Size / Hats
Hat Size Converter: Head Circumference to US, UK and EU
Enter head circumference in cm or inches to estimate US fitted sizes to the nearest eighth, UK sizes and European centimetre sizes.
Hat Size Converter: Head Circumference to US, UK and EU: Divides circumference in inches by pi and rounds to the nearest eighth for US fitted sizing. UK is US minus one eighth; European metric sizing is circumference in cm. Numeric S/M/L/XL bands are omitted because an eligible source was not established. Runs 100% locally in your browser with zero server file uploads.
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Run a soft tape horizontally around the widest part of your head, above the eyebrows and ears where the hat will sit. Keep it snug without compressing the skin or hair.
| System or measurement | Result |
|---|---|
| Head circumference | 58 cm / 22.83 in |
| US fitted — common convention | 7¼ |
| UK — common convention | 7⅛ |
| EU / metric (cm) | 58 |
Approximate sizes: brands vary; measure and check the maker's chart.
US = head circumference in inches ÷ π, rounded to the nearest 1/8. UK = US − 1/8. EU = head circumference in cm. At a rounding midpoint, the larger fitted size is shown.
S/M/L/XL labels are a common convention with varying cut-offs. Use the maker's letter-size chart with your circumference.
Sources: DOI/USFS Aviation Helmet Standard v4.3, size definitions; Russ Rowlett, How Many?, hat size entry. This is a hat size estimate, not a helmet fit assessment.
Published size definitions
DATA_CHECKED 2026-10-04; accessed 2026-10-04. DOI/USFS Aviation Helmet Standard v4.3, section 4, defines the American diameter-in-inches and European circumference-in-cm conventions. Only these definitions are used; this converter does not apply its helmet performance requirements or ASTM F2220 / EN 960 headform tables. https://edit.doi.gov/sites/default/files/uploads/doi-usfs-aviation-helmet-standard-rev4.3.pdf
UK common convention
Russ Rowlett, How Many? A Dictionary of Units of Measurement, hat size entry, gives US = circumference/(2.54 × pi) for cm input and UK = US − 1/8. This is a common convention, not an ISO hat sizing standard. Accessed 2026-10-04. https://www.ibiblio.org/units/dictH.html
How to use it
- Measure horizontally where the hat will sit, above the eyebrows and ears.
- Enter head circumference in cm or inches.
- Read approximate fitted sizes and compare your measurement with the maker's chart.
Privacy & limitations
Your head measurement stays in your browser. Nothing is uploaded.
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Frequently asked questions
Are US and UK hat sizes the same?
In the common convention used here, UK is one eighth below US for the same circumference.
Why is there no S/M/L/XL result?
Letter-size cut-offs vary. Use the maker's letter-size chart with your head circumference.
Can I use this to fit a protective helmet?
This is a hat sizing estimate. Helmet fit needs the maker's fitting procedure.
How exact is the result?
It is approximate. Brands vary; measure and check the maker's chart, especially between fitted sizes.
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