Hat Size Converter – US, UK, EU, Japan, and Australia
Shopping for a hat online or traveling abroad can be confusing when the size label uses a system you do not recognise. This Hat Size Converter instantly translates between the five major international sizing systems — US/Canada, UK, EU/Europe, Japan (JP), and Australia (AU) — as well as direct head circumference entry in both centimetres and inches.
How Hat Sizes Are Measured
Every hat sizing system ultimately derives from one measurement: the circumference of your head, taken at the widest point just above the ears and about 2.5 cm (1 in) above the eyebrows. A flexible tape measure gives the most accurate result. Measure in the afternoon when slight swelling is at its peak, and take two or three readings to confirm consistency.
The International Sizing Systems Explained
US and Canada
The US system expresses hat size as head circumference in inches divided by pi (π ≈ 3.14159). The result is rounded to the nearest one-eighth fraction: 6 3/8, 6 1/2, 6 5/8, up to 8. A head circumference of 58 cm (22.83 in) gives US size 7 1/4. Australia uses exactly the same scale as the US, so US and AU sizes are always identical.
UK
The UK system is almost identical to the US system but is defined as exactly 1/8 of a size smaller than the US equivalent. A US 7 1/4 hat is a UK 7 1/8, and a US 7 1/2 hat is a UK 7 3/8. This small offset reflects historical differences between American and British millinery traditions.
EU / Europe
European hat sizes are the most straightforward: the EU size number is simply the head circumference in centimetres, rounded to the nearest whole number. A 58 cm circumference gives an EU 58, a 59 cm circumference gives an EU 59, and so on. This makes EU sizing very easy to work with when you know your head circumference.
Japan (JP)
Japanese hat sizes use the same centimetre-based value as the EU system. An EU 58 hat and a JP 58 hat are the same size. Japan-market hats will often display the size in centimetres on the inner label (e.g. 58 cm).
Conversion Formulas
The core formulas used by this converter are:
- Circumference (in) → US size:
US = circumference_in ÷ π - US size → circumference (in):
circumference_in = US × π - Circumference (cm) → EU size:
EU = round(circumference_cm) - UK size:
UK = US − 1/8 (0.125) - Inches ↔ centimetres:
cm = in × 2.54
How to Measure Your Head for a Hat
Wrap a soft measuring tape around your head at the largest point — typically 2–3 cm (about 1 in) above your eyebrows and ears. If you do not have a flexible tape, wrap a piece of string around your head, mark the overlap, then measure the string against a ruler. That measurement in centimetres is your EU hat size and your starting point for converting to any other system.
What If My Size Falls Between Two Sizes?
Hat sizing uses discrete 1/8-inch steps. If your head circumference falls between two standard sizes, most hat makers recommend sizing up for a comfortable fit. Many quality hats also include a sweatband or size-reducer strip that can be added to fine-tune the fit by up to half a size.
Size Categories
Hat size ranges loosely correspond to descriptive categories used in retail:
- XS — EU 53 and below
- S — EU 54 – 55
- M — EU 56 – 57
- L — EU 58 – 59
- XL — EU 60 – 61
- XXL — EU 62 – 63
- XXXL — EU 64 and above
These descriptive labels vary between brands, so always rely on the numeric size or head circumference for accurate hat sizing rather than S/M/L alone.
Common Uses
This converter is useful for online hat shopping from international retailers, theatrical and costume departments managing large hat inventories, hat manufacturing and labelling across export markets, and travellers who want to purchase a local hat while abroad. Enter your head circumference in centimetres for the fastest result across all systems simultaneously.