How do I measure my head for a hat?

Wrap a soft tape measure around the largest part of your head, roughly one centimetre above your ears and eyebrows, which is where most hats rest. Keep the tape level and snug but not tight. The circumference in centimetres or inches drives every hat size system.

Why are US and UK hat sizes different?

US and UK fitted hat sizes are both based on head diameter in inches but round differently, so a US 7 1/8 is a UK 7. The EU system uses the head circumference in centimetres directly, so an EU 57 hat fits a 57 cm head. This chart aligns all three.

What is the formula behind fitted hat sizes?

Fitted hat size equals the head circumference in inches divided by pi, giving an effective head diameter. A 22.4 inch circumference divided by 3.1416 is about 7.1 inches, which rounds to a US 7 1/8 hat. EU sizing skips this and uses the raw circumference in centimetres.

What if my head is between two sizes?

For fitted hats, round up to the next size for comfort, then use the internal sizing tape or foam inserts to fine-tune. For S/M/L adjustable hats, choose the band your measurement falls into; the strap or snapback handles small differences.

Are stretch and structured hats sized the same?

No. Stretch-fit and knit hats run smaller and pull to fit a range, so go by their stated S/M/L range. Structured fitted caps and formal hats follow the exact numeric sizing in this chart, so measure carefully for those.

Hat Size Conversion Chart

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A hat that fits depends on a single measurement — the circumference of your head — but it gets labelled four different ways. This chart converts your head measurement into US fractional sizes, UK fractional sizes, EU numeric sizes (the circumference in centimetres), and the S/M/L bands used on adjustable hats.

How it works

Fitted hat sizes are derived from head circumference. The numeric US/UK size is the head diameter in inches, found by dividing the circumference by pi:

diameter (in)  = circumference (in) / π        (π ≈ 3.1416)
US / UK size   = diameter rounded to the nearest 1/8 inch
EU size        = circumference in cm, rounded   (ISO standard)
S / M / L      = band the circumference falls into

The tool stores a fixed table linking circumference to all four systems. Enter a measurement and it converts to a common unit, finds the closest row, and shows every equivalent size.

Tips and notes

Measure where the hat actually sits — just above the ears and brow.
Hair volume matters; measure with your usual style.
If between sizes on a fitted hat, size up and use the internal tape.
Leather and wool hats may shrink slightly; allow a hair of extra room.

For example, a head circumference of 57 cm (about 22.4 in) is an EU 57, a US 7 1/8, a UK 7, and sits in the Medium band.