CM Head Size To UK Hat Size Converter
Convert head circumference in centimetres to a UK hat size, with rounding to eighth sizes and a simple S/M/L guide.
Convert Head Measurement
UK Hat Size Result
Hat sizing is a guide. Head shape, hair, lining, brim weight and brand block can change the best choice.
Recent Conversions
Direct Hat Size Conversion
UK hat size is traditionally based on head circumference in inches divided by pi. That makes a 58 cm head roughly a UK 7 1/8 or UK 7 1/4 depending on rounding and allowance. Modern retailers often show centimetres and S/M/L labels as well, but formal hats, bowlers, trilbies, flat caps and some riding or country hats may still use the numbered UK size.
This converter starts with the actual head circumference, applies any comfort allowance, converts centimetres to inches and then divides by pi. The result is rounded to the eighth size by default because many traditional hat sizes use eighth steps. For soft caps, the S/M/L guide may matter more than the exact fraction. For structured hats, the fraction is more useful.
Formula And Steps
Adjusted head size = measured centimetres + comfort allowance
Head circumference in inches = adjusted centimetres / 2.54
UK hat size = head circumference in inches / pi
Rounded result = nearest 1/8, 1/4 or next 1/8
Measure level
Wrap a soft tape around the head where the hat band will sit, usually above the ears and across the forehead.
Repeat twice
Take two measurements. If they differ, use the larger value for structured hats and check the brand chart.
Check the style
A rigid felt hat and a stretch beanie do not need the same precision, even when the centimetre number matches.
CM To UK Hat Size Table
| Head circumference | Approx UK hat size | Common label guide |
|---|---|---|
| 53 cm | UK 6 1/2 | Small |
| 54 cm | UK 6 5/8 | Small |
| 55 cm | UK 6 3/4 | Small to medium |
| 56 cm | UK 6 7/8 | Medium |
| 57 cm | UK 7 | Medium |
| 58 cm | UK 7 1/8 | Medium to large |
| 59 cm | UK 7 1/4 | Large |
| 60 cm | UK 7 3/8 | Large |
| 61 cm | UK 7 1/2 | Large to extra large |
| 62 cm | UK 7 5/8 | Extra large |
| 63 cm | UK 7 3/4 | Extra large |
Related Hat Size Conversions
| Conversion | Method | Use it for |
|---|---|---|
| cm to inches | cm / 2.54 | Checking US or older hat charts. |
| inches to UK size | inches / pi | Traditional numbered hats. |
| UK size to inches | UK size x pi | Reverse check from a hat label. |
| UK size to cm | UK size x pi x 2.54 | Compare a numbered hat with a cm chart. |
| cm to S/M/L | Retailer band table | Soft caps and casual hats. |
| helmet size | Manufacturer chart | Safety helmets and riding helmets. |
| child hat size | cm chart by age and brand | Age alone is not reliable. |
| women’s fashion hats | cm plus brand label | Style may be one-size or adjustable. |
| flat cap size | cm or UK number | Check lining and peak shape. |
| formal hat resizing | Professional adjustment | Useful when a valuable hat sits between sizes. |
Measurement Mistakes To Avoid
Do not pull the tape tight enough to press into the hair or skin. Do not measure around the crown where the hat will not sit. A tape angled down at the back can add extra centimetres, while a tape placed too high on the forehead can produce a smaller result. If you have thick hair, measure in the style you expect to wear under the hat.
For helmets and protective headwear, use the manufacturer’s size chart and safety guidance rather than relying only on traditional hat size. This converter is most suitable for ordinary hats, caps and retail size checking. Protective equipment has fit and safety rules that sit outside a simple circumference conversion.
When To Size Up Or Down
Choose the next size up if the hat is rigid, the measurement is exactly on a boundary, or you plan to wear a thin liner. Choose the closer size if the hat has an internal adjustment band. Do not size up to solve a width or shape issue if the hat then moves when you walk. For expensive felt hats, a hatter can advise on stretching, padding or block shape.
If you are buying a hat as a gift, keep the centimetre measurement and the rounded UK size together. A returns policy matters because two people with the same circumference can have different head shapes. For vintage hats, ask for the internal band measurement rather than relying only on the label.
FAQs
What UK hat size is 58 cm?
With no allowance, 58 cm is about UK 7 1/4 when rounded to the nearest eighth. Some charts may show 58 cm as 7 1/8 or 7 1/4 depending on rounding.
Is UK hat size the same as US hat size?
Traditional UK and US numbered hat sizes are often very close because both use head circumference in inches divided by pi. Retailer labels can still differ.
Should I round up?
Round up for rigid hats when you sit between sizes, but use the brand chart if it offers a precise centimetre band.
Can I use this for helmets?
Use it only as a rough circumference conversion. Helmets should be chosen from the manufacturer’s chart and safety fitting instructions.
How do I measure without a tape?
Wrap string around the head where the hat sits, mark the overlap, then measure the string against a ruler. A soft tape is easier and usually more accurate.
Why does my hat label show only S, M or L?
Many casual hats use broad size bands. Use the centimetre result to choose the band, then check adjustment and return options.
Sources
- Lock & Co. Hatters. (2026). Hat size guide. Lock & Co. https://www.lockhatters.com/pages/hat-size-guide
- Christys’ London. (2026). Hat size guide. Christys’ London. https://www.christys-hats.com/pages/hat-size-guide
- National Institute of Standards and Technology. (2008). Guide for the Use of the International System of Units (SI), Special Publication 811. NIST. https://www.nist.gov/pml/special-publication-811