Hat sizing is more confusing than it needs to be because multiple incompatible sizing systems are in use simultaneously, the same hat can be labelled differently by different brands, and the conversion between systems involves a step that most people get wrong. A buyer who sees a hat labelled '7 1/4' may not know whether that refers to head circumference in inches (it does not), an arbitrary code (partially correct), or a size in a specific national system (yes). This guide resolves the confusion by explaining exactly what each sizing system measures and how to convert between them.
The Fundamental Measurement: Head Circumference
All hat sizing ultimately derives from head circumference -- the measurement in centimetres (or inches) of the widest path around the head. This is measured with a flexible tape measure at approximately 2-3 cm above the eyebrows at the front and following the widest path around the sides and back of the head. The resulting measurement in centimetres is your baseline hat size and is the number from which all hat sizing conversions start.
How to take the measurement accurately:
- Use a flexible tape measure (fabric or soft plastic, not a metal carpenter's tape)
- Measure over any hair that will be present when wearing the hat
- Take the measurement at the widest point -- this is typically slightly above the eyebrows and around the fullest part of the back of the head
- Take the measurement at least twice and use the larger reading if they differ
- If between two hat sizes, size up (a hat that is slightly loose can be adjusted with a sizing insert; a hat that is too small cannot be adjusted)
The US/UK Hat Sizing System
The traditional US and UK hat sizing system expresses hat size as a fraction derived from dividing the head circumference in inches by pi (approximately 3.14159).
The formula: Head circumference (inches) ÷ 3.14159 = Hat size
Example: 22 inches circumference ÷ 3.14159 = 7.00 (size 7)
This is why hat sizes in the US/UK system run as 6 5/8, 6 3/4, 6 7/8, 7, 7 1/8, 7 1/4, 7 3/8, 7 1/2, and so on in eighth-inch increments. The number is the head diameter in inches (derived from the circumference divided by pi), not the circumference itself.
The typical range of adult hat sizes runs from approximately 6 5/8 (the smallest common adult size) through 7 5/8 (a large head). Sizes below 6 5/8 exist for children; sizes above 7 5/8 require specialist retailers.
The European Hat Sizing System
European hat sizes are the head circumference in centimetres, which makes them considerably more intuitive than the US/UK system. A size 56 hat is designed for a head with a circumference of 56 cm. The European size range for adults runs approximately from 52 (a small head) through 62 (a large head).
The conversion between European and US/UK sizes:
- European size (cm) = US/UK size × pi × 2.54
- Or more practically: European size = US/UK size × 8.0 (approximate)
Example: US/UK size 7 × 8.0 = 56 European size
S/M/L/XL Sizing
Many casual and fashion hat brands use S/M/L/XL sizing because it is simpler to produce and market than precise size increments. The problem is that S/M/L ranges are not standardised across brands -- an 'M' from one brand may be the equivalent of an 'S' from another. When S/M/L sizing is used, always check whether the brand provides head circumference ranges for each size category, and use those ranges rather than assuming 'M' means the same thing universally.
Typical S/M/L/XL adult hat size ranges (these vary by brand):
- S: 52-54 cm (approximately US 6 5/8 - 6 7/8)
- M: 55-57 cm (approximately US 6 7/8 - 7 1/8)
- L: 58-60 cm (approximately US 7 1/4 - 7 1/2)
- XL: 61-63 cm (approximately US 7 5/8 - 7 7/8)
Adjustable vs Fitted Hats
Many modern hats are adjustable rather than sized to a specific head circumference. Adjustable hats use one of several mechanisms:
- Snapback: a plastic snap closure at the rear that adjusts in defined increments. Typically fits heads from approximately 55-60 cm
- Velcro (or hook-and-loop): allows fine adjustment across a range. Similar fit range to snapback
- Fitted: a hat constructed to a specific size with no adjustment mechanism. Requires accurate sizing but provides a cleaner appearance and typically better fit
- Flexfit: a brand name for a hat construction using an elastic band in the interior that allows one size to fit a range of circumferences (typically S/M fits 55-57 cm, L/XL fits 58-60 cm)
Browse hats in sized and adjustable options at Hatloom.
Frequently Asked Questions
How do I convert my head circumference to a hat size?
If you measured your head circumference in centimetres, your European hat size is that number directly (e.g., 57 cm circumference = European size 57). To convert to US/UK size, divide by 8: 57 ÷ 8 = 7.125, which rounds to US/UK size 7 1/8. For the conversion in the other direction: multiply your US/UK size by 8 to get the approximate European size (7 1/8 × 8 = 57 European). These are approximate conversions sufficient for hat selection purposes.
What if my head circumference falls between two hat sizes?
Size up. A hat that is slightly too large can be made to fit correctly with a hat sizing insert -- a foam or felt strip added to the interior sweatband that fills out the extra space and creates a snugger fit. A hat that is too small cannot be expanded without damaging the hat, and wearing a too-small hat is uncomfortable and can deform the hat over time. If you are consistently between sizes, having a sizing insert available is standard practice.
Why do some vintage hats fit differently than their size label suggests?
Vintage hats may fit differently from modern hats of the same stated size for several reasons: sizing standards have shifted slightly over the decades; the hat may have shrunk or expanded through use and moisture exposure; the leather sweatband may have stretched or contracted with age; and the hat may have been blocked or reshaped at some point in its history. When buying vintage hats, always ask for the interior sweatband circumference measurement rather than relying on the stamped size number. This gives you the current actual fit size of the hat, which may differ from its original size.