Hat Size Converter Calculator

Work out hat size converter instantly with clear inputs, formula shown and shareable results.

About Hat Size Converter Calculator

Traditional hat sizes are the head diameter in inches, obtained by dividing the circumference by pi. A 57 cm head is 22.44 inches around, giving a UK size of about 7.14 - normally sold as 7 1/8. US sizes run an eighth larger than UK for the same head, and letter sizing bands every 2 cm.

Formulas

Hat size

size = circumference(in) / pi

US offset

US size = UK size + 1/8

Worked example

With the default values below, Hat Size Converter Calculator returns UK Hat Size of 7.14. Change any input above and every figure here is recalculated in your browser.

Inputs used

Head circumferenceminimum 30 cm: 57 cm
Show size asone of UK hat size (inches), US hat size (inches), Letter size (S/M/L): UK hat size (inches)

Results

UK Hat Size: 7.14
US Hat Size: 7.27
Head Circumference In Inches: 22.44
Letter Size: M

Step by step

1Values used
2Hat size
3US offset
4UK Hat Size= 7.14
5US Hat Size= 7.27
6Head Circumference In Inches= 22.44
7Letter Size= M

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Where should I measure?

Around the head about a centimetre above the ears and eyebrows, at the widest point, with the tape snug but not tight.

Why are hat sizes in eighths?

Because they descend from diameter in inches, and an eighth of an inch is about 1 cm of circumference - the practical resolution of a fitted hat.