Hat Size From Head Measurement

This page starts from a tape measure. If you already know a hat size and simply want the same number in another scale, the hat size converter takes any of US, UK, EU or circumference and fills in the rest as you type. Both pages use the same arithmetic, so they agree.

Hat size is based on head circumference, the measurement around the widest part of your skull. The three systems in common use are not three different measurements of your head. They are three different ways of writing down the same one.

The Measurement Formula:

The American number is the diameter of your head in inches, on the assumption that a head is a circle:

US size = Head circumference (inches) ÷ π, rounded to the nearest eighth

The European number is simpler than people expect. It is just your head circumference in centimetres, rounded to the nearest whole one. There is no arithmetic to do. A 58 cm head is an EU 58.

The British number is where it goes wrong for people ordering across the Atlantic. UK sizes run one eighth smaller than US sizes for the same head, a hangover from a different assumption about head shape in the original blocks. A head that takes a US 7¼ takes a UK 7⅛. Order a British hat off your American size and it will be tight.

Measurement Technique:

Wrap a soft tape measure around the head about 2.5 cm (1 inch) above the eyebrows and ears, at the widest point. Do not pull it tight. You want a natural resting fit.

Hat Size Conversion Chart:

Every row of that chart comes straight out of the formula rather than from a chart somebody copied, which is worth saying because the two most commonly reproduced hat charts on the internet disagree with each other by a full size at the small end.

One thing the calculator does differently from the formula, and it is worth knowing about. Hats are blocked and stocked by the centimetre, so the calculator finds the row nearest your circumference in centimetres rather than rounding the inch figure to an eighth. The two grids are within a quarter of a percent of each other, because an eighth of an inch of diameter works out at 0.9975 cm of circumference. They can still land on different rows for a head that sits almost exactly between two sizes, and when that happens the calculator says so rather than pretending the choice is obvious.

Worked Example:

Head circumference = 58 cm.

Convert to inches first: 58 ÷ 2.54 = 22.83 inches. Divide by π: 22.83 ÷ 3.14159 = 7.27. Round to the nearest eighth: 7.27 is closest to 7.25, so the answer is US 7¼. The British size is one eighth below that, UK 7⅛, and the European size is the raw measurement, EU 58.

Hat Fit Tips:

Practical Tips:

• Measure head circumference twice and take the larger of the two measurements
• Head size changes slightly through the day, so measure in the afternoon when it is at its largest
• Wool felt hats stretch 1–2 sizes after wear; buy fitted or slightly firm