Head circumference

Head circumference is the distance around the head: across the forehead just above the eyebrows, just above the tops of the ears, and around the fullest part of the back. It is what a hat or helmet size measures. A US hat size is the same length in inches divided by pi: a 57 cm head is 22.44 in, and 22.44 divided by 3.1416 is 7.14, a 7 1/8.

How to measure

1. Use a soft tape, or a string marked where it overlaps and laid on a ruler.
2. Place it across the forehead just above the eyebrows, just above the ears, around the back of the head.
3. Keep it snug, not tight. Read, move it a little, measure again, and keep the largest reading.
4. For a helmet, go around the widest part of the head, "about 2 cm above the eyebrows and ears", as the helmet template says.

On a size chart

The Hats and caps template gives letter sizes with a row of US hat sizes:

The Helmets template runs from 53 to 54 cm (XS) to 63 to 64 cm (XXL), and Kids' hats from 40 to 43 cm at 0 to 6 months to 54 to 55 cm at 8 to 12 years.

In Ruban Size

Find my size asks for Head. Between two sizes it takes the larger one, on hats and helmets alike, and says "Between two sizes, we suggest the larger one." Hats offer "Fit I like"; helmets don't, and add the advice "A helmet sits snug and level, and does not move when you shake your head. Adjust it with the pads or the dial." The Hats and caps template shows it working.