Hat Size Converter
Convert head circumference in cm to US hat size, EU cm hat size, and UK hat size.
How to use this tool
- Enter head circumference in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your us hat size and the full breakdown beneath it.
Measure your head circumference in centimetres and convert to US, EU, and UK hat sizes.
Formula
Conversion is performed via a lookup table keyed on head circumference (cm):
{ US hat size, EU hat size, UK hat size } = table[head_cm]
US/UK hat sizes are traditionally expressed as fractions of an inch (e.g. 7-1/8) derived from: US size = head circumference (inches) ÷ π.
How it works
Hat sizing uses the head circumference as the universal starting point. The EU system simply states the circumference in centimetres, while US and UK sizes convert that circumference to inches, divide by π (approximately 3.14159), and round to the nearest standard fraction. The converter maps whole-centimetre circumference values from 53 cm to 62 cm to their corresponding fractional hat sizes.
Common mistake: People often confuse head circumference with head diameter. To measure correctly, wrap a flexible tape around the widest part of the head (typically about 2.5 cm above the ears and eyebrows), keeping the tape parallel to the ground. Measuring the diameter of the head and multiplying by π will give the same result only if the head were a perfect circle, which introduces error for elongated head shapes.
Worked example
Find hat sizes for a 57 cm head circumference
- Input: head circumference = 57 cm.
- Look up 57 in the hat size table.
- Read off: US = 7-1/8, EU = 57, UK = 7-1/8.
57 cm head circumference = US hat size 7-1/8, EU hat size 57 cm, UK hat size 7-1/8.
Common mistakes to avoid
- Measuring across the head rather than around it -- hat size requires head circumference (a tape around the widest part of the forehead), not the diameter.
- Treating US hat sizes as simple fractions of inches without knowing they represent head diameter in inches -- a size 7 hat fits a head diameter of 7 inches (circumference about 22 inches or 55.9 cm).
- Ignoring hat style differences -- a fitted baseball cap runs a half size smaller than a fedora for the same head circumference because of brim and lining construction.
Key terms
- Head circumference (cm)
- The distance around the largest part of the head, measured in centimetres. It is the universal input for hat fitting across all regional sizing systems.
- US/UK hat size
- Expressed as a fraction (e.g. 7-1/8), derived by converting head circumference to inches and dividing by π. US and UK hat sizes are numerically identical, unlike shoe sizes.
- EU hat size
- Simply the head circumference in centimetres, rounded to the nearest centimetre. The most straightforward hat sizing system with no conversion required once the measurement is known.
- Paris point (hat context)
- Not directly used in hat sizing, but the underlying principle of basing size on a physical measurement (circumference) rather than an arbitrary scale is common to both EU hat and EU shoe sizing.
Frequently asked questions
- How do I measure my hat size?
- Use a flexible tape measure around your head about 1 cm above your ears and eyebrows. Round to the nearest centimetre and use the table above.