Hat Size Converter (Head Circumference → US/UK/EU/JP)
Hat sizes everywhere come from the same physical measurement — head circumference — but the systems express it differently. The US and UK use fractional numeric sizes (e.g. 7, 7 1/8) derived from circumference ÷ π in inches. Europe and Japan label hats by head circumference directly in centimetres. Sportswear and stretch-fit lines (adidas, Nike, New Era stretch fit) use the S/M/L/XL letter scale. This tool applies the standard hat-industry conversion table (New Era 59FIFTY, Stetson, Bailey of Hollywood) and returns five interchangeable sizes — US fitted (fractional), UK, EU cm, JP cm and the letter band — plus a direct cm/inch readout, so online buyers can pick the right fitted cap, fedora, beanie or helmet liner with confidence.
Enter a valid head circumference (40–70 cm / 15.7–27.6 in).
US (fitted)
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UK
—
S / M / L
—
EU (cm)
—
JP (cm)
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CM / IN
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How to measure: run a soft tape around the head about 1 cm above the eyebrows, snug enough to fit a finger underneath. Individual brand fit varies.
Formula
US fitted = (head_in) ⁄ π, rounded to nearest 1/8 UK = US − 1/8 EU = JP = round(head_cm)
- · References: New Era 59FIFTY Size Chart (fitted 7 = 22 in head = 55.88 cm); Stetson Size Guide; Bailey of Hollywood Specs; the UK numeric convention UK = US − 1/8.
- · How to measure: (1) use a cloth or soft measuring tape; (2) place it around the widest part of the head — about 1 cm above the eyebrows and the top of the ears; (3) keep it snug but not tight (one finger should slip under); (4) measure three times and take the average; (5) heads can be ~0.5 cm larger first thing in the morning due to mild overnight fluid retention — measure during the day for everyday wear.
- · Reference ranges: men average 56–58 cm (US 7 to 7 1/4, letter M–L); women average 54–57 cm (US 6 3/4 to 7 1/8, letter S–M); children and slim-profile heads can fall to ≤ 53 cm (XS).
- · Fit tolerance by style: (1) Fitted (New Era 59FIFTY) — exact size, no adjustment; buy the matched size; (2) Snapback / strapback — about 3 cm of adjustment; size down by ~0.5 from your measured size; (3) Flexfit / stretch — S/M fits 55–57 cm, L/XL fits 58–61 cm; (4) Fedora / dress hat — fitted is exact but most include a sweatband for minor adjustment — measure precisely.
- · Helmet liners: bicycle, ski and motorcycle helmets generally use the EU cm system. Measure the head the same way, but try on in person — wearing position matters (no sweat dripping into the eyes).
- · Japan vs Europe: both use cm so the numbers match, but Japanese brands (Coen, Beams) typically cut smaller than European brands. Size down or choose stretch fits when buying from Japan.
Frequently asked
Is US "7" the same as EU "57"?
They come from the same head circumference but at different rounding precision. US 7 = 22.0 in = 55.88 cm, so US 7 strictly maps to EU 56 (rounded), not EU 57. EU 57 maps to roughly US 7 1/8 (22.44 in, 57 cm). A 1 cm gap is enough to make a fitted cap feel wrong. Practical advice: (1) measure your head — 56 cm → US 7, 58 cm → US 7 1/4, 60 cm → US 7 1/2; (2) trust the measured number, not the converted label; (3) when shopping cross-border, double-check the brand's own chart — Japanese reproductions of US labels sometimes have their own offset.
I cannot try the hat on before buying. How do I deal with ±1 cm measurement error?
Strategy: (1) use a proper cloth tape — error < 0.3 cm vs 1–2 cm for a hard ruler or eyeball estimate; (2) measure three times across the day (morning, afternoon, evening) and average — heads are ~0.5 cm larger in the morning; (3) prefer styles with built-in fit tolerance: snapbacks and stretch-band fedoras tolerate ±2 cm; fitted 59FIFTYs tolerate only ±0.5 cm; (4) size up rather than down — you can shim a loose hat with a sweatband or foam strip, but a tight hat almost always has to be returned; (5) buy from sellers offering free size exchange on your first order with a new brand; (6) when your measurement falls between two sizes (e.g. 57.5 cm is between US 7 1/8 = 57 and 7 1/4 = 58), choose the larger for fitted styles.
How do I size a hat for a child?
Children's head circumference by age (WHO growth standards): newborn 33–36 cm; 6 months 42–45 cm; 1 yr 45–47 cm; 2 yr 47–49 cm; 4 yr 50–51 cm; 6 yr 51–52 cm; 8 yr 52–54 cm; 10–12 yr 53–55 cm; teens are essentially adult sizes. Practical advice: (1) do not "size up to leave room to grow" — a too-loose hat falls into the eyes and is unwearable; (2) 0–2 yr babies wear stretch beanies (no sizing); after 1 yr use an adjustable snapback or elastic-back that can be retuned every 4–6 months; (3) sun hats need a 7 cm+ brim and a chin strap; (4) bicycle and skate helmets must be fitted exactly (CPSC / EN 1078 — no slack), so try them on in person rather than ordering online; (5) children above the 90th percentile in head circumference can move into adult XS/S.
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