What Is the Hat Size Calculator?
This tool converts the measurement around your head into the hat sizes used by retailers. In the United States, fitted hat sizes are based on the diameter of your head — which equals your head circumference divided by pi (π ≈ 3.14159). European hats are sized directly by the head circumference in centimeters, so a 58 cm head wears a size 58 hat.
How to Use It
Wrap a soft tape measure around the widest part of your head — about an inch (or 2 cm) above your eyebrows and ears. Enter that measurement, choose whether it is in inches or centimeters, and the calculator returns both your US fitted hat size and your EU hat size.
The Formula Explained
Because a hat is essentially a circle that sits on your head, its diameter (the US size) is the circumference divided by \(\pi\). For example, a head circumference of 22 inches gives \(22 \div 3.14159 = 7.003\), which rounds to a US size 7. The EU size is simply the circumference converted to centimeters and rounded: \(22 \text{ in} \times 2.54 = 55.88 \text{ cm} \approx 56\).
$$\begin{gathered} \text{US Size} = \frac{\text{Circumference (in)}}{\pi} \\[1.5em] \text{EU Size} = \text{Circumference (in)} \times 2.54 \end{gathered}$$
Worked Example
Suppose you measure 23.25 inches.
$$\text{US size} = 23.25 \div 3.14159 = 7.40$$rounded to the nearest 1/8 gives 7 3/8 (7.375). In centimeters:
$$23.25 \times 2.54 = 59.06 \text{ cm}$$rounded to EU size 59.
FAQ
What if my size falls between two values? The US result is rounded to the nearest eighth of a size; if you are exactly between, size up for comfort, especially for stretch-free fitted caps.
Why does the US size use pi? US hat sizing historically refers to the head's diameter in inches, and diameter equals circumference \(\div\) \(\pi\).
Are these sizes universal? US and EU sizing are standardized as shown, but individual brands vary slightly, so always check a brand's chart when possible.
