Hat Size Guide: Measure Your Head Correctly
Measure your head circumference to find the right hat and cap size in US numeric, UK letter and EU centimeter systems. Includes fitted vs adjustable sizing and common fit issues.
Hat sizing can be surprisingly tricky because the same head circumference maps to different numbers depending on whether you are looking at US numeric sizing, UK letter sizing or EU centimeter sizing. US hat sizes are expressed as fractions of an inch of head circumference, such as 6 and 7 eighths or 7 and 1 eighth. UK and EU hats often use S/M/L/XL or a letter scale tied to centimeter measurements. Our Hat Size Converter translates between all three systems so you can order hats from international brands with confidence.
Measuring Head Circumference for Hats
To measure your head accurately, place a flexible tape measure around your head at the level where the hat will sit — typically about half an inch above the eyebrows and just above the ears. This is the frontal bone position and gives the largest circumference measurement, which is what hat manufacturers use. Keep the tape snug but not tight, and take the measurement three times to ensure consistency. The average adult head circumference is about 22 to 23 inches, which maps to a US hat size of 6 and 7 eighths to 7 and 1 quarter.
Fitted Hats vs Adjustable Hats
Fitted hats, most commonly associated with baseball caps, are sized to a specific head circumference and have no adjustment strap. They offer the cleanest look and most secure fit but require an accurate measurement. Adjustable hats use a snapback or strapback closure at the rear and are sold in one-size-fits-most ranges such as S/M (about 21 to 22 inches) or L/XL (about 22 to 23 inches). Beanies and knitted hats are usually one-size-fits-most because the stretchy fabric accommodates a wide range of head sizes naturally.
US Numeric Sizing vs S/M/L Letter Sizing
US numeric hat sizes use eighths of an inch increments — for example, 7 and 1 eighth means the hat fits a 22 and 5 eighths inch head circumference. These numeric sizes correspond roughly to letter sizes: a small is about 6 and 7 eighths to 7, a medium is about 7 to 7 and 1 eighth, a large is about 7 and 1 quarter to 7 and 3 eighths, and extra large is above 7 and 1 half. Our Hat Size Converter makes it easy to jump between numeric and letter systems without the mental math.