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.

What is the Hat Size Conversion Chart?

Convert hat sizes across US fractional, UK fractional, EU numeric, and S/M/L systems using head circumference. Measure around your head and read every matching size, or browse the full reference table for fitted and adjustable hats. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Hat Size Conversion Chart

Name: Hat Size Conversion Chart
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

A hat that fits depends on a single measurement — the circumference of your head — but that measurement gets labelled four completely different ways depending on where the hat was made and sold. 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 hat size systems work

Fitted hat sizes are derived from head circumference. The numeric US and UK size is the effective 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 EU system is the simplest because it skips the pi conversion: an EU 57 hat is made for a 57 cm head circumference, exactly. US and UK both use the diameter figure, but they round to 1/8-inch steps. A US 7 1/8 and a UK 7 refer to the same physical size — the fraction notation is just how English-speaking markets historically labelled hat sizes.

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

How to measure your head correctly

Getting the measurement right is the most important step — a millimetre or two of measurement error can put you between sizes.

Use a soft tape measure (or a strip of paper you measure against a ruler).
Wrap it around the largest part of your head — typically about 1 cm above your ears and eyebrows, following the slight ridge above the forehead.
Keep the tape level all the way around; tilting it up at the back gives a shorter reading.
Note the circumference in centimetres or inches. For fitted hats, measure to the nearest half-centimetre.
Measure with your usual hair style — a change from flat to voluminous hair can mean a full size difference.

What the S/M/L bands mean

Adjustable hats (snapbacks, fitted-back caps, most knit beanies) use a size band rather than an exact circumference. Typical ranges vary slightly by brand, but common conventions are:

Band	Approximate circumference
XS / S	Under 54 cm
S / M	54–57 cm
M / L	57–59 cm
L / XL	59–62 cm
XL / XXL	Over 62 cm

These are starting points; always check the specific brand’s size guide for adjustable hats since the strap, snapback, or elastic can compensate for an inch or two.

Size quirks to know

Baseball caps (structured) — these come in exact fitted sizes (6 7/8 through 8) or one-size-adjustable. For fitted baseball caps, your exact circumference matters.
Fedoras and dress hats — traditionally sized in US fractional or EU numeric. No stretch; exact size is critical.
Knit and stretch beanies — very forgiving; one “OS” or “S/M” covers a wide range because the fabric has 20–30% negative stretch.
Wool hats — wool felts and woven wool hats can shrink slightly after getting wet. If between sizes, go up half a size.
Children’s sizing — uses actual head circumference in cm in most markets, rather than fractional sizing.

Full reference example

A head circumference of 57 cm (about 22.4 in) is:

EU 57 — the circumference in centimetres
US 7 1/8 — the diameter (18.13 cm / π ≈ 7.11 in, rounded to nearest 1/8)
UK 7 — same as US in this system
Medium in S/M/L bands

If your circumference is 57.5 cm, round up to an EU 58 for fitted hats to avoid a tight fit.