MasterTool

Hat Size Calculator

Convert head circumference (inches or cm) to US fitted hat size, UK size and EU size using the standard fitted-hat chart.

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What this tool does

Convert head circumference (inches or cm) to US fitted hat size, UK size and EU size using the standard fitted-hat chart. Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.

How to use the Hat Size Calculator

  1. Enter or select measurement unit.
  2. Enter or select head circumference.
  3. Read the calculated result; change any measurement to compare alternatives.

Formula

Chart lookup (src/data/sizes.ts): US fitted size is approximately circumference in inches divided by pi, snapped to the standard eighth-size steps; EU size is the circumference in cm.
unit
Measurement unit
value
Head circumference

Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.

Worked example

For hat size calculator, the following measurements illustrate the exact method: Measurement unit: in; Head circumference: 22.75.

Inputs

  • Measurement unitInches
  • Head circumference22.75

Result

  • US fitted hat size7 1/4
  • US size (decimal)7.25
  • UK size7.13
  • EU size (cm)58
  • Head circumference (inches)22.75
  • Head circumference (cm)57.8

Results explained

US fitted hat size
US fitted hat size from the formula above. Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.
US size (decimal)
US size (decimal) from the formula above. Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.
UK size
UK size from the formula above. Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.
EU size (cm)
EU size (cm) from the formula above. Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.
Head circumference (inches)
Head circumference (inches) from the formula above. Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.
Head circumference (cm)
Head circumference (cm) from the formula above. Measure around the head just above the eyebrows and ears, where a hat sits. Between sizes, fitted-hat wearers usually take the larger size.

Frequently asked questions

Wrap a soft tape around your head about an inch above the eyebrows and ears, keeping it level and snug but not tight. Enter the circumference — 22 3/4 inches is a US 7 1/4, the most common adult size.

Roughly circumference (inches) / pi: 22.75 / 3.14159 = 7.24, which snaps to the standard size 7 1/4. The chart also gives UK (one eighth smaller) and EU (= cm) equivalents.

Choose the larger size for fitted hats — a slightly loose hat can be padded with sizing tape, while a tight hat cannot be stretched much. Adjustable caps make the question moot.

Yes, the fitted scale is unisex and based purely on circumference; styles differ, sizes do not.

One-size hats labelled M usually cover about 7 1/8 to 7 1/4 (57-58 cm). If your measurement falls outside a labelled range, prefer a fitted size.