Question 1

How accurate is this hat size calculator?

Accepted Answer

The calculator uses the international standard hat size conversion (circumference ÷ π, rounded to the nearest 1/8 inch). Match a correct tape measurement and it's exact. The most common source of error isn't the math — it's measuring at the hairline instead of ~2.5 cm above the eyebrows, which underestimates by a full size.

Question 2

Should I round up or down if I'm between sizes?

Accepted Answer

Always up. A hat can be padded down with a $3 self-adhesive sizing strip in 60 seconds; a hat that's a full size too small will squeeze, leave a red ring, and can only be stretched 3–4 mm without deforming.

Question 3

Does the calculator work for baseball fitted caps?

Accepted Answer

Yes — fitted caps use the same US fractional sizing. Add one half-size if you like a very relaxed fit, since fitted-cap polyester/wool shells run about 3–5 mm tighter than felt at the same labelled size.

Question 4

What if my head is bigger than 66 cm?

Accepted Answer

Above 66 cm (US 8¼) you're in made-to-order territory. Optimo (Chicago) and Bencraft (London) will block a felt hat to spec; expect a 6–12 week wait and starting prices around $400.