# Hat Size Calculator

URL: https://speedycalculator.com/calculators/everyday-life/hat-size/
Last updated: 2026-10-25

> The hat size calculator converts head circumference into US and UK fitted hat sizes, letter sizes (XS to XXXL) and metric size, for adults and children.

## Inputs

- Head circumference (cm, in)

## Results

- Metric size
- US fitted size
- UK fitted size
- Letter size
- Circumference in inches

## Formula

```
Metric hat size = head circumference in cm
US fitted size = circumference (in) ÷ π, to the nearest ⅛
UK size = US size − ⅛
```

## Explanation

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 size28.52942.7543575771.257185.586114114

## Worked 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.

## FAQ

**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.