our charts are at types of hat sizes and fitted hat sizes explained.
Record measurements in centimeters even if you took them in inches. Whole numbers survive being written down and repeated in a way eighths do not.
Converting Between Them
The arithmetic is simple. Multiply inches by 2.54 for centimeters, or divide centimeters by 2.54 for inches.
Two shortcuts that are accurate enough for headwear. An inch is roughly two and a half centimeters, so 22 inches is about 56. And each centimeter is a bit under half an inch, so a two-centimeter difference is roughly three quarters of an inch.
Where conversion goes wrong is rounding at the wrong point. Converting 22 3/8 inches to 56.8 centimeters and then rounding to 57 is fine. Rounding 22 3/8 to 22 1/2 first and then converting gives 57.15, which reads as 57 anyway — but the same sloppiness on a borderline measurement can land a size out.
The habit that avoids it. Convert first, round last.
Convert first and round last. Rounding an inch measurement before converting is where most unit errors originate, and on fitted sizing it can shift a full size.
When Each Unit Is Fine
- Adjustable caps. Either. One size spans two and a half inches, so precision is irrelevant.
- Knit beanies. Either, and barely needed at all.
- Stretch-fit. Centimeters, since band boundaries are where people get caught.
- Fitted. Centimeters. Eighth-inch gradations are exactly where rounding errors live.
- International orders. Centimeters, without exception.
- Children's sizing. Centimeters, since the bands are narrow.
The pattern is that the more precise the construction, the more centimeters help. On forgiving constructions the unit is genuinely a matter of preference.
Why International Sizing Uses Centimeters
European and Japanese hat sizes are the centimeter measurement itself — a 57 is a 57-centimeter head. There is no derivation, no offset and nothing to convert.
That makes them the clearest system in use, and it is why working in centimeters removes friction on any non-US order.
The contrast is instructive. US fitted sizes derive from head diameter, and UK fitted sizes use the same notation with a different derivation that runs one eighth smaller. Converting between those two through inches compounds two sources of error at once.
Converting through centimeters avoids it, because every system agrees on what a centimeter is. Our detail is at the hat size conversion chart.
Converting a US fitted size to a UK one through inches compounds two errors — the rounding and the one-eighth offset between the systems. Convert through centimeters instead.
- Rounding inches before converting. Convert first, round last.
- Reading a European size as a US fitted number. A 57 is a measurement, not a size.
- Recording measurements as approximate inches. Eighths get lost when written down.
- Using inches on children's sizing. The bands are too narrow for the rounding.
Collecting Measurements from a Group
A practical point for anyone gathering sizes rather than taking them.
People reporting their own measurement round generously and inconsistently — 22 1/2 covers anything from 22 3/8 to 22 5/8 in practice. In centimeters the same person reports 57, which is a single value.
So asking for centimeters produces cleaner data even from people who measured in inches, because there is less room to be vague.
Two habits help further. Ask for a number rather than a size name, since size names carry whatever assumption the person is making. And ask them to measure twice and give the larger reading, since the common measuring errors all read short.
For adjustable programs none of this is needed. Our guide is at group order sizing.
Millimeters and Why Nobody Uses Them
A reasonable question if centimeters are better for being finer: why not go finer still?
The answer is that precision beyond the centimeter exceeds what the measurement can support. A soft tape around a head, taken by a person, on hair that compresses differently each time, is not reliable to the millimeter — repeat the measurement and it will vary by more than that.
Fitted sizes also advance in roughly one-centimeter steps, so millimeter precision offers nothing to act on. A reading of 567 millimeters and one of 572 both land on the same hat.
The practical resolution is that centimeters to the nearest whole number is the right level of precision for headwear — fine enough to distinguish sizes, coarse enough to be repeatable.
Where millimeters do appear is in manufacturing specifications for blanks, where a machine rather than a tape is doing the measuring and consistency between units is the point.
What the Unit Does Not Fix
Worth saying, because precision in the wrong dimension does not help.
Circumference is one dimension. Crown depth is the other, and no unit makes it more measurable — it is rarely quoted on blanks and rarely measured on heads.
So a measurement recorded to the nearest millimeter still will not tell you whether a cap will perch or sit low. That depends on the blank's crown profile against the wearer's head height, which is a matching problem rather than a measuring one.
Our detail is at custom dry fit hats for performance builds and kids hat sizes by age for the children's ranges.
What We Tell Customers Who Ask
Working in centimeters costs nothing and prevents a class of error that shows up as wasted fitted hats. It is the cheapest accuracy improvement available.
If you know your size in one unit, note both. A measurement recorded in only one unit gets converted badly later, usually by someone in a hurry.
For any order from a non-US supplier, specify in centimeters. Every system agrees on what a centimeter is, and none of them agree on how to label it.
For adjustable programs use whichever unit is convenient. For fitted, stretch-fit, children's or international orders, work in centimeters throughout.
We ask for centimeters on precise orders because whole numbers survive being written down and repeated where eighths do not. Free digital proof, free artwork setup on twelve or more, no minimums.
Types of hat sizes, the hat size conversion chart, fitted hat sizes explained.
Frequently Asked Questions
Should I measure my head in cm or inches?
Centimeters for precision. One fitted size step is about one centimeter but only three eighths of an inch, and eighths are where rounding errors happen.
How do I convert inches to centimeters?
Multiply by 2.54, or use the shortcut that an inch is roughly two and a half centimeters. Convert first and round last, never the other way round.
What is 22 inches in hat sizing?
About 56 centimeters, which is a US fitted size 7 and a European 56.
Why do European hat sizes use centimeters?
Because the size is the measurement. A European 57 fits a 57-centimeter head, with no derivation or offset, which makes it the clearest system in use.
Does the unit matter for adjustable caps?
Barely. One adjustable size spans about two and a half inches, so precision is irrelevant. The unit matters most on fitted, stretch-fit and children's sizing.
What should I ask for when collecting sizes from a group?
Centimeters, as a number rather than a size name, and ask people to measure twice and report the larger reading since the common errors all read short.
Is there a minimum order?
No. One hat is a real order, and artwork setup is free on orders of twelve or more.
Centimeters where it matters, either where it does not. Order custom hats — start on the quick order page — fitted, stretch-fit and adjustable in both measuring systems. Free digital proof before production, free artwork setup on twelve or more, no minimums. Browse custom hats.
Popular blank: Flexfit 6511 Trucker.
Related blanks: Richardson 112 Snapback Trucker.
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