Hat Size Calculator
Convert your head circumference into US, UK and EU hat sizes, plus the head diameter and a general S–XL category. Enter your measurement in cm or inches and pick a sizing system.
Find Your Hat Size from a Head Measurement
Hat Size Formula
Where:
- \(C\) — head circumference, measured where a hat would sit
- \(\pi \approx 3.14159\) — the constant used to convert circumference to diameter
- \(d\) — head diameter, i.e. circumference divided by \(\pi\)
- \(d_{\text{in}}\) — the same diameter expressed in inches
US hat sizes are defined as the head's diameter in inches, rounded to the nearest eighth. The UK convention is one eighth smaller, and the EU (metric) size is simply the head circumference in centimetres.
Hat Size – Calculation Example
Example: your head measures 57 cm around, and you want to know your hat size in every system.
\[ d = \frac{C}{\pi} = \frac{57}{3.14159} = 18.14 \text{ cm} = 7.14 \text{ in} \]
\[ \text{US} = \text{round}(7.14 \times 8) / 8 = 57/8 = 7\tfrac{1}{8} \]
\[ \text{UK} = 7\tfrac{1}{8} - \tfrac{1}{8} = 7 \quad \text{EU} = \text{round}(57) = 57 \]
Result: US 7 1/8, UK 7, EU 57 (head diameter 18.1 cm), general size M.
How to use the calculator
- Measure around your head with a soft tape, about 1 cm above your ears and eyebrows, keeping the tape snug but not tight.
- Enter the value in the Head circumference field.
- Choose the Unit of your measurement — cm or in.
- Select the Sizing system you need: US, UK, EU/metric, or "Show all systems" to compare them side by side.
- Read your size from the results, together with the head diameter and the general S–XL category.
What hat size means and why it matters
A hat that fits well stays put without pressing on your forehead or slipping over your ears. Because heads are roughly circular, hatters traditionally size hats by treating the head as a circle: the circumference you measure with a tape is converted to a diameter by dividing by \(\pi\). That diameter, in inches, is the basis of the traditional US sizing system.
The three main systems describe the same head in different units. A US size such as 7 1/8 means the head's diameter is seven and one eighth inches. UK sizes follow the same logic but are conventionally one eighth smaller in number, so a US 7 1/8 equals a UK 7. The EU or metric system skips the conversion entirely and uses the head circumference in centimetres — a 57 cm head takes an EU 57 hat. The letter categories (S, M, L, XL) group circumferences into broad bands: below 55 cm is S, 55–57 cm is M, 58–60 cm is L, and above 60 cm is XL.
Knowing your size in more than one system is practical because brands label hats differently. An American maker may print "7 1/8", a British maker "7", and a European maker "57" on what are essentially the same hat. With your circumference and this conversion, you can shop across all of them confidently.
Reference table
| Head circumference | US | UK | EU | Category |
|---|---|---|---|---|
| 52 cm (20.5 in) | 6 1/2 | 6 3/8 | 52 | S |
| 54 cm (21.3 in) | 6 3/4 | 6 5/8 | 54 | S |
| 55 cm (21.7 in) | 6 7/8 | 6 3/4 | 55 | M |
| 57 cm (22.4 in) | 7 1/8 | 7 | 57 | M |
| 59 cm (23.2 in) | 7 3/8 | 7 1/4 | 59 | L |
| 61 cm (24.0 in) | 7 5/8 | 7 1/2 | 61 | L |
| 63 cm (24.8 in) | 7 7/8 | 7 3/4 | 63 | XL |
Assumptions, limitations and practical tips
- The calculator assumes a roughly circular head. Real heads are slightly oval, so the diameter derived from \(\pi\) is an equivalent circle, not an anatomical measurement. This is exactly how traditional hat sizing works, so the result matches standard size charts.
- Sizing conventions vary between manufacturers. One brand's 7 1/8 may fit differently from another's. Use the calculated size as a starting point and check the maker's own chart before buying, especially for fitted caps and dress hats.
- Measure carefully. Take two or three readings to be consistent, and measure snugly against the head — thick hair or a ponytail can add a centimetre or more. If you are between sizes, most hatters recommend going up and tightening the fit with sizing tape or foam strips inside the sweatband.
- Materials matter. Wool felt and straw hats stretch and conform slightly over time; stiff structured caps do not, so precision matters more for the latter.
If you track other body measurements — for example with our A Body Shape Index calculator or the Hair Density calculator — you can note your head circumference alongside them for future reference.
Frequently Asked Questions
What is the average hat size for men and women?
The average adult male head circumference is about 57–59 cm, which corresponds to a US hat size of 7 1/8 to 7 3/8 (EU 57–59). Women average slightly smaller, around 55–57 cm, or US 6 7/8 to 7 1/8 (EU 55–57). These are only averages — adult head circumferences commonly range from 52 to 63 cm, so always measure rather than assume an average size.
How do I calculate my hat size by hand without a calculator?
Measure your head circumference where a hat sits, then divide by π to get the diameter. If you measured in inches, round the diameter to the nearest eighth — that is your US size. If you measured in centimetres, divide by π and then by 2.54 to convert to inches. Subtract 1/8 for the UK size, or simply use your circumference in cm as the EU size.
How do I measure my head without a tape measure?
Use a piece of string or a non-stretch cord: wrap it around your head about 1 cm above your ears and eyebrows, mark where it completes the circle, then lay it flat against a ruler. Measure twice and use the larger, consistent reading. A metal tape or a strip of paper works too, but avoid stretchy materials like elastic, which will understate your size.
Can I convert a hat size back into a head circumference?
Yes. For a US size, multiply the size by π to get the circumference in inches (e.g. a US 7 1/8 is about 22.4 in), then multiply by 2.54 for centimetres — roughly 57 cm. For an EU size, the number is already the circumference in cm; divide by 2.54 for inches. This is useful when a shop lists sizes in a different system than your measurement.
Are children's hat sizes calculated the same way?
For older children, yes — the same circumference-to-diameter logic applies, and many brands label kids' hats in centimetres or with age brackets (e.g. 2–4 years, 4–6 years) rather than US eighths. Children's heads grow quickly, so an age label is less reliable than an actual measurement. Measure the child's head and buy with a little growing room for soft hats, but a snug fit for sun hats that must stay on in wind.
Do fitted baseball caps use the same sizes as dress hats?
Not always. Traditional fitted caps are often labeled in eighths of an inch just like dress hats, so a 7 1/8 fitted cap matches a US 7 1/8 hat. However, many cap brands use S/M/L/XL or one-size-fits-all with adjustable bands instead. Structured caps do not stretch the way felt and straw do, so a precise measurement matters more — check the specific brand's chart before ordering.
What should I do if my measurement falls between two sizes?
Most hatters recommend rounding up rather than down. A slightly loose hat can be tightened with sizing tape, foam strips, or a thicker sweatband inside the band, but a hat that is too tight cannot be made larger and will cause headaches. The exception is snug elasticated caps and beanies, where stretching is expected and you can choose the closer or smaller size.