Fitted caps, fedoras and helmets are sized by head circumference measured about 1 cm above the ears and eyebrows. US and UK fitted sizes are an old system based on the diameter of the head in inches, which is why they look like 7⅛ or 7¼.
How to use the Hat Size Calculator
- Head circumference
Results update as you type, and every field accepts the units shown in its menu.
How it works
Wrap a soft tape around your head where the hat will sit, just above the ears and eyebrows, and enter the measurement.
Tips
- Measure over thin clothing with a soft tape held snug but not tight, and keep the tape level all the way round.
- Take each measurement twice; if you are between sizes, the larger size is usually the safer choice.
- Size charts vary by brand and country – check the maker’s own chart when it is available.
- For sewing projects, pre-wash fabric before cutting and buy 10% extra for shrinkage, pattern matching and mistakes.
Common mistakes to avoid
- Mixing inches and centimetres between the tape and the size chart.
- Forgetting seam and hem allowances when cutting fabric.
- Measuring an old, stretched garment instead of your body.
What each result means
- Metric size = round([Head circumference]÷0.01)
- US fitted size = round([Head circumference]÷0.0254÷pi×8)÷8
- UK fitted size = round([Head circumference]÷0.0254÷pi×8)÷8 – 0.125
Example values
How the main result changes when Head circumference changes and everything else stays at the example values above:
| Head circumference (cm) | Metric size |
|---|---|
| 28.5 | 29 |
| 42.75 | 43 |
| 57 | 57 |
| 71.25 | 71 |
| 85.5 | 86 |
| 114 | 114 |
Formula
Metric hat size = head circumference in cmUS fitted size = circumference (in) ÷ π, to the nearest ⅛UK size = US size − ⅛Worked example
Inputs — Head circumference: 57 cm.
Results — Metric size = 57; US fitted size = 7 1/8; UK fitted size = 7; Letter size = M; Circumference in inches = 22.44 in.
Frequently asked questions
How do I measure hat size?
Measure around your head about 1 cm above the ears and eyebrows, where the hat sits.
What hat size is 57 cm?
Metric 57, US 7⅛, UK 7, letter M.
What if I am between sizes?
Go up a size; hats can be made smaller with sizing tape but not larger.
What formula does the Hat Size Calculator use?
Metric hat size = head circumference in cm.