Hat sizes: head circumference to US hat size

Measure around your head, just above the ears and eyebrows, and read the row. The US hat size is the head circumference in inches divided by pi, rounded to the nearest eighth. Letter sizes and the centimeter number do the rest.

The values below are copied from Ruban Size's Hats, Kids' hats and Helmets templates, with inches added (centimeters divided by 2.54). They are typical values that a store replaces with its own, so the brand's chart wins for a given hat.

Hat size chart

From the Hats and caps template:

Size	US hat size	Head (cm)	Head (in)
S	6 3/4 - 6 7/8	54-55	21.3-21.7
M	7 - 7 1/8	56-57	22.0-22.4
L	7 1/4 - 7 3/8	58-59	22.8-23.2
XL	7 1/2 - 7 5/8	60-61	23.6-24.0

Each letter covers two centimeters of head and two US sizes: S is 54 cm (6 3/4) and 55 cm (6 7/8).

How the US hat size works

A US hat size is the diameter of a round head with your circumference, in inches. Divide the circumference in inches by pi (3.1416) and round to the nearest eighth.

Worked through for a 57 cm head:

57 cm ÷ 2.54 = 22.44 in.
22.44 ÷ 3.1416 = 7.14.
The nearest eighth is 7 1/8.

You can check every cell of the template this way. It also gives a handy rule: one eighth of a US hat size is pi ÷ 8 = 0.39 in of head, which is 1.0 cm. Each centimeter of head moves the US size by one eighth, which is why the table runs from 6 3/4 at 54 cm to 7 5/8 at 61 cm in steps of one eighth.

For shoppers outside the US, show the centimeter number itself, with letters as the size names. The cm to inches guide for size charts covers how to write the inch values, fractions included.

How to measure, and what to do between two sizes

The template's step: "around your head, just above the ears and eyebrows." Keep the tape level and snug, not tight. The full method, with the helmet version, is in how to measure your head for a hat or a helmet.

Between two sizes, take the larger one. The template's note says so, and Find my size does the same: a 57.5 cm head, between M (56 to 57) and L (58 to 59), gets L.

Kids' hats by age

From the Kids' hats template:

Age	Head (cm)	Head (in)
0-6 M	40-43	15.7-16.9
6-12 M	44-46	17.3-18.1
1-2 Y	47-49	18.5-19.3
2-4 Y	50-51	19.7-20.1
4-8 Y	52-53	20.5-20.9
8-12 Y	54-55	21.3-21.7

The note: "Between two sizes? Take the larger one: little heads grow fast." By 8 to 12 years, the range meets the adult S, 54 to 55 cm.

Helmet sizes

From the Helmets template:

Size	Head (cm)	Head (in)
XS	53-54	20.9-21.3
S	55-56	21.7-22.0
M	57-58	22.4-22.8
L	59-60	23.2-23.6
XL	61-62	24.0-24.4
XXL	63-64	24.8-25.2

Helmets are measured a little higher: "around the widest part of your head, about 2 cm above the eyebrows and ears." The note: "A helmet should feel snug all around, without pressure points, and stay in place when you shake your head. Between two sizes? Take the larger one and adjust it with the pads."

The letters don't match hat letters. A 57 cm head is a hat M (56 to 57) and also a helmet M (57 to 58), but a 56 cm head is a hat M and a helmet S. Always read the helmet's own chart.

Why hat sizes are approximate

One centimeter is a whole eighth, so a loose or tight tape moves the answer by a size. Hats also differ by shape and material, which no conversion captures. And every brand sets its own letter ranges. Use the conversion to understand a chart, then follow the chart.

How a store should present hat sizes

The head circumference as the measurement, in centimeters with a switch to inches, or inches first for shoppers in the US.
The US hat size beside the letter sizes, as a second name for the same size, not as a measurement to convert.
One measuring step, close to the table, since a centimeter matters.
A note for between two sizes.
Kids' hats and helmets on charts of their own.

How Ruban Size does it

The Hats template has three columns: the letter size, the US hat size and the head in centimeters. The US hat size column is marked as size names, so it is shown as written and never converted, while the head column switches between cm and in. Like every template, it opens as a draft and shows nowhere until you save it and choose its products.

Find my size for hats and helmets asks for the head circumference. Between two sizes, it suggests the larger one and says so: "Between two sizes, we suggest the larger one." The Helmets template offers no fit choice (no Snug or Loose for a helmet) and ends its answer with a reminder: "A helmet sits snug and level, and does not move when you shake your head. Adjust it with the pads or the dial."

On the Kids' hats template, the child's measurement is saved apart from the parent's, so each finds their own size on every product.

The real size guide: Hat and cap size chart. Switch units, or open Find my size.

Size	US hat size	Head (cm)
S	6 3/4 - 6 7/8	54-55
M	7 - 7 1/8	56-57
L	7 1/4 - 7 3/8	58-59
XL	7 1/2 - 7 5/8	60-61

How to measure

Head: around your head, just above the ears and eyebrows.

Between two sizes? Take the larger one.

Questions

What hat size is 58 cm?

A US 7 1/4, size L in Ruban Size's Hats template (58 to 59 cm, 7 1/4 to 7 3/8).

How do I convert head circumference to a US hat size?

Divide the circumference in inches by pi (3.1416) and round to the nearest eighth. A 57 cm head is 22.44 in, divided by pi 7.14, so a 7 1/8.

What size hat does a 1-year-old wear?

Measure first: in the Kids' hats template, 44 to 46 cm is 6-12 M and 47 to 49 cm is 1-2 Y. Between two, take the larger one.

Is a helmet the same size as a hat?

Not always. Both go by head circumference, but helmets are measured at the widest part of the head and use their own ranges: a 56 cm head is a hat M and a helmet S.