Enter your head circumference in centimeters or inches and click "Calculate" — the calculator will instantly determine your hat or cap size in US, UK, Metric, and letter formats for adults and kids.
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How to Measure Your Head Circumference
What you need. A soft measuring tape, or a string/thread and a ruler.
How to measure. Wrap the tape around your head about 1.5–2 cm (0.5–0.75 inches) above your eyebrows and slightly above your ears — this is the widest part of your head where a hat typically sits.
Tip. Keep the tape snug but comfortable, not too tight. If your measurement falls between two sizes, it is generally recommended to choose the larger size.
How Hat Sizing Works
Hat sizes are determined by head circumference in centimeters or inches. International letter sizes (XS–XXXL) correspond to specific range brackets, which differ for adults and kids. Traditional US and UK sizes use a numerical scale (such as 7 or 7 1/8) representing the nominal hat diameter.
Adult Hat Size Chart
| Head Circumference | US Size | UK Size | International Size | |
|---|---|---|---|---|
| cm | inches | |||
| 53 | 20 7/8 | 6 1/2 | 6 3/8 | XS |
| 54 | 21 1/8 | 6 5/8 | 6 1/2 | S |
| 55 | 21 1/2 | 6 3/4 | 6 5/8 | S |
| 56 | 22 | 6 7/8 | 6 3/4 | M |
| 57 | 22 1/4 | 7 | 6 7/8 | M |
| 58 | 22 5/8 | 7 1/8 | 7 | L |
| 59 | 23 | 7 1/4 | 7 1/8 | L |
| 60 | 23 1/2 | 7 3/8 | 7 1/4 | XL |
| 61 | 23 7/8 | 7 1/2 | 7 3/8 | XL |
| 62 | 24 1/4 | 7 5/8 | 7 1/2 | XXL |
| 63 | 24 5/8 | 7 3/4 | 7 5/8 | XXL |
| 64 | 25 | 7 7/8 | 7 3/4 | XXXL |
| 65 | 25 1/2 | 8 | 7 7/8 | XXXL |
Children's Hat Size Chart
| Head Circumference | US Size | UK Size | Approx. Age | International Size | |
|---|---|---|---|---|---|
| cm | inches | ||||
| 40–44 | 15 3/4 – 17 3/8 | 5 – 5 1/4 | 4 7/8 – 5 1/8 | 0–6 months | XXS |
| 44–46 | 17 3/8 – 18 1/8 | 5 1/4 – 5 1/2 | 5 1/8 – 5 3/8 | 6–12 months | XS |
| 46–48 | 18 1/8 – 18 7/8 | 5 3/4 | 5 5/8 | 12–24 months | S |
| 48–50 | 18 7/8 – 19 5/8 | 6 | 5 7/8 | 2–4 years | M |
| 50–52 | 19 5/8 – 20 1/2 | 6 1/4 – 6 1/2 | 6 1/8 – 6 3/8 | 4–8 years | L |
| 52–54 | 20 1/2 – 21 1/4 | 6 1/2 – 6 3/4 | 6 1/2 – 6 5/8 | 8+ years | XL |
Frequently Asked Questions
What is the difference between a helmet size and a hat size?
While both are based on head circumference, helmets require a snug, secure fit for safety and often include a built-in adjustment wheel at the back. Hats are typically chosen for a looser fit to prevent pressure marks on the forehead.
How do I choose a hat size for a child?
Because children grow rapidly, it is best to rely on their current head circumference in centimeters rather than their age. If the measurement is between sizes, always size up — you can adjust the fit using internal drawstrings or chin straps.
How can I make a hat smaller if it is slightly too big?
You can use adhesive hat sizing tape (foam or felt inserts) placed under the sweatband. These inserts can reduce the hat size by about 1–1.5 cm for a better fit.
Is hat sizing consistent across different brands?
Generally, yes — hat sizing is based on head circumference and is fairly standardized. However, some brands (especially premium hatmakers) may use unique molds or blocks that affect the fit. If you are buying an unfamiliar brand online, always check their specific size chart.
Looking for a helmet size for cycling or skiing? Use our helmet size calculator.