How this instrument works
Hat sizing has no ANSI or ISO standard behind it at all — no government or standards body has ever defined what a 'size 7' hat means. Instead, the numbering used across the English-speaking hat trade traces back to reference charts kept by hatters like Lock & Co. Hatters of London, founded in 1676 and still one of the oldest operating hat shops in the world, whose sizing table remains a widely used industry reference today.
The underlying relationship is simple once you have the reference points: US hat size is approximately your head circumference in centimeters divided by 8. That's not a physical law — it's a close approximation of the historical chart, born from the fact that hat sizes were originally set in inches of head diameter (not circumference), and the diameter-to-circumference-in-cm math happens to land very close to a clean divide-by-8 relationship across the sizes that matter for adult hats.
Because it's chart-based rather than formula-derived, this convention is most reliable in the everyday adult range — roughly 54 to 64 cm, spanning small through XX-large. Outside that band, or when comparing across UK, EU, and US labels on imported hats, expect small rounding differences between retailers, since (as with shoe and ring sizing) no single body audits or enforces consistency across brands.
- Wrap a soft tape measure around your head at its largest point — typically about an inch above your eyebrows and ears.
- Keep the tape snug but not tight, and note the circumference in centimeters.
- Enter that figure into Head circumference (cm).
- Read US hat size — head circumference divided by 8, matching the Lock & Co. reference chart.
- If you land between two common retail sizes, size up slightly rather than down — a marginally loose hat is more comfortable than a tight one.
Worked example — a 58 cm head circumference
A 58 cm head circumference, a common adult size, divides out to 58 ÷ 8 = 7.25 — a US hat size of 7 1/4. That matches the Lock & Co. Hatters reference chart exactly for this measurement, which also lists it as EU size 58 and UK size 7 1/8.
For comparison, a smaller 56 cm head circumference lands on exactly US size 7.0, while a larger 62 cm head circumference lands on US size 7.75 — each additional centimeter of head circumference moves the US size by an eighth, roughly the spacing between adjacent retail hat sizes.
Questions
Is there an official standard that defines hat sizes?
No. Unlike, say, screw threads or paper sizes, hat sizing has never been codified by ANSI, ISO, or any government body — it's purely an industry convention that traces back to reference charts kept by historic hatters, most notably Lock & Co. Hatters of London (established 1676). Different hat makers can and occasionally do publish slightly different charts, though the core US/UK/EU conversion has stayed fairly consistent for decades.
Why is US hat size roughly head circumference divided by 8?
Hat sizes were historically set from the hat's inner diameter in inches, not its circumference — an old hatter's block was labeled by the diameter of head it fit. Converting that diameter-based inch scale into a circumference-in-centimeters measurement happens to land very close to a clean divide-by-8 relationship across the adult sizing range, which is why the approximation works well even though it isn't derived from a physical formula.
How do I measure my head correctly for a hat?
Wrap a soft (cloth or paper) tape measure around your head at its widest point — for most people, that's about an inch above the eyebrows in front and just above the ears on the sides, then around the back of the head. Keep the tape snug against your skin without pulling it tight, and take the measurement in centimeters for the most precise result with this calculator.
What if my calculated size falls between two hat sizes a store sells?
Most hat retailers sell in eighth- or quarter-size steps, so round to the nearest size they offer. If you're exactly between two sizes, sizing up slightly is the safer choice for structured hats like fedoras or flat caps, since a hat that's marginally loose can often be adjusted with a sizing tape or sweatband insert, while one that's too tight generally can't be stretched without damaging the material.
Do US, UK, and EU hat sizes use the same numbering?
No — they're related but distinct scales. US sizes are typically given in eighths (7, 7 1/8, 7 1/4...), UK sizes historically ran in a similar inch-based scale but are sometimes shown differently on imported hats, and EU sizes are usually just the head circumference in centimeters rounded to the nearest even number. This calculator returns the US figure; check a specific retailer's conversion chart if you're buying a hat labeled in UK or EU sizing.