Enter the head circumference and PI into the calculator to determine the hat size. This calculator can also evaluate any of the variables given the others are known.

Hat Size Formula

The following formula is used to calculate the hat size.

HS = (H / PI)


  • HS is the hat size H is the head circumference in inches PI is the mathematical constant approximately equal to 3.14159

To calculate the hat size, measure the head circumference in inches. Then divide the head circumference by PI (approximately 3.14159). The result is the hat size.

What is a Hat Size?

A hat size is a measurement used to determine the appropriate size of a hat for an individual. It is typically calculated by measuring the circumference of a person’s head at the widest point, usually about an inch above the ears and across the mid-forehead. This measurement, usually given in inches or centimeters, corresponds to specific sizes (small, medium, large, etc.) or numerical hat sizes, depending on the country and the hat manufacturer.

How to Calculate Hat Size?

The following steps outline how to calculate the Hat Size using the formula: HS = (H / PI).

  1. First, determine the head circumference in inches (H).
  2. Next, gather the value of the mathematical constant PI (approximately equal to 3.14159).
  3. Next, use the formula HS = (H / PI) to calculate the Hat Size.
  4. Finally, insert the values of H and PI into the formula and calculate the Hat Size.
  5. After calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

head circumference in inches (H) = 22

mathematical constant PI (approximately equal to 3.14159)